SYSTEM for EXERCISE STRESS CARDIAC MAGNETIC RESONANCE IMAGING THESIS Presented in Partial Fulfillment of the Requirements for Th

SYSTEM FOR EXERCISE STRESS CARDIAC MAGNETIC RESONANCE

IMAGING

THESIS

Presented in Partial Fulfillment of the Requirements for

the Degree Master of Science in the Graduate

School of The Ohio State University

Eric L. Foster, B.S.

* * * * *

The Ohio State University

2009

Thesis Committee: Approved by:

Professor Orlando P. Simonetti, PhD,

Advisor

Professor Robert A. Siston, PhD Adviser

Professor John W. Arnold Mechanical Engineering Graduate

Program

ABSTRACT

Coronary artery disease (CAD) is the leading cause of death in the United States.

Early detection provides the opportunity to initiate medical treatment and lifestyle changes that can save lives. Cardiac imaging and treadmill exercise stress testing are the primary noninvasive methods of diagnosing CAD. Echocardiography and single photon emission computed tomography (SPECT) are imaging methods combined with exercise stress testing clinically; however both suffer from limitations in image quality and diagnostic ability. In addition, SPECT involves the injection of a radioisotope. Magnetic resonance imaging (MRI) can provide a comprehensive cardiac examination (function and perfusion) with higher quality images without the use of radiation.

The challenges of the MRI environment have prevented the successful combination of treadmill exercise stress and MRI. Standard treadmills contain ferromagnetic components and use electromagnetic motors. The strong magnetic field of the MRI environment can turn ferromagnetic objects into projectile hazards and can disrupt the operation of electromagnetic motors. A treadmill exercise cardiac magnetic resonance imaging (CMR) system requires a completely MR compatible treadmill which is able to sit safely in the MR environment and not affect MRI image quality. The treadmill must be capable of automatically running the Bruce treadmill protocol and

ii allow for the acquisition of function images within 60 s of peak stress, according to

American Heart Association (AHA) guidelines.

The feasibility of the exercise stress CMR method was tested using a “semi- compatible” treadmill that could be placed in the outer corner of the MRI room. Ten cardiac patients successfully completed the study with an average time to start function imaging of 71±7 s.

A MRI compatible treadmill was developed, which used water hydraulics to power the drive and elevation systems. This design allows a traditional electromagnetic motor to be placed outside of the MR environment, and power is transferred through fluid-filled hoses running through an opening in the MRI room to MR compatible hydraulic actuators located on the front of the treadmill. A hydraulic motor powers the treadbelt through a shaft and pulley system. A hydraulic cylinder acts on a lever arm to raise and lower the treadmill elevation. All structural components were fabricated from nonferromagnetic aluminum and stainless steel.

MR safety tests performed with a powerful hand magnet confirmed only a small amount of ferromagnetic components in the drive bearings and cylinder tie rods.

Substitute components have been located for replacement, but the system was deemed safe under supervision. MRI system diagnostic testing confirmed that the presence of the treadmill system in the room does not affect image quality, and tests comparing treadmill performance inside and outside of the MRI room confirmed that the MR environment did iii not significantly affect treadmill performance (no worse than 1.6% difference at any stage). The treadmill was tested in the MR environment under subject load, and was able to successfully operate through the range of the Bruce protocol. Three subjects performed maximal exercise testing, and function imaging was successfully completed in an average of 45 s, meeting the 60 s AHA guideline.

ACKNOWLEDGMENTS

I would like to recognize and thank the people who have supported and assisted me through my graduate studies. A project of this scope would not have been possible without the combined knowledge, expertise, guidance, and support from a variety of sources.

To Danielle Komarek, my partner who has been by my side through this whole experience. Knowing that I could come home to your smile has made each day a bit easier. Your love and patience have made this all worthwhile, and having you in my life means the world to me.

I will always be thankful for the opportunity to work with Lon Simonetti, my

advisor. I would not have considered graduate studies if I didn’t feel like I’d found the

right person to guide me through the experience. Lon, you’ve had my back each step of

the way, and you are the type of supervisor that inspires hard work without having to

demand it. Working with you has given me the opportunity to observe the qualities

needed for success in action, and I hope to carry that into our company.

This experience has been much more beneficial to me than just a degree. It has surrounded me with the right combination of talent to push this work forward. Along with myself and Lon, two other individuals have played a major role in launching a

v company to commercialize this effort. Thanks to John Arnold, whose hydraulic expertise has enabled this project to get off the ground. Whenever I got stuck on part of the design, the answer was always just a phone call away. Without clinical guidance, the treadmill developed in this thesis would not have been very useful. The guidance and advice of Dr.

Subha Raman is enabling this to become a fully working system capable of making a difference in peoples’ lives.

Thanks also to Dr. Robert Siston, one of my committee members. Rob’s advice was among the first I received when beginning to consider graduate school and I have found myself repeating it to others over the course of the last couple of years. In addition, the life he has breathed into the Mechanical Engineering department has been a welcome change. Both in and out of the classroom, he is a model professor whose style others could benefit from observing.

I would like to extend thanks to the rest of the treadmill development team. Jacob

Bender, a fellow member of the Cardiac MRI research team was enormous help in the effort to make this a functional system. Jake only recently started working on the control side of this project, but his knowledge, experience, and dedication is one of the reasons we were able to get the treadmill working. His was there with us late nights and on weekends making sure that everything on the electrical/control side of the system was performing well enough to test the mechanical systems. Thanks also to Mihaela Jekic for

vi her work on control system and imaging development. You were my introduction to this project, and I would not have been involved if not for your recommendation.

This project has required a great deal of cooperation from the OSU Medical

Center, and certain individuals have been instrumental in enabling the clinical testing of this concept. Thanks to Beth McCarthy, for recruiting subjects and assisting with each test. Michelle Ballinger and Jennifer Dickerson helped a great deal in the early stages of development to make this a clinically viable system. Anne Garcia allowed us to perform research in Ross Heart Hospital MRI area and provided guidance on how our system could be worked into the clinical environment.

Finally, thanks to the rest of the CMR/CT research team: Yu Ding, Harris Lin,

Georgetta Mihai, Shivraman Giri, and Yiu-Cho Chung. Your knowledge and friendship have made the lab an inspiring and fun place to work.

vii

VITA

September 11, 1975 Born – Rochester, NY, USA

December, 2005 B.S. Mechanical Engineering, The Ohio State University

June, 2006 - Dec 2006 Student Researcher, Cardiac Imaging Research Center, The Ohio State University

January, 2007 - March 2009 Graduate Research Associate Cardiac Imaging Research Center, The Ohio State University

PUBLICATIONS

Research Publications

1. Raman, SV, Jekic, M , Dickerson, JA, Foster, EL and Simonetti, OP, “In-room treadmill exercise stress cardiac magnetic resonance in patients with suspected ischemic heart disease” Journal of Cardiovascular Magnetic Resonance 2009, 11(Suppl 1):O40

2. Raman, SV, Richards, DR, Jekic, M, Dickerson, JA, Kander, NH, Foster, EL, Simonetti, OP, “Treadmill Stress Cardiac Magnetic Resonance Imaging: First In Vivo Demonstration of Exercise-Induced Apical Ballooning ” J. Am. Coll. Cardiol. 2008;52;1884

3. Jekic, M, Foster, EL , Raman, SV, Simonetti, OP, “Cardiac function and myocardial perfusion immediately following maximal treadmill exercise inside the MRI room” J Cardiovasc Magn Reson , 2008. 10 (1): p. 3.

FIELDS OF STUDY

Major Field: Mechanical Engineering

viii

TABLE OF CONTENTS

ABSTRACT ...... ii

ACKNOWLEDGMENTS ...... v

VITA ...... viii

TABLE OF CONTENTS ...... ix

LIST OF TABLES ...... xvi

LIST OF FIGURES ...... xviii

1 INTRODUCTION ...... 1

1.1 Research Goals ...... 1

1.2 Organization of Thesis ...... 2

2 BACKGROUND ...... 3

2.1 Coronary Artery Disease - Problem and Significance ...... 3

2.2 Detection – Current Imaging Methods ...... 5

2.2.1 Echocardiography ...... 7

2.2.2 Nuclear Scintigraphy ...... 9

2.2.3 Magnetic Resonance Imaging (MRI) ...... 11

2.3 Exercise Stress Testing...... 14 ix

2.4 Desired Method – Treadmill Exercise Stress MRI ...... 17

2.4.1 Benefits ...... 17

2.4.2 Challenges ...... 19

3 THE MR ENVIRONMENT ...... 22

3.1 Introduction ...... 22

3.2 Definitions ...... 24

3.3 Materials ...... 28

3.3.1 Aluminum ...... 28

3.3.2 Stainless Steel ...... 29

3.3.3 Other metals ...... 31

3.3.4 Composites ...... 31

3.4 Actuators ...... 31

3.4.1 Drive shaft/belts ...... 32

3.4.2 Hydraulic...... 33

3.4.3 Pneumatic ...... 34

3.4.4 Electric ...... 35

3.5 Electrical/Power Issues ...... 37 x

3.5.1 Heating ...... 37

3.5.2 RF Noise / Image Quality ...... 38

3.5.3 Solutions ...... 39

4 PRELIMINARY TESTING...... 40

4.1 Initial Prototype Treadmill ...... 40

4.2 Methods ...... 44

4.3 Results ...... 46

4.4 Conclusions ...... 47

5 MRI COMPATIBLE HYDRAULIC TREADMILL ...... 48

5.1 Structural Components ...... 48

5.1.1 Frame ...... 48

5.1.2 Rollers ...... 49

5.2 Hydraulic Power Systems Overview...... 50

5.3 Drive System ...... 55

5.3.1 Drive Requirement Tests ...... 56

5.3.2 Drive System Requirements ...... 60

5.3.3 Hydraulic Motor...... 67 xi

5.3.4 Hydraulic Power Pack...... 68

5.3.5 Addition drive system components ...... 70

5.3.6 Drive System Component Summary ...... 76

5.4 Elevation System ...... 77

5.4.1 Alternative strategies: ...... 77

5.4.2 Lever Arm ...... 78

5.4.3 Cylinder...... 82

5.4.4 Accumulator ...... 84

5.4.5 Other elevation system components ...... 87

5.4.6 Fluid Control Valves ...... 89

5.5 Additional Hydraulic Components...... 90

5.5.1 Hydraulic hoses ...... 90

5.5.2 Power Pack Cart ...... 94

5.6 Control System ...... 96

6 PERFORMANCE TESTING AND REDESIGN ...... 98

6.1 Initial System Testing...... 98

6.1.1 Drive System Performance ...... 98 xii

6.1.2 Elevation System Performance ...... 105

6.2 Second Design Iteration and Testing ...... 106

6.2.1 Braking Valve ...... 106

6.2.2 Noise and Vibration ...... 108

6.2.3 Valve leakage ...... 109

6.2.4 Maximum Elevation...... 111

7 ADDITIONAL SYSTEM COMPONENTS ...... 113

7.1 480V Power ...... 113

7.2 Penetration Panel ...... 114

7.3 Patient Monitoring...... 115

8 VALIDATION ...... 121

8.1 MR Compatibility Testing ...... 121

8.1.1 MR Safety/Conditionality ...... 121

8.1.2 MR Compatibility ...... 124

8.1.3 Conclusions ...... 130

8.2 Performance testing inside and outside of the MRI room ...... 131

8.3 Performance testing with human subject inside the MRI room ...... 134 xiii

8.3.1 Methods and Materials ...... 134

8.3.2 Results and Conclusions ...... 136

8.4 Demonstration of Treadmill Exercise Stress Cardiac MRI with MRI Compatible

Treadmill ...... 137

8.4.1 Methods and Materials ...... 138

8.4.2 Results and Conclusions ...... 139

9 CONCLUSIONS AND FUTURE WORK ...... 144

9.1 Conclusions ...... 144

9.2 Drive and Elevation System Improvements ...... 145

9.3 Next-generation Prototype ...... 147

9.3.1 Structural Improvements ...... 147

9.3.2 Treadmill Height Adjustment ...... 148

9.3.3 Control System...... 149

9.4 Significance of the work ...... 150

APPENDIX A ...... 153

A.1 Results of shim test with no treadmill in room ...... 154

A.2 Results of shim test with treadmill in room ...... 156

xiv

A.3 Results of spike test with no treadmill ...... 158

A.4 Results of spike test with treadmill in room ...... 160

A.5 Results of artifacts calculation – without treadmill ...... 162

A.6 Artifacts Calculations – With Treadmill ...... 168

A.7 RF Noise Test with Hydraulic Hoses ...... 174

A.8 RF Noise Test with Hydraulic Hoses and Treadmill ...... 176

A.9 RF Noise Test with Hydraulic Hoses and Fiber-Optic Cables...... 178

APPENDIX B ...... 193

BIBLIOGRAPHY ...... 215

LIST OF TABLES

Table 2.1: Parameters for Bruce Treadmill Exercise Protocol ...... 14

Table 3.1: MR labels introduced with ASTM F2503-05 ...... 27

Table 3.2: Material properties of common grades of stainless steel ...... 30

Table 4.1 - Results of preliminary testing in 10 patients with suspected CAD. MPHR = maximum age-predicted heart rate...... 46

Table 5.1: Comparison of densities of Plain Steel and Type 316 Stainless Steel (source:

[48, 49])...... 50

Table 5.2 - Current Measurement for Landice L7 running Bruce Protocol ...... 58

Table 5.3 - Force measurements obtained from Landice L7 Treadmill at each Bruce

Protocol elevation stage ...... 59

Table 5.4: Drive system requirement calculations performed using force data ...... 62

Table 5.5: Summary of drive system requirements ...... 66

Table 5.6: Kinetic Energy stored by flywheel ...... 71

Table 5.7: Results from study by Gottshcall et al. with treadmill speed of 2.8 MPH and varying uphill elevations. (Source [50]) ...... 72

Table 5.8: Part list for drive system ...... 76

Table 5.9: Summary of elevation system calculations...... 86

xvi

Table 5.10: Summary of Elevation System Components ...... 87

Table 6.1: Flow rate for examining hydraulic motor leakage ...... 99

Table 6.2: Relief valve testing at 50 Hz. electric motor speed ...... 101

Table 8.1: Results of MRI system performance testing. (+) indicates that specification has been met...... 128

Table 8.2: Results of RF noise tests. (+) indicates specifications have been met. (-) indicates specifications were not met ...... 129

Table 8.3: Results of performance testing with no subject inside and outside the MRI room...... 132

Table 8.4: Results from performance testing inside MRI room ...... 136

Table 8.5: Timing results of maximal stress treadmill exercise CMR study ...... 140

Table 8.6: Comparison of results obtained in preliminary data and with MR compatible treadmill ...... 142

xvii

LIST OF FIGURES

Figure 2.1: A normal artery with normal blood flow is shown in A. B shows an artery with plaque buildup (source [2]]) ...... 4

Figure 2.2: Short axis view of the heart obtained with MRI showing A) normal rest function, B) stress function abnormality, C) normal rest perfusion, D) stress perfusion defect ...... 6

Figure 2.3: Cardiac echo image showing a short-axis view of the left ventricle ...... 8

Figure 2.4: Cardiac Nuclear SPECT short-axis image of the heart. Decreased signal intensity in the inferior wall may indicate ischemia, or attenuation artifact...... 10

Figure 2.5: Delayed Enhancement CMR image showing signal enhancement in region of anterior myocardial infarction...... 12

Resting CMR perfusion image shows uniform enhancement without artifact. E: End-

xviii systolic cine CMR image at stress shows diminished thickening of the basal inferior wall

(arrow). F: Resting end-systolic cine CMR image shows normal uniform thickening of heart muscle. G: Invasive X-ray coronary angiogram confirming total occlusion of the right coronary artery (arrow) responsible for findings on stress imaging. H: Scar CMR image obtained at rest 10 minutes after contrast injection shows uniformly black myocardium, indicating viable heart muscle and no evidence of prior myocardial infarction...... 13

Figure 2.7: ECG tracing showing ST-segment depression indicative of ischemia.

Horizontal ST segment depression (A=at rest, B=after three minutes' exercise, C=after six minutes' exercise) and downsloping ST segment depression (D=at rest, E=after six minutes' exercise) Source [9] ...... 15

Figure 2.8: Typical layout for exercise stress echocardiography test. The patient bed is immediately adjacent to the treadmill to minimize time between exercise and imaging, and risk of patient injury...... 18

Figure 3.1: Model MR Environment. Zones III and IV notate the potentially hazardous area of the MRI environment. (Source: [29]) ...... 23

Figure 3.2: Phantom MRI image showing a band of RF noise artifact. Source [46] ...... 38

Figure 4.1: Commercial treadmill schematic showing ferromagnetic components ...... 40

Figure 4.2: Magnetic field plot and exam room layout for the MR system (1.5 Tesla

MAGNETOM Avanto, Siemens Healthcare, Malvern, PA) in the Ohio State University xix

Ross Heart Hospital showing the position of the prototype treadmill inside of MRI exam room...... 42

Figure 4.3: Room setup and patient monitoring equipment for preliminary data. Patient physiologic data is displayed on computer monitor and recorded to a PC. ECG, monitor and bluetooth extension cables are brought into the MRI room through wave guide...... 43

Figure 4.4 -Timeline for the treadmill MRI test, including slice localization, rest and stress function, rest and stress perfusion, and viability. The estimated cumulative time for the entire procedure is shown...... 44

Figure 5.1: Exam room layout for exercise stress MRI. Electric motor driven pump and monitoring system computer remain outside of exam room. All other equipment, including treadmill, ECG display, BP monitor, and MRI control console are within the room...... 51

Figure 5.2: Layout of Drive Motor and Elevation Cylinder for MR-Compatible Treadmill

...... 52

Figure 5.3: Electric motor driven pump, positioned outside of MRI exam room, powers treadmill mounted hydraulic motor and cylinder through hoses...... 54

Figure 5.4: Hydraulic drive system components ...... 55

Figure 5.5: Torque measurement for preliminary data ...... 57

Figure 5.6: Specifications for M6 hydraulic motor supplied by The Water Hydraulics Co,

Ltd...... 63 xx

Figure 5.7: Specifications for P6 hydraulic pump supplied by The Water Hydraulics Co.,

Ltd...... 65

Figure 5.8: Hydraulic motor (P/N: M6, The Water Hydraulics Co. Ltd) ...... 67

Figure 5.9: Hydraulic power pack (P/N P6, The Water Hydraulics Co. Ltd)...... 69

Figure 5.10: Belt Tensioner ...... 74

Figure 5.11: Shaft coupler...... 75

Figure 5.12: Schematic of elevation lever arm ...... 78

Figure 5.13: Side view of treadmill showing critical dimensions for elevation calculations ...... 79

Figure 5.14: Relationships used for elevation system calculations ...... 79

Figure 5.15: Elevation system legs in both flat and extended positions ...... 81

Figure 5.16: Front end of the treadmill showing cylinder and upper leg of elevation system ...... 82

Figure 5.17: Relationships for determining cylinder stroke length ...... 83

Figure 5.18: Relationships for determining effective weight ...... 83

Figure 5.19: Section view of Figure 5.3 emphasizing control valves ...... 89

Figure 5.20: Hydraulic hoses and stainless steel quick connectors ...... 90

Figure 5.21: Nomogram for sizing hydraulic hoses, supplied by Parker Hydraulics. The dotted line is from their example. The three solid lines represent the hose ID’s investigated for the treadmill system...... 92 xxi

Figure 5.22: Power pack cart with prototype controls attached ...... 94

Figure 5.23: Control System schematic for MRI-Compatible treadmill ...... 96

Figure 6.1: Hydraulic motor disassembled for inspection ...... 100

Figure 6.2: Specifications for Lovejoy Oldam Style shaft coupler ...... 102

Figure 6.3: Image of Treadmill Drive system with new coupler and drive system mount.

...... 103

Figure 6.4: Cross Sectional view of Parker Parflex 540N Hydraulic Hose ...... 105

Figure 6.5: Performance testing of system with new system relief valve, fixed orifice braking and no feedback. “Loaded” curves represent tests performed with a 200 lb subject...... 107

Figure 6.6: Drive shaft showing evidence of wear with first bearing configuration ..... 108

Figure 6.7: Section view of Figure 5.3 indicating problematic valves ...... 109

Figure 6.8: Bearing crafted from aluminum stock ...... 111

Figure 7.1: 480V fuse box and receptacle installed in W080 Scott Lab ...... 113

Figure 7.2: Existing penetration panel with new waveguide contained in storage cabinet

...... 114

Figure 7.3: MRI compatible patient monitoring equipment for use with exercise stress

CMR...... 116

Figure 7.4: Screen shot of GE Cardiosoft stress test program ...... 117

xxii

Figure 7.5: 12-lead ECG system will connect patient to PC located outside of magnet

room ...... 118

Figure 7.6: Graphical representation of the ischemic cascade, showing perfusion defect and systolic dysfunction (observable as wall motion abnormality) occurring earlier than

ECG changes (Source [53]) ...... 119

Figure 8.1: Treadmill system placed in MRI room...... 123

Figure 8.2 Sample image from RF noise test with hydraulic hose, fiber-optic cable, and antenna ...... 130

Figure 8.3 Treadmill set up in room for performance testing ...... 134

Figure 8.4: Volunteer subject performing maximal exercise stress on MR compatible treadmill ...... 137

Figure 8.5: Ross Heart Hospital MRI room setup for cardiac stress MRI with MR compatible treadmill ...... 138

Figure 8.6: Vertical long-axis and short-axis frames acquired at end-systole at rest and after treadmill exercise stress. Increased myocardial thickening clearly shown in stress images acquired after exercise...... 143

Figure 9.1: Proposed hydraulic system redesign ...... 146

Figure B.1: Hydraulic Motor (supplied by The Water Hydraulics Co, Ltd) ...... 194

Figure B.2: Hydraulic Power Pack (supplied by The Water Hydraulic Co. Ltd.) ...... 195

Figure B.3: Bearing for Drive Shaft (supplied by McMaster-Carr) ...... 196 xxiii

Figure B.4: Tensioner Wheel ...... 197

Figure B.5: Tensioner Body ...... 198

Figure B.6: Flywheel ...... 199

Figure B.7: Motor Shaft ...... 200

Figure B.8 Drive Bearing Mount ...... 201

Figure B.9: Drive Roller ...... 202

Figure B.10: Shaft Collar ...... 203

Figure B.11: Motor Mount ...... 204

Figure B.12: Drive Pulley ...... 205

Figure B.13: Hydraulic Cylinder (supplied by Lehigh Fluid Power) ...... 206

Figure B.14: Bearing for Elevation System (supplied by McMaster-Carr) ...... 207

Figure B.15: Upper Leg ...... 208

Figure B.16: Lower Leg ...... 209

Figure B.17: Clevis Mount ...... 210

Figure B.18: Cross Brace ...... 211

Figure B.19: Lower Brace...... 212

Figure B.20: Upper Brace ...... 213

Figure B.21: Trunion Mount ...... 214

xxiv

CHAPTER 1

INTRODUCTION

1.1 Research Goals

Coronary artery disease (CAD) is characterized by plaque buildup in the coronary arteries. CAD is the number one cause of death in the United States [1], however early detection provides an opportunity to initiate medical treatment and lifestyle changes that can reverse the effects of the disease. Several methods are currently used to detect CAD, however each of these methods suffer from technical limitations or harmful side effects.

Exercise Stress Cardiac Magnetic Resonance Imaging (CMR) would provide a safe, noninvasive method to accurately diagnose CAD, but technical challenges have prevented the development of a system to perform this type of examination. The large static magnetic field of an MRI system prevents the placement of standard exercise equipment in the vicinity of the machine. For this reason, we sought to develop a novel

MRI-compatible treadmill to allow for placement directly adjacent to an MRI machine.

The successful implementation of such a system will allow for a higher standard of clinical care for cardiac patients, and allow for a safe, flexible examination method for

MRI system owners.

1.2 Organization of Thesis

This thesis is divided into nine chapters. Chapter 1 provides a top level overview of

the problem to be investigated and informs the reader on the layout of the text. Chapter 2

offers background into the nature of CAD, the size and scope of the problem, and an

overview of current imaging methods. The proposed new diagnostic modality of exercise

stress CMR is introduced along with the challenges of developing such a method.

Chapter 3 describes the terminology and challenges of the MR environment along with a

review of MR safe materials and actuation methods currently used in MR equipment

design. The preliminary development and feasibility testing of the exercise CMR

procedure using a “semi-MR compatible” treadmill is described in Chapter 4. It is shown

that the method appears promising, and provides further justification for a completely

MR compatible treadmill. Chapter 5 describes the initial design and fabrication of a

water hydraulically powered, MR compatible treadmill. Initial lab testing and design

refinements are described in Chapter 6. In Chapter 7, the other components of the

exercise stress CMR system developed by the author are described. Chapter 8 describes

the testing performed on the treadmill system to determine MR safety and compatibility,

and the results of performance testing inside and outside of the MR environment.

Chapter 9 contains conclusions and a description of future work required to make the

described system clinically realizable.

CHAPTER 2

BACKGROUND

2.1 Coronary Artery Disease - Problem and Significance

Coronary artery disease (CAD) is a condition which affects 13.2 million individuals in the United States, leading to 500,000 deaths annually, and resulted in an estimated

$142.5M economic burden in 2006 [1]. The disease is characterized by plaque buildup in the coronary arteries, which restricts blood flow to the myocardium (heart muscle) [2].

This reduced blood flow causes a deficit of oxygen to the myocardium (ischemia) and may result in cell death of the affected parts of the heart muscle. Besides the ischemia that directly results from blockages, the built-up plaque may rupture, causing thrombosis and total occlusion of the coronary arteries. Either type of blockage may lead to angina

(chest pain) or myocardial infarction (heart attack). CAD often goes undetected until the disease is in its advanced stages, and often only after a cardiac event.

Figure 2.1: A normal artery with normal blood flow is shown in A. B shows an artery with plaque buildup (source [2]])

2.2 Detection – Current Imaging Methods

Two thirds of sudden cardiac deaths occur without prior recognition of disease

[1]. Despite the severity of CAD, in many instances the disease is reversible. For this reason, early detection is extremely important in order to initiate pharmacologic treatment and lifestyle changes. These changes offer the opportunity to abate or reverse the effects of CAD.

Cardiac imaging and exercise stress testing are the primary noninvasive methods of detecting CAD. Imaging methods allow physicians to visually and quantitatively assess the level of ischemia using functional (wall motion) or perfusion (blood flow) imaging. In some cases, severe ischemia may be observable at rest; however more typically ischemia and symptoms of ischemia are only apparent with stress. As physical stress level increases, so does the required amount of oxygen required by the heart.

Many cardiac patients experience no angina or other symptoms until stress levels rise.

Stress function imaging examines the thickening of the myocardium under stress conditions. Figure 2.2a shows a short axis cardiac view of the myocardium at rest. A uniform myocardial thickness can be observed in the image. In Figure 2.2b, a nonuniform thickening can be seen under stress conditions. This nonuniformity indicates a deficiency in blood flowing to the affected area which is preventing normal myocardial contraction and thickening. Stress perfusion imaging allows for a real-time look at the

5 blood flowing through the myocardium. Figure 2.2c shows a perfusion image under rest conditions on an MRI scan. Uniform image intensity can be observed in each myocardial segment. Figure 2.2d shows a perfusion defect under stress conditions. The dark spot in the myocardium demonstrates that injected contrast agent has not yet arrived in that region, indicating a lack of blood flow to that area.

Figure 2.2: Short axis view of the heart obtained with MRI showing A) normal rest function, B) stress function abnormality, C) normal rest perfusion, D) stress perfusion defect

Three noninvasive imaging modalities, echocardiography, nuclear scintigraphy, and magnetic resonance imaging (MRI), are commonly used for imaging the heart at stress.

2.2.1 Echocardiography

Echocardiography (echo) is a safe, non-invasive imaging method which uses ultrasound technology to examine the wall motion (function) of the heart. Acoustic waves propagate through the chest cavity and are received using an ultrasound transceiver. The acoustic signal is then reconstructed into an image. Echo uses no contract agents or radiation, and is a rapid, safe method of cardiac imaging. The use of microbubble contrast agents for echocardiography is coming into more widespread clinical practice, but there are some associated safety concerns [3]. These agents can improve the quality of cardiac function studies, and offer the potential to perform myocardial perfusion imaging by echocardiography.

Echocardiography does suffer from several limitations. The modality suffers from low signal-to-noise and contrast-to-noise ratios and relies on an “acoustic window” to send and receive its signal. Several conditions may infringe upon this window and affect image quality and diagnostic ability. Body habitus, prior surgery and lung disease may all affect the acoustic window required to obtain images. When combined with exercise stress, heavy breathing and heart motion may affect the sonographer’s ability to locate an effective acoustic window, which can prolong image acquisition and decrease the effectiveness of the diagnosis. It is extremely important in echocardiography to image as close as possible to peak stress in order to prevent the resolution of stress-

7 related wall motion abnormalities. Greater discussion of this topic may be found in

Section 2.4.2.1. Echocardiography is currently unable to image perfusion, and consequently relies on a single method of diagnosis to detect CAD. The sensitivity and specificity of stress echo wall motion diagnosis of ischemic heart disease is 76% and

88%, respectively [4]. An example of an echocardiographic image can be seen in Figure

2.3.

Figure 2.3: Cardiac echo image showing a short-axis view of the left ventricle

2.2.2 Nuclear Scintigraphy

Single Photon Emission Computed Tomography (SPECT), or nuclear scintigraphy, is an imaging method which uses an injected radioisotope to examine perfusion (blood flow through the myocardium). It is the most frequently used noninvasive method of detecting CAD and has sensitivity and specificity of 88% and

77% respectively [4]. Like echocardiography, SPECT imaging is easily paired with exercise stress testing because special exercise equipment is not required to perform the study. The radioisotopes commonly used for nuclear perfusion imaging are actively and rapidly taken up by viable myocytes and remain in cells for an extended time. Thus, a

“snapshot” of the perfusion of the heart muscle at the point of peak cardiovascular stress can be obtained by injecting the radioisotope just as the patient reaches peak stress on a treadmill. Unlike echocardiography, there is no rush to image the patient within seconds of peak stress; the radioisotope remains in the cells long enough that a patient can be imaged after recovery, some 30 min to 60 min after exercise. The treadmill stress can therefore by performed in a separate room from the nuclear camera. Despite its widespread use, SPECT imaging suffers from several major limitations and harmful side effects. Spatial resolution is limited, on the order of 10–14 mm, and degrades with distance [5]. Image quality is frequently degraded by attenuation artifacts and photon scatter. In addition, a SPECT exam may cause radiation exposure of between 10-28

9 miliseverts [6], which may not only be harmful on its own, but compounds with radiation exposure from other exams likely for a given patient and limits its use in serial follow-up of chronically ill patients.

Figure 2.4 shows an image from a typical cardiac perfusion SPECT exam with clearly limited spatial resolution. The low resolution makes diagnosis of a perfusion defect difficult.

Figure 2.4: Cardiac Nuclear SPECT short-axis image of the heart. Decreased signal intensity in the inferior wall may indicate ischemia, or attenuation artifact.

2.2.3 Magnetic Resonance Imaging (MRI)

Cardiac Magnetic Resonance Imaging (CMR) provides the opportunity to offer a safe examination with improved diagnostic capability over both SPECT and echo. MRI does not use ionizing radiation to obtain images and allows for imaging of both cardiac function and perfusion. Sensitivity and specificity values for CMR pharmacological stress perfusion imaging in diagnosis of CAD in at least 1 coronary artery has been reported at 90% and 85% [7]. In addition, CMR offers the ability to image previous scar tissue in the myocardium in order to differentiate between new ischemia and past myocardial damage. Figure 2.5 shows an example of this type of imaging, called delayed enhancement. The white area of the myocardium demonstrates where the contrast agent has been taken up by necrotic tissue. The healthy tissue appears black because contrast agent has washed out of the area.

Necrotic

Healthy

Figure 2.5: Delayed Enhancement CMR image showing signal enhancement in region of anterior myocardial infarction.

The results of a comprehensive stress CMR exam including function, perfusion and delayed enhancement imaging can be seen in Figure 2.6. Because of the challenges

of developing exercise equipment that is compatible with the MR environment, stress

CMR is performed in clinical practice using pharmacological agents which attempt to

reproduce stress conditions in the patient.

A C E G

SPECT and MR images acquired with stress, plus X-ray angiography.

B D F H

SPECT and MR images at rest.

Figure 2.6 - Case demonstrating positive CMR exercise stress test with ambiguous nuclear SPECT results. A: Stress SPECT nuclear perfusio n image suggests a defect involving the inferior wall (arrow), but clinical interpretation was that this was caused by attenuation artifact. B: Resting SPECT nuclear perfusion image shows relatively uniform myocardial uptake of the radioisotope. C: Stress CMR perfusion image shows a clearly demarcated basal inferior sub-endocardial dark region of absent perfusion (arrow). D: Resting CMR perfusion image shows uniform enhancement without artifact. E: End-systolic cine CMR image at stress shows diminished thickening of the basal inferior wall (arrow). F: Resting end-systolic cine CMR image shows normal uniform thickening of heart muscle. G: Invasive X- ray coronary angiogram confirming total occlusion of the right coronary artery (arrow) responsible for findin gs on stress imaging. H: Scar CMR image obtained at rest 10 minutes after contrast injection shows uniformly black myocardium, indicating viable heart muscle and no evidence of prior myocardial infarction.

2.3 Exercise Stress Testing

Exercise stress electrocardiography (ECG) was the primary non-invasive method of detecting CAD until recent imaging methods were developed offering improved diagnostic accuracy. The stress ECG test involves a patient being connected to a 12-lead electrocardiogram system and running through a preset exercise protocol on a treadmill.

The most common protocol is the Bruce, in which the treadmill automatically changes speed and elevation in preset intervals every three minutes [8]. Table 2.1 shows the parameters for the Bruce Protocol.

Stage Time (min) Speed (mph) Elevation (%) 1 0 1.7 10 2 3 2.5 12 3 6 3.4 14 4 9 4.2 16 5 12 5.0 18 6 15 5.5 20 7 18 6.0 22

Table 2.1: Parameters for Bruce Treadmill Exercise Protocol

Changes in ECG readings indicate a variety of cardiac abnormalities. Figure 2.7 shows an ECG tracing from a patient with ischemic heart disease. Standard criterion for

CAD diagnosis is a depression or elevation of the ST segment greater than 1mm [9].

Other important information obtained from the Bruce treadmill test include exercise

14 capacity, blood pressure response, development of arrhythmias, and the presence or absence of symptoms such as chest pain during exercise [10]. Exercise ECG testing alone has a sensitivity of 78% and specificity of 70% for detecting CAD. Because of these low values, results must be combined with a probability prediction of CAD based on patient history and physiologic response to exercise in order to effectively diagnose

CAD [9].

Figure 2.7: ECG tracing showing ST-segment depression indicative of ischemia. Horizontal ST segment depression (A=at rest, B=after three minutes' exercise, C=after six minutes' exercise) and downsloping ST segment depression (D=at rest, E=after six minutes' exercise) Source [9]

Exercise stress may be performed independently or in combination with cardiac imaging for improved diagnostics. Because exercise stress testing using the Bruce treadmill protocol offers a wealth of information independently of imaging, its use is preferred over pharmacologic stress testing. This is evidenced by the breakdown of stress testing procedures performed at the OSU Medical Center in 2007. Out of 4692 nuclear stress tests, 3102 (68%) were exercise stress vs. 1500 (32%) pharmacological. Of 2157 echocardiographic stress tests, 1714 (79%) were exercise stress vs. 444 (21%) pharmacological. Pharmacological testing is beneficial in patients unable to exercise due to orthopedic problems or poor conditioning, but exercise testing is preferred in all others. Exercise testing has the ability to link physical activity to symptoms and ischemia

[10]. Factors such as drop in blood pressure, inability to reach a workload of 6 mets

(completion of stage 2 of the Bruce Protocol) or 85% of age-predicted maximum heart rate, and onset of angina accompanied by ST depression indicate a potential for CAD [9].

2.4 Desired Method – Treadmill Exercise Stress MRI

2.4.1 Benefits

The combination of the prognostic capability of exercise stress testing and the superior image quality of MRI provides the potential for a much improved diagnostic modality. The group led by Dr. W. Gregory Hundley at Wake Forest University has pioneered this type of examination and has shown the feasibility of detecting severe coronary artery stenosis by exercise stress MRI using a treadmill positioned outside the magnet room [11]. In this setup, the patient was required to walk about 20 feet from the treadmill to the MRI system, resulting in a time-to-image of 61±24 s from the end of exercise. The sensitivity and specificity to detect >70% coronary artery diameter narrowing in 27 patients were 79% and 85% respectively, using only stress function imaging, although the AHA guidelines of imaging within 60 s was not met [11]. While

Hundley proved the feasibility of this type of imaging, rapid real-time cine imaging and the addition of stress perfusion and delayed enhancement imaging should improve the prognostic capability of the test. In addition, the placement of a totally MRI-safe treadmill directly adjacent to the MRI scan table would enable the MRI scanner room to

17 be configured similarly to a stress-echocardiography lab (Figure 2.8). This configuration provides familiarity to clinical staff who have been trained in stress echo and the positioning most likely to allow the completion of function imaging within 60 s.

2.4.2 Challenges

Despite its benefits, treadmill exercise presents significant challenges for use within

the MR environment. Treadmills are typically powered by electromagnetic motors, and

use ferromagnetic structural materials, precluding their use in close proximity to an MRI

magnet. The operation of electromagnetic motors may be affected by the magnetic field

of the MRI machine, and the magnetic field induced by the motor can affect image

quality. Ferromagnetic materials may become projectiles in the large magnetic field

around the MRI machine. A detailed description of these issues may be found in Chapter

2.4.2.1 Time to Image

The American Heart Association has recommended that cardiac stress echocardiography be completed within 60-120 s of peak stress conditions, with 60 s being the preferred target [12]. The justification of this guideline is that exercise induced wall motion abnormalities (WMA’s) generated under stress conditions will begin to disappear immediately as ischemia is reversed upon recovery of the stress condition [13-

20]. The persistence of WMA’s is most likely related to the severity of CAD (number of vessels involved, percent stenosis), the presence of coronary collateral flow, and the duration of ischemia [21-24]. In order to accurately diagnose patients with less severe single-vessel CAD, associated with a high ischemic threshold and rapid WMA resolution,

19 imaging must be performed as close as possible to peak exercise, and ideally with no delay. Dagianti et al. [17] observed that 26% of WMA’s disappeared within 1 minute of the end of exercise. Presti et al. [16], Ryan et al. [25], Hecht et al. [18], Dymond et al.

[15] all showed a greater sensitivity for imaging at peak exercise than post-exercise. In the Presti study, of the 29 patients who had a new WMA detected with exercise echocardiography, six would have been misclassified as normal if imaging had been performed only after exercise. Thus, every effort must be made to minimize the time between end of exercise and imaging in order to maximize the sensitivity of the test.

2.4.2.2 Safety and Practicality

Due to the electromagnetic and ferromagnetic components in standard treadmills, the only way to perform an exercise stress CMR study using standard equipment would be to locate a treadmill either in a hallway or in the control room outside the MRI examination room. This setup would create conditions with are both unsafe for the patient and impractical from a clinical standpoint.

Deconditioned cardiac patients may become lightheaded or dizzy following an

exam that pushes them to maximal stress. For this reason, it is essential to locate the

treadmill as close as possible to the MRI machine in order to minimize locomotion or

20 turning while moving from the treadmill to the examination table. Preferably, the treadmill could be located at a height such that it would be necessary for the patient to simply sit down on the table immediately following the exercise portion of the exam.

In addition to the safety issues, there are practical reasons that the treadmill should be located within the MRI examination room. Positioning the treadmill just outside the door of the MRI room will likely place it in a hallway. The busy hallways of a hospital are far from ideal locations to perform diagnostic exams. HIPPA regulations require privacy for patients and the amount of foot and hospital bed traffic make finding the space to perform the exams difficult.

In order to develop an examination that is safe for the patient and clinically practical, a completely MRI compatible treadmill must be developed which is able to sit safely in the magnetic field of the MRI machine in order to create a private and safe environment for the clinical patient.

CHAPTER 3

THE MR ENVIRONMENT

3.1 Introduction

ASTM Standard F2503-05 defines the MR environment as a volume within the 5

Gauss (G) line of an MR system, which includes the entire 3-D volume of space surrounding the MR scanner [26]. If the 5G line is contained within a Faraday shielded volume, the entire room is considered an MR environment [27]. The Faraday cage provides for magnetic field homogeneity and eliminates radiofrequency (RF) noise from entering the MRI environment. The 5G line or Faraday shielded room additionally presents a delineation point from the magnetic field effects of the MRI scanner, however wires passing through the Faraday cage may act as antennae, allowing outside RF noise to enter the MR environment. The static and gradient magnetic fields of the MRI system can cause potential hazards due to magnetic attraction, induced electrical currents, and heating. The static field of a 1.5T MRI scanner is about 30,000 times that of the earth’s magnetic field [28].

Figure 3.1: Model MR Environment. Zones III and IV notate the potentially hazardous area of the MRI environment. (Source: [29])

The two types of forces induced on an object by the scanner’s static magnetic field are torque, in which the object tries to align itself with the scanner’s magnetic field, and displacement, whereby an object is attracted directly towards the scanner magnet. Both types of forces occur due to any ferrous elements in the material being attracted by the

MRI scanner’s static magnetic field. 23

RF heating is caused by the RF excitation pulses generated during an MRI session.

These pulses may cause metallic objects to warm up due to induced electrical currents.

IEC 60601-2-33 defines acceptable levels of RF heating for clinical use [28]. For the studies described in this thesis, the problem of heating is most likely to occur in the electrocardiogram (ECG) leadwires. Peripheral nerve stimulation, which occurs due to

RF energy being directly deposited into the patient, may also occur due to the pulsed gradient field [30]. These effects cause challenges in developing the rapid scanning sequences required for stress cardiac imaging, however they will not have an effect on the hardware development discussed in this thesis.

3.2 Definitions

In 1997, The Food and Drug Administration (FDA) Center for Devices and

Radiologic Health proposed that devices to be used in the MRI environment should be labeled either as MR-safe or MR-compatible [31]. A device was considered MRI-safe if it experiences no attraction towards the MRI magnet [32]. Conversely, items introduced into the MRI environment can cause artifacts in the acquired images. MR-compatibility refers to the effect of a device on MRI image quality as well as the effect of the magnetic field on device operation [32]. The FDA defined the terms as follows:

MR Safe : The device, when used in the MRI environment, has been demonstrated to present no additional risk to the patient or other individual, but may affect the quality of the diagnostic information. The MRI conditions in which the device was tested should be specified in conjunction with the term MR safe, since a device which is safe under one set of conditions may not be found to be so under more extreme MRI conditions [31].

MR compatible : A device shall be considered “MR compatible” if it is MR safe and the device, when used in the MRI environment, has been demonstrated to neither significantly affect the quality of the diagnostic information nor have its operation affected by the MR system. The MRI conditions in which the device was tested should be specified in conjunction with the term MR safe, since a device which is safe under one set of conditions may not be found to be so under more extreme MRI conditions [33].

Both terms required the inclusion of the conditions in which the device had been tested to meet the conditions listed with the classification. The MR-community felt that these definitions did not adequately provide sufficient warning to cover the range of MR environments, and were often used interchangeably or incorrectly. As of Aug. 2005, a new set of standards (ASTM F2052, F2213, F2182, F2119, F2503) were introduced to address the testing and labeling of MR devices. The new definitions are as follows:

MR Safe : An item that poses no known hazards in all MR environments. MR safe items

include nonconducting, nonmagnetic items such as a plastic petri dish. An item may be

determined to be MR Safe by providing a scientifically based rationale rather than test

data [26].

MR Conditional : An item that has been demonstrated to pose no known hazards in a

specified MR environment with specified conditions of use. Field conditions that define

the specified MR environment include field strength, spatial gradient, dB/dt, RF fields,

and specific absorption rate. Additional conditions, including specific configurations of

the item (eg, the routing of leads used for a neurostimulation system), may be required.

For MR Conditional items, ASTM F2503 requires the item labeling to include

results of testing sufficient to characterize the behavior of the item in the MR

environment. In particular, the testing should address magnetically induced displacement

force and torque, and RF heating. Other possible safety issues include, but are not limited

to, thermal injury, induced currents/voltages, electromagnetic compatibility,

neurostimulation, acoustic noise, interaction among devices, and the safe functioning of

the item and the safe operation of the MR system. Any parameter that affects the safety

of the item should be listed, and any condition that is known to produce an unsafe

condition must be described [26]. 26

MR Unsafe: An item that is known to pose hazards in all MR environments. MR Unsafe items include magnetic items such as a pair of ferromagnetic scissors [26].

Because of the issues described above, special care must be utilized in choosing materials or actuators for use in the MRI environment. Any device to be used in the MR environment must display one of the labels shown in Table 3.1.

MR Safe

MR Compatible

MR Unsafe

Table 3.1: MR labels introduced with ASTM F2503-05

3.3 Materials

Many standard devices use ferromagnetic components due to their desirable

mechanical properties such as strength, rigidity and machinability; however these

materials are unsafe for use in the MR environment. Objects made of ferromagnetic

materials are subject to projectile and torsional forces resulting from their introduction to

the high magnetic field of the MR environment. In addition, the use of conductive

materials may result in the generation of eddy currents, which may generate external

forces when trying to move the object through the static magnetic field. Material which

contains no ferromagnetic components may be used to fabricate MR safe devices, which

can be used without fear of attraction towards the MRI machine. A limited quantity of

ferromagnetic material may be used in some cases, provided it is either located

sufficiently outside the zone of magnetic attraction or is anchored to a permanent

structure. Some typical MR safe materials are described below.

3.3.1 Aluminum

Aluminum is considered a paramagnetic material, which implies that that it may become slightly magnetic over time when exposed to a large magnetic field. The degree of magnetic attraction for aluminum is considered too small to be a factor in MRI- compatible equipment. In fact, many MR-safe devices are made of aluminum for this reason. A drawback to aluminum is its conductivity. As aluminum approaches the bore

28 of the MRI machine, where the strongest magnetic fields exist, an induced current, resulting in a magnetic field, may occur. This is a result of the large magnetic field gradients that exist in the area near the MRI machine. As a piece of aluminum is moved through this space, the static gradients act as a changing magnetic field, which induces a resultant electric field in the aluminum. This current induces a magnetic field within the aluminum, and according to Lenz’s law, creates a repulsive force from the MRI machine.

Moving the aluminum slowly or staying outside of the large magnetic field gradients will reduce this effect to a point where it is not an issue, although this effect must be considered when designing a treadmill with moving parts (rollers, flywheels, etc) made of non-ferromagnetic, but conductive, materials such as aluminum.

3.3.2 Stainless Steel

Stainless steels are iron-based alloys which contain 10-30% chromium and 0.03-

1.2% carbon, along with other added elements such as nickel, molybdenum, titanium, and aluminum [34]. Martensitic (400-series) stainless are ferromagnetic at room temperature and are therefore incompatible with the MRI environment. Austenitic stainless steels include the 300-series, which are comprised of chromium (16-20%), nickel (up to 35%), manganese (up to 15%), and other alloying elements [34]. These materials are paramagnetic at room temperature, showing no signs of magnetic attraction. Austenitic stainless steels have a face-centered-cubic structure, which is thermodynamically

29 metastable, so upon cold-working, the material may revert to the more stable martensitic structure, which is ferromagnetic [34]. This property allows for a range of ferromagnetism in these materials depending on the forming process. Table 3.2 shows the material properties of select grades of stainless steels.

Hardness Hardness Tensile (ksi) Yield (ksi) Elongation (Brinell) (Rockwell B) Type UNS min min min max max

300 Series Austinetic 302 S30200 75 30 40% in 2" 183 88 304 S30400 75 30 40% in 2" 183 88 304L S30403 70 30 40% in 2" 183 88 316 S31600 75 30 40% in 2" 217 95

400 Series Martensitic 410 S41000 65 30 20% in 2" 217 95 410S S41008 60 30 22% in 2" 183 88 430 S43000 65 30 22% in 2" 183 88

Table 3.2: Material properties of common grades of stainless steel

3.3.3 Other metals

Brass and copper and have been used previously in MR safe components. These materials offer ease of machinability and lack of magnetic attraction; however they sacrifice strength and stiffness compared to aluminum and stainless steel. Brass does contribute to imaging artifact, and therefore must be placed at least 90 mm away from the region of interest (ROI) [35]. Titanium and tungsten both offer metallic alternatives, but are generally much more expensive than aluminum or stainless steel and do not provide the structural strength of those materials.

3.3.4 Composites

Polymers offer ease of machining and negligible imaging artifacts, however strength

and stiffness do not in general match those of metal [27]. Several commonly used

polymers include Delrin, PET, PEEK, and polyoxymethylene. Fiberglass and carbon

fiber materials offer higher stiffness than polymers, but suffer from greater difficulty in

machining.

3.4 Actuators

Due to their ferromagnetic and electromagnetic nature, traditional electromagnetic

motors will not function safely in the magnetic field of the MRI environment. These

motors are inherently made from ferromagnetic components, which may be attracted to

31 the MRI magnet. This projectile hazard may be reduced by fastening the motor to a immovable object, however other problems may persist. The large (1.5-7T) static magnetic field and switching gradient fields may interfere with motor operation by affecting the magnetic field required to drive the motor. Conversely, the magnetic field generated by the motor may affect the magnetic field used in image acquisition, resulting in image artifacts.

Although to date not routinely used in cardiac MRI, actuators have begun infiltrating devices used in interventional and functional MRI procedures. Robotic arms are used during image guided surgery, in which the surgeon performs the procedure from the control room. Haptic interfaces are used in functional MRI (fMRI) studies to judge neural response to stimulation or degree of recovery from stroke or other mental incapacitation. In either case, fine degrees of control and feedback are required. Neither application requires the power necessary to run a treadmill; however the type of actuators used for these purposes form a good starting point for investigation.

3.4.1 Drive shaft/belts

One possible way to use an electromagnetic motor in the MRI environment would be through remote operation. The motor could be located externally and a drive shaft extended through a waveguide in the wall of the MRI room. While there is nothing technically wrong with this concept, it presents many practical drawbacks. If used for a

32 treadmill, it allows for only a single location of operation for the device. This location is either dependent on a preexisting opening in the room or the installation of a new one. If should be noted that the installation of a waveguide to provide an opening through RF shielded walls of the MRI room may cost in the thousands of dollars. A drive shaft also may present an obstacle for the medical team working in proximity to the patient on the

MRI table.

3.4.2 Hydraulic

Hydraulic systems offer the ability to transmit large forces over long distances and offer a combination of power and control in a flexible environment. Hydraulic power systems can be engineered using standard ferromagnetic components and electromagnetic actuators, which can be safely located outside of the MR room.

Pressurized fluid is transmitted through hoses to passive actuators that can be construced of nonferromagnetic materials and safely located in the MRI room Flexible hoses allow for a wide variety of configurations within the MR environment. Drawbacks include the possibility of fluid leakage, fluid and joint friction, fluid viscosity and compressibility

[36]. Problems with leakage, viscosity and compressibility may be minimized by using water as the hydraulic fluid, rather than traditional hydraulic oil. Tradeoffs come in the form of increased design complexity, more difficulty in sourcing components and the risk of corrosion within the system. Proper hose length selection also plays an effect on

33 system performance, since longer hoses reduce the efficiency of the system and cause nonlinear dynamics [35]. Ganesh el al. found that, in a hydrostatic system, the energy transmission nearly doubled as hose length was reduced from 10m. to 6m [37]. Early work with hydraulic systems in the MR environment has focused on linear operation, in which a cylinder is driven by external actuation. An early example of this mode of operation is the development of an MR compatible manipulator, in which Moser et al. used hydrostatic transmission to drive a master-slave cylinder system from outside the

MRI environment [38]. A linear motor was used to actuate the master cylinder. Kim et al developed a similar system using an ultrasonic motor located remotely to actuate the master cylinder [39]. Mendolwitz et al. at M.I.T. developed a MR compatible wrist robot. Two standard electric motors, located outside the MRI room, drive an MRI compatible hydraulic system consisting of two pairs of custom designed and fabricated vane motors [40]. The hydraulic motors actuate a handle grasped by the patient via a friction drive or geared differential.

3.4.3 Pneumatic

The use of pneumatic actuators eliminates some of the problems associated with hydraulic power. Pneumatic systems are in general cleaner and operate at higher speeds than hydraulics [41]. Many pneumatic components are readily available in MR compatible materials and the flexibility of air hoses allows for a wide variety of routing

34 configurations. As with hydraulics, the power source may be remotely located outside of the MR environment. Furthermore, system leakage is not a problem and system design is less complicated due to the lack of need for a return path for the air. Drawbacks include low force generation and lack of system stiffness due to the compressibility of air, making accuracy and control problematic [36]. A recently developed pneumatic system for automated brachytherapy seed placement uses a pneumatic stepper motor that operates completely free of electricity [42]. This novel concept uses pressure waves created by a pneumatic distributor remotely located in the controller unit and transmitted to the robot through air hoses. The actuation is encoded by fiber optics so that the motors use pressure and light but no electricity. These features prevent the robot from creating interference during imaging [42].

3.4.4 Electric

Piezoelectric elements and ultrasonic motors are widely used in the development of MR compatible devices for situations that require low force generation and pulsed actuation. Piezoelectric actuators are, by nature, MR safe due to their use of piezoelectric crystals as the power generation source and the piezoelectric material may be housed in a

MR compatible material. When a voltage is applied to the crystal, the material expands, creating an actuator. Because of the low power output of piezoelectric materials, mechanical linkages involving levers and springs are frequently used to amplify the

35 actuation when using these devices [36]. A vibration mechanism may be created by providing alternating current to the crystal, causing oscillations in the displacement of the crystal. Harrington el al. developed a vibrotactile stimulator for functional MRI (fMRI) procedures [43]. A piezoelectric wafer was used, but resulted in difficulty producing sufficient power from deflection of the piezoelectric material. Large batteries were required, which were located outside of the MR environment. Uffman el al. have used this technology to develop a MR compatible system for tissue excitation during MR elastography, using a stack of piezoelectric crystals and a lever mechanism for amplification [44]. The entire device is able to be placed inside the bore of the MR scanner without concern for patient safety or image artifacts.

Ultrasonic motors use piezoelectric elements to develop power generation and are the most widely used type of MR compatible actuator [36]. A piezoelectric element in the stator is excited, and the vibrations are transferred to the rotor, amplifying the signal.

Because friction is the main source of motion, ultrasonic motors are not affected by the static field of the MR environment. In addition, they offer little backlash and rarely require gears [35]. Drawbacks include the requirement of a high frequency power source and image artifacts when used too close to the ROI. The latter are caused by eddy current effects in the rotor housing and can be eliminated by placing the ultrasonic motor remotely from the scanner bore and actuating through a series of mechanical linkages.

The Shinsei Corp has developed two nonmagnetic ultrasonic motors which are 36 accelerating the development of ultrasonic actuator systems for the MR environment

[45]. Initial solutions using ultrasonic motors were developed by groups seeking methods of positioning tools during interventional MRI [27], however this type of motor is now used in the majority of MR compatible devices requiring actuation.

3.5 Electrical/Power Issues

Electrical power causes two issues in the MRI environment. The first is heating of electrical conductors caused by induced currents, which may cause burns in patients or damage to the electrical components. The second is in the form of radiofrequency (RF) noise, which may cause imaging artifacts. These issues are examined further and potential solutions explained below.

3.5.1 Heating

The RF transmitters used to generate pulses that “excite” the protons in the body causing them to emit a detectable magnetic resonance signal will also generate current in any conductive material located within the imaging region [35]. This electrical current leads to heating within the material, which is amplified if the material forms a loop. If this material contacts the patient, severe burns may occur. The temporal gradient fields may also produce currents in inductive materials, but the heating in these cases is much less than that caused by RF pulses [35].

3.5.2 RF Noise / Image Quality

As long as the power source initiates and terminates within the MRI environment, it cannot carry RF noise from outside of the shielded magnet room; however, any electrical devices within the room may be a source of RF noise A 1.5T MRI receiver system is tuned to receive signals at 63.5 MHz; any noise with frequency components in this range will interfere and create image artifacts. The MRI room is shielded with a copper mesh designed to prevent RF waves from entering the room. Any breaks in this shielding or conductive wires passed through a waveguide from outside the room provide an avenue for RF noised to be introduced. This RF noise is picked up by the RF receiver coils used to obtain the image, resulting in characteristic image artifacts. This type of interference appears as vertical lines of image noise at discreet frequencies (Figure 3.2).

Figure 3.2: Phantom MRI image showing a band of RF noise artifact. Source [46]

3.5.3 Solutions

Remote location of sensors and actuators provides a solution to many electrical issues in the MR environment. Fiber optic or wireless transmission allows signal to be carried through the MR environment without wires acting as RF antennae. This also prevents interference in the sensor signals due to the electromagnetic pulses emitted from the MR scanner. The location of the electrical power source inside the MRI environment reduces the risk of external RF noise; however the power source must be properly shielded for use in the MR environment. Fiber optic or wireless transmission likewise reduces the risk of wire heating by eliminating conductive components.

CHAPTER 4

PRELIMINARY TESTING

Figure 4.1: Commercial treadmill schematic showing ferromagnetic components

4.1 Initial Prototype Treadmill

To create an initial prototype, a standard rehabilitation treadmill (Landice 8700,

Randolph, NJ) was modified to allow it to sit in the far corner of the MRI room at a safe distance from the MRI system. This particular treadmill was chosen because it has an

40 extruded aluminum frame, making the majority of the treadmill already MRI compatible.

Figure 4.1 shows the components of the treadmill which were modified in order to allow it a degree of MR compatibility. The steel handrails were replaced with aluminum tubing which were bent into an ergonomic shape. The steel crossbars used for stability were replaced with new aluminum supports. The aluminum supports were made twice as thick as the original steel supports to provide equivalent stability using lower strength material.

The supporting uprights on the crossbars were replaced with 300-series stainless steel equivalents. The rear roller was replaced with a conveyor roller made from 300-series stainless steel, containing a stainless steel axle and bearings, and bronze bushings. The remaining ferromagnetic components (front roller, drive and elevation systems) were located in the front portion of the treadmill.

The treadmill also had to be modified to allow it to reach the upper stages of the

Bruce Protocol [8], since as a rehabilitation treadmill it was not designed to reach the

levels of elevation required by the protocol. Three inch leg extensions were crafted from

300-series stainless steel to allow the treadmill to reach the maximum required elevation

and measurements were performed to re-calibrate the built-in treadmill display with the

new actual elevations. The Bruce Protocol was programmed into a user-program slot on

the treadmill.

Figure 4.2: Magnetic field plot and exam room layout for the MR system (1.5 Tesla MAGNETOM Avanto, Siemens Healthcare, Malvern, PA) in the Ohio State University Ross Heart Hospital showing the position of the prototype treadmill inside of MRI exam room.

This treadmill was placed in the outer corner of the MRI examination room, such that the remaining ferromagnetic components were located outside of the 5G line (Figure

4.2). Figure 4.3 shows the room setup for obtain preliminary data, including patient monitoring and other required hardware. These components will be further discussed in

Chapter 7.

This system was used to optimize the Exercise Stress MRI procedure and to obtain exercise stress function and perfusion images in 20 healthy subjects. The procedure and results were published [47] and the system was then tested on patients with known or suspected coronary artery disease (CAD).

4.2 Methods

10 patients referred for a treadmill exercise SPECT exam due to suspected CAD were recruited for the study. The MRI stress exam described in [47] and summarized in

Figure 4.4 was incorporated into the clinical exercise SPECT protocol.

Resting Treadmill exercise Recovery MRI Scan #4 ECG 12-lead ECG and 12-lead ECG (Viability) (Supine) blood pressure (Supine) monitoring MRI Scan #2 MRI Scan #3 MRI Scan #1 Stress Function (Resting Slice Localization and Perfusion Perfusion) Resting Function

10 20 30 min 40 min45 min 50 min 55 min Cumulative min min time Subject quickly transferred from Table immediately treadmill to MRI; pulled out; 12-lead IV connected ECG reconnected

The examination was designed so that the clinical and research studies could be performed simultaneously without requiring the patient to perform multiple stress exams.

When the patient reached target heart rate on the treadmill in the MRI room, the radioisotope was injected in order to capture a “snapshot” of myocardial perfusion under peak stress conditions for the clinical SPECT stress perfusion study. MRI function and perfusion imaging was performed immediately following the patient reaching peak stress, with the SPECT imaging completed following the CMR study. The combined

SPECT+MRI protocol allowed subjects to complete both studies within a single exercise test, providing a direct comparison of SPECT and MRI in the same subjects, under the same cardiovascular stress conditions.

Time was recorded from end of exercise to start of imaging, end of function imaging, and end of perfusion imaging. Image quality was scored by two independent expert reviewers. Scores of 1 to 5 (1=unacceptable, 2=suboptimal, 3=adequate, 4=good,

5=excellent) were assigned for the following image qualities: noise, contrast, sharpness, artifact level, and slice coverage. The images were also evaluated for the presence or absence of wall motion and perfusion defects.

4.3 Results

Exercise CMR was successfully completed in all 10 subjects. The entire procedure, from initial setup to completion of the delayed enhancement scan, took an average of one hour. MRI commenced an average of 45±6 seconds after exercise while function imaging was completed an average of 71±7 seconds post-exercise. Image quality was sufficient for visual assessment of wall motion and perfusion in all left ventricular segments. The results are summarized in Table 4.1.

Max predicted Max HR Time to start Time to end Time to HR Bruce achieved MRI cine end perf PatientGender Age (MPHR) Stage (%MPHR) (s) (s) (s) 1 F 56 164 3 84 44 61 85 2 M 55 165 4 98 50 79 100 3 M 47 173 5 105 34 61 80 4 M64 156 3 72 45 70 96 5 M62 158 4 94 44 71 87 6 M66 154 3 92 39 68 86 7 M66 154 2 95 56 81 99 8 M59 161 4 93 47 75 88 9 M 37 183 2 79 46 71 100 10 M 53 167 4 95 46 72 91 avg 57 164 3 91 45 71 91 std 9 9 1 10 6 7 7

Table 4.1 - Results of preliminary testing in 10 patients with suspected CAD. MPHR = maximum age-predicted heart rate.

4.4 Conclusions

The ability to acquire stress cardiac function and myocardial perfusion images in

CAD patients immediately following maximal exercise on a treadmill positioned inside the MRI room was successfully demonstrated. All studies were completed successfully and when the results were compared to the concurrent SPECT study, there were two cases in which the stress SPECT exam was incorrect, while the CMR results have been in agreement with the gold standard catheterization x-ray angiography in each case it has been performed (3 out of 10 patients). The preliminary test results did not meet the AHA recommendation that cardiac function imaging be completed within 60 seconds of exercise, however by positioning the treadmill immediately adjacent to the MRI table, patients will have less distance to travel following the end of exercise and these guidelines should be met. Safety concerns were present due to the risk of attraction of the ferromagnetic treadmill motor. A totally MR safe treadmill that can be placed immediately adjacent to the MRI patient table will minimize the time delay between exercise and imaging, and provide a safe means of CMR exercise stress testing at any

MRI field strength.

CHAPTER 5

MRI COMPATIBLE HYDRAULIC TREADMILL

The prototype treadmill (Landice 8700, Randolph, NJ) used to obtain preliminary data was further modified by replacing the electromechanical drive and elevation systems with hydraulic systems to allow for complete MRI compatibility. Additional modifications were performed on the structural components to ensure that the treadmill would operate safely in the MRI environment.

5.1 Structural Components

5.1.1 Frame

The basic outer frame used in the concept prototype treadmill (Landice 8700,

Randolph, NJ) served as the main body of the new treadmill. It was known from the preliminary study described in Chapter 4 that all treadmill components behind the front roller were already MRI compatible. The front display panel was removed and a handrail was constructed from aluminum tubing to provide a safety handle for the user. Any ferromagnetic fasteners not replaced in the first prototype were replaced with stainless

48 steel equivalents. Using this frame, a modular design process was used that allowed the original drive components to be removed, and new motor and elevation units to be inserted into the original structure.

5.1.2 Rollers

The original rollers were made of steel, and therefore required replacement. Each roller weighed 14 lbs., and this inertia was used to create a secondary flywheel effect for the belt drive system. In order to replicate the flywheel effect, the new rollers had to be of similar weight. Table 5.1 shows a comparison between the densities of plain carbon steel and Type 316 stainless steel. It can be seen there that there is sufficient density equivalence that the use of Type 316 stainless steel will maintain the proper secondary flywheel effect when using similar dimensions. A supplier was found (Rolcon Venix,

Cincinatti, OH) to custom fabricate these rollers. The rear roller was made from a 2.5 in

X 20-7/8 in 316 stainless steel tube with ¼ in thick wall. It contains a ¾ in diameter stainless steel axle and stainless steel bearings with bronze bushings. The front roller has similar characteristics, but has a 2.5 in x 19-1/8 in face and a 0.785 in shaft.

Material Type Density (g/cm 3) Plain carbon steel AISI-SAE 1020 7.9 Stainless steel type 316 8.0

Table 5.1: Comparison of densities of Plain Steel and Type 316 Stainless Steel (source: [48, 49])

5.2 Hydraulic Power Systems Overview

Due to the large magnetic field present in the MRI environment, the primary challenges in developing a MRI-compatible treadmill are the drive and elevation systems.

Traditional electric motors operate on electromagnetic principles and use ferromagnetic components with significant mass. These devices can pose a severe projectile hazard if brought into close proximity to the MRI magnet. Static and gradient magnetic fields may negatively affect their operation, and conversely, the magnetic field generated by an electromagnetic motor may distort the main magnetic field of the MRI system. For this reason, a totally non-ferromagnetic hydraulic motor system was developed to power the treadmill. After investigating several types of MRI compatible motors (see Chapter 2 for discussion), it was determined that a hydraulic motor would offer the greatest power and stability with the lowest cost and would interface positively with the MRI environment.

Further investigation revealed that this strategy had been employed in underwater treadmills used for aqua-therapy (Aquagaiter, Ferno Performance Pools, Wilmington,

OH).

The power unit, consisting of an electrical motor driven pump, will be located outside the MR exam room (Figure 5.1). The pump forces hydraulic fluid from a reservoir to the drive and elevation systems. Hydraulic hoses carry the pressurized fluid into the magnet room to a non-ferromagnetic hydraulic motor mounted on the front of the treadmill. The

51 flow of this fluid powers the motor, turning the drive shaft which contains a nonferromagnetic stainless steel flywheel to attenuate inertial differences during footplant and speed change (Figure 5.2). The drive shaft is connected to the drive roller through a belt and pulley system. Return hoses cycle the hydraulic fluid back to the reservoir. The hoses are attached to the treadmill via MR-compatible, valve-loaded quick-connects to allow for quick, clean setup and teardown.

Figure 5.2: Layout of Drive Motor and Elevation Cylinder for MR-Compatible Treadmill

The Bruce Treadmill exercise protocol requires the treadmill to attain a maximum grade of 22% to accommodate patients with a wide range of physical conditions. An elevation system was developed which uses a nonferromagnetic hydraulic cylinder acting on a lever arm to raise and lower the treadmill body (Figure 5.2). Prior to each exercise test, an accumulator, located on the power pack, will be charged via the electric motor driven pump. At each protocol stage, a portion of water will be discharged from the accumulator using solenoid valves and sent through hoses to a nonferromagnetic stainless steel cylinder located at the front of the treadmill.

Due to the placement of the treadmill system within a healthcare facility, the hydraulic power system was designed to use water rather than traditional oil-based hydraulic fluids. This will ensure simple cleanup of any accidental fluid leakage from the system. It will also make the system more universal by eliminating the need for on-hand stock of hydraulic fluid. Figure 5.3 shows the hydraulic schematic for the drive and elevation systems.

Figure 5.3: Electric motor driven pump, positioned outside of MRI exam room, powers treadmill mounted hydraulic motor and cylinder through hoses.

5.3 Drive System

Drive roller (P/N 4)

Tensioner Wheel (P/N 1)

Hydraulic motor (P/N M6, The Water Tensioner Body Hydraulics Co, Ltd) (P/N 2)

Shaft Coupler (P/N MOL-32C, Lovejoy) Drive belt (P/N 9003K109 McMaster- Carr) Motor Mount (P/N 8) Drive pulley (P/N 9)

Speed sensor Flywheel (P/N 42EFG1JBAF4, (P/N 3) Rockwell Automation)

Figure 5.4: Hydraulic drive system components

The treadmill design utilizes basic fluid power components currently available in

industrial applications. The power flow starts with a variable speed electric motor

capable of supplying the power to the treadmill system. Power from the electric motor is

55 supplied via a shaft and flexible coupler to a submerged, fixed displacement hydraulic pump. These components are located together in a hydraulic power pack which will be installed outside of the MRI environment (Figure 5.1). The power is converted to flow proportional to rotational speed and pressure proportional to the treadmill load. The fixed displacement hydraulic motor, connected to the treadmill belt by pulleys and belt, will convert the fluid power to rotational power. The pump output flow connects directly to the motor inlet. Treadmill speed feedback will be provided by a photoelectric sensor aimed at a pattern on the flywheel (Figure 5.4). The motor outlet will flow through a hydraulic valve capable of maintaining the appropriate pressure on the motor outlet to control speed and prevent inlet cavitation when operated at high gradients.

5.3.1 Drive Requirement Tests

Rationale: To meet specifications, the treadmill must be able to execute the standard

Bruce stress test protocol up to Stage 7 (6 mph and 22% incline) for subjects up to 400 lbs of body weight. System requirements for current and required motor drive force needed to be determined in order to size an appropriate drive system.

Figure 5.5: Torque measurement for preliminary data

Methods: To measure the current draw, three subjects (weight = 150, 165 and 200 lb) walked at speeds and elevations corresponding to the stages of the Bruce Protocol on a standard treadmill (Landice L7, Randolph, NJ), both individually and two at a time in

57 order to place a larger load on the treadmill. A clamp-on ammeter was used to determine the current draw of the standard treadmill. The meter was positioned around the power cord of the treadmill during these tests and the current draw was recorded.

To perform the motor torque tests, the drive belt was removed from the drive roller, and a spring-force gauge was attached to the front roller with a rope. The rope was tied to the front roller and wound several times to prevent slippage. One subjects

(Weight = 125) stepped onto the treadmill belt and the steady state force on the drive shaft, as indicated on the spring scale, was recorded as gravity pulled the subject down the belt (Figure 5.5). The test was repeated with two subjects (Weight =290 lbs) together on the belt.

Results and Conclusions:

Current (amps) Subject Weight (lbs) (150) (165) (200) (365) Stage (elevation) 0 (0%) - - - 5.2 M1 (3%) - - - 4.5 M2 (5%) - - - 3.5 1 (10%) 1.3 1.5 1.5 1.0 2 (12%) 1.2 1.5 1.2 0 3 (14%) 1.1 1.2 0 0 4 (14%) 1.2 1.2 0 0 5 (16%) 0.1 0.5 0 0 6 (18%) 0.5 0 0 0

Table 5.2 - Current Measurement for Landice L7 running Bruce Protocol

The condition of maximal current draw was maximum weight at lower angles

(Table 5.2) and the current draw decreased as angle increased. This was attributed to increased energy input by the subject at the higher angles of elevation. In essence, the subjects were assisting the motor in driving the treadbelt while walking uphill. By supplying power to the electric motor, they were causing it to act as a generator, creating a back EMF, and creating current in the negative direction. The measured current was the total current through the power cable, so when input current and back current were equal, it resulted in a measurement of zero. The test was repeated with both subjects together performing the modified Bruce Protocol stages, and it was confirmed that the largest current draw, and therefore greatest motor power, occurred with the greatest load on the treadmill at zero elevation.

Force (lbf) Subject Weight 125 lbs 290 lbs Grade 0 15 30 5 15 20 10 10 15 12 7 10 14 5 5 16 5 0 18 0 -5 20 -5 -10

Table 5.3 - Force measurements obtained from Landice L7 Treadmill at each Bruce Protocol elevation stage

A similar phenomenon occurred with the force measurement tests (Table 5.3). Up to

a certain angle of elevation (dependent on the weight of the subject), the force on the

drive roller decreased as elevation increased. Upon reaching this critical point, the force

began to act in reverse. This suggests that the motor is actually acting as a brake to

control the speed of the treadmill under these conditions, so a solution was required to

allow the hydraulic motor to act as a brake at higher elevations. To account for this, a

braking valve needed to be installed on the hydraulic power pack to create sufficient

backpressure to prevent the hydraulic motor from “freewheeling”.

5.3.2 Drive System Requirements

The data obtained in Section 5.3.1 was used to determine the drive system

requirements for the hydraulic treadmill. The following system properties were used in

these calculations:

Front Roller Radius (R roller )= 1.25 in

Belt Thickness (T belt ) = 0.1 in

Drive Ratio (DR)= 3:1

Using the linear speed of the treadbelt (V belt ), the rotational drive roller speed

(Nroller) was determined at each stage using:

1 V (MPH ) 1 056 in/min N (RPM ) = × belt × roller 2π 1 MPH R + T 5.1 roller 2 belt

Using a 3:1 drive ratio between the front pulley and the drive roller sheave, the

required hydraulic motor speed (N hyd. motor ) at each stage was calculated by:

Nhyd. motor = Nroller x DR 5.2

Next, the experimentally determined pull forces were extrapolated to represent values obtained using the maximum subject weight of 400 lbs (F pull ) (Table 5.4). These

values were used to determine the maximum required hydraulic motor torque (T hyd mot ) at each stage, using the following relationship:

T hyd mot = F pull x (R roller + .5*T belt ) / DR 5.3

The results of these calculations are summarized in Table 5.4. As previously

shown in the preliminary data, the hydraulic motor torque drops to zero at the higher

Bruce Protocol stages due to the subject supplying the torque required to turn the

treadbelt.

Pull force for Hyd. Hyd. . Hyd. 400 Motor Motor Motor Speed Gradient lbs Speed Torque Torque Stage (mph) (%) (lbf) (rpm) (lb-ft) (Nm) Start 0 0 73 0 2.648 3.590

1 0 60 388 2.167 2.938

mB1 1.7 0 60 659 2.167 2.938

mB2 1.7 5 40 659 1.444 1.958

1 1.7 10 27 659 0.963 1.306

2 2.5 12 13 970 0.481 0.653 3 3.4 14 7 1319 0.241 0.326 4 4.2 16 0 1629 0.000 0.000 5 5 18 0 1939 0.000 0.000 6 5.5 20 0 2133 0.000 0.000 7 6 22 0 2327 0.000 0.000

Table 5.4: Drive system requirement calculations performed using force data

The remaining calculations to select appropriate components were performed by obtaining information from specs supplied by the manufacturer for the hydraulic motor and pump (Figure 5.6, Figure 5.7).

Figure 5.6: Specifications for M6 hydraulic motor supplied by The Water Hydraulics Co, Ltd.

Knowing the required or estimated hydraulic motor torques and speeds at each stage (Table 5.4), allows the use of Figure 5.6 to determine the differential pressure required to the motor at each stage. An assumption was made that torque is independent of speed for hydraulic motor speeds for speeds of greater than 500 RPM (the minimum recommended by the manufacturer). As seen in Table 5.4, the minimum hydraulic motor speed required to run the first stage of the Bruce protocol (1.7 MPH) is 659 RPM.

However, if we want to operate at speeds lower than 1.7 MPH, our performance

63 estimates may be inaccurate because torque may influence speed under low flow conditions. Actual performance at low speeds will need to be characterized experimentally because performance data was not provided for these conditions.

Knowing the desired output torque, the corresponding point on the “4.6cc Torque” line is found in Figure 5.6. The point is projected down to determine the required differential pressure corresponding to this point. Values were reported from the manufacturer for only 2 operating speeds (1500 RPM and 3000 RPM, Table 5.4), so extrapolation had to be performed to determine input flows at each stage. It can be seen in Table 5.4 that hydraulic motor operating speeds for the Bruce Protocol range from 659 RPM to 2327

RPM. Knowing the required differential pressure at each stage, the corresponding input flow could be determined by projecting up from the pressure value on Figure 5.6 to the appropriate approximation of the 4.6cc speed lines. From this point, the corresponding input flow at each stage could be determined. The values found from this procedure for each stage may be found in Table 5.5

Figure 5.7: Specifications for P6 hydraulic pump supplied by The Water Hydraulics Co., Ltd.

Figure 5.7 can be used to determine the pump torque and power inputs. Knowing the output flow and its corresponding pressure at each stage, the input torque may be determined for each stage. Given the required input torque, the required input power at each stage was found using:

Power = Torque x Rotational Speed 5.4

Pump speed can then be found by using the appropriate approximation of the 6cc speed curve. Pump performance curves were only given for a single pump speed (1500

RPM, Figure 5.7), so the location of curves corresponding to other speeds had to be estimated. A summary of the drive system design requirements is found in Table 5.5.

Motor Pressure Differential Pump Pump Pump Requirement Pump Torque Input Input Speed Gradient for 400 lb Speed Input Power Power Stage (mph) (%) load (bar) (rpm) (Nm) (kW) (hp)

Start 0 0 91 10 11 0.012 0.016

1 0 51 367 6.5 0.251 0.336

mB1 1.7 0 51 620 6.5 0.424 0.568

mB2 1.7 5 39 615 5 0.324 0.434

1 1.7 10 25 612 3.6 0.232 0.311 2 2.5 12 21 900 3.4 0.322 0.432 3 3.4 14 19 1224 3.3 0.425 0.57 4 4.2 16 0 1515 3 0.478 0.641 5 5 18 -16 1800 2.5 0.473 0.635 6 5.5 20 -18 1900 2.5 0.5 0.67 7 6 22 -24 2040 2.5 0.537 0.719

Table 5.5: Summary of drive system requirements

At this point, the determination of a 3:1 drive ratio was finalized. A first attempt

using a 2.5:1 drive ratio resulted in the manufacturer’s components not being able to

provide sufficient torque at low speeds. A reanalysis using a 3:1 drive ratio placed our

66 requirements in line with what the manufacturer’s components could provide. These calculations were made merely to determine appropriate component selection, not to provide a detailed analysis at each stage. The components selected based on the design criteria are described in detail below.

5.3.3 Hydraulic Motor

Figure 5.8: Hydraulic motor (P/N: M6, The Water Hydraulics Co. Ltd)

The hydraulic motor will be required to operate at a maximum speed of 2327

RPM (6 mph treadmill speed, Table 5.4) and produce a maximum of 3.6 N-m of torque.

A motor was selected capable of turning at 3000 RPM with a displacement of 4.6 cc/rev, and producing 10.5 N-m of torque (M6, The Water Hydraulic Co, Ltd, East Yorkshire,

UK). The motor was mounted in a custom ½ in thick aluminum bracket (P/N 8) in order to keep the shaft height at 3.5 in from the bottom of the treadmill to allow space for the flywheel (Figure 5.4).

5.3.4 Hydraulic Power Pack

A hydraulic power pack (Figure 5.9) was selected to provide sufficient flow and

to operate the hydraulic motor (P6 3kW Power Pack w/ 480V motor, The Water

Hydraulics Co., Ltd., East Yorkshire, UK). The power pack contains an electromagnetic

motor, a hydraulic pump, and the system fluid reservoir. In addition, valves required for

controlling pressure and elevation and safety switches are contained in the unit. This

power pack will sit external to the MR environment.

Figure 5.9: Hydraulic power pack (P/N P6, The Water Hydraulics Co. Ltd)

The hydraulic pump is required to displace 6.0 cc/rev of fluid to the hydraulic

motor. A 480V, 3-phase, 5HP electric motor operates the pump. The electric motor is

controlled using an Allen-Bradley PowerFlex 70 AC Drive (480VAC, 3 phase, 8A, 5HP ).

System cooling is provided by the reservoir and monitored with a temperature switch.

Filtration was built into the power unit to filter the fluid returning from the circuits as well as fluid added to the system. Water level and temperatures sensors, and system pressure switches are located within the power pack to ensure safe operating conditions.

5.3.5 Addition drive system components

Additional drive system components include the flywheel, front pulley, drive belt, bearings, and belt tensioner. These components are coupled to the hydraulic motor via the drive shaft (Figure 5.2).

5.3.5.1 Flywheel

A flywheel (P/N 3) was added to help reduce the inertial changes associated with footfall during running on the treadmill. Because this was not a critical component, its size selection was based on available physical space rather than optimization for performance. After selection of the motor shaft bearing (P/N 5192K26, McMaster-Carr,

Figure 5.2), the motor shaft was placed 3.5 in from the bottom of the treadmill, which would allow for a 6 in diameter flywheel. Sufficient space existed axially on the motor shaft to allow for the flywheel to be 2 in wide. Stainless steel was chosen as the material for its MR compatibility and to maximize the inertial properties of the flywheel. Using a flywheel radius of 3in (R wheel ), a thickness of 2 in (T wheel ), and density ( ρ) of stainless steel = 0.29 lb/in 3, the amount of kinetic energy stored by the flywheel was found as

follows:

The volume of flywheel (V wheel ) is:

2 2 3 Vwheel = π T wheel Rwheel = π x (2 in) x (3in) = 56.5 in 5.5

The mass of the flywheel (M wheel ) can be found from:

3) 3 Mwheel = ρVwheel = (0.29 lb/in x (56.5 in ) = 16.4 lb 5.6

The flywheel inertia can be found as:

2 2 2 Iwheel = 0.5*M *R = 0.5*(16.4 lb)*(3 in) = 73.8 lb-in 5.7

Finally, the kinetic energy of the flywheel is determined. Results are found in Table 5.6:

for a solid disk: K.E. = 0.5 *I ω2 5.8

Speed Elev. Flywheel Kinetic Energy Stage (MPH) (deg) Speed (RPM) (Joules) 1 1.7 5.7 659 51.9 2 2.5 6.8 970 112.4 3 3.4 7.8 1319 207.8 4 4.2 9.1 1629 316.9 5 5.0 10.2 1939 449.0 6 5.5 11.3 2133 543.4 7 6.0 12.4 2327 646.7

Table 5.6: Kinetic Energy stored by flywheel

Gottshcall et al. performed experiments to characterize the mechanical energy exchange during uphill and downhill running [50]. Subjects with an average mass of 63 kg (138 lb) walked uphill and downhill at varying slopes ranging from -9° to +9° at a speed of 1.25 ms -1 (2.8 MPH). They found that the instance of greatest mechanical energy transfer due to ground reaction forces was at the maximum elevation. The values increased consistently with elevation when performing tests (Table 5.7). Comparison 71 with the values in Table 5.6 show that the flywheel is providing compensation for the energy transferred to the treadbelt. In future designs, it may be possible to reduce the flywheel size in order to reduce load on the hydraulic motor at low speeds.

Elevation (deg) 3 6 9 Maximum Mechanical Energy Transfer 90 120 140 (Joules)

Table 5.7: Results from study by Gottshcall et al. with treadmill speed of 2.8 MPH and varying uphill elevations. (Source [50])

5.3.5.2 Pulley System

A 3:1 drive ratio between the pulley on the motor shaft (P/N 9) and the pulley on the front roller was used to ensure the proper drive speed for the hydraulic motor, as explained in detail in Section 5.3.2. The sheave from the original front roller was pressed onto the new stainless steel roller. A pulley was connected to the motor shaft. Using the following properties, the diameter of the new pulley is 1.45 in.

Roller sheeve diameter (D sheave ) = 4.5 in

Drive belt thickness (T belt ) = 0.14 in

The total effective outer diameter around front roller sheeve (ODsheave ) can be found from:

OD sheave = D sheave + 0.5 Tbelt = 4.57 in 5.9

A 3:1 ratio is required, so total the total effective outer diameter of the drive pulley

(OD pulley ) is:

OD pulley = OD pulley /DR = 4.57 in/3 = 1.52 in 5.10

Accounting again for the belt thickness, the required diameter of the front pulley (D pulley ):

(D pulley ) = OD pulley - 0.5T belt = 1.52 in – 0.07 in = 1.45 in 5.11

A front pulley (P/N 9) having an O.D. of 1.45 in was machined with grooves to fit the poly-v drive belt (Figure 5.4). A 0.25 in x 0.125 in keyway was machined in the I.D. to allow access by the same key used in the flywheel. A ¼”-20 hole was drilled and tapped to allow for a set screw to hold the pulley in position.

A collar (P/N 7) was machined for the opposite side of the flywheel to prevent movement in the direction of the hydraulic motor. A 1/4”-20 stainless steel set screw was used to hold the collar in place.

The motor shaft is supported on both sides by 0.75 in bronze bearings (5192K26,

McMaster-Carr) mounted on a custom aluminum support fastened to the bottom of the treadmill (P/N 5, Figure 5.2).

A poly-V belt (P/N 9003K109,McMaster-Carr) was chosen for the primary drive belt based on its strength and gripping ability. It connects the front pulley located on the motor shaft to the sheave on the front roller (Figure 5.2). Similar to the belt on the original treadmill, a 0.14 in thick, 10-rib poly-V belt was used.

5.3.5.3 Tensioner

Tensioner Wheel, P/N 1

Tensioner Body, P/N 2

Figure 5.10: Belt Tensioner

To prevent drive belt slippage and allow for ease of assembly, a slack side belt tensioner was fabricated (Figure 5.10). The main upright (P/N 2) and wheel (P/N 1) were machined from aluminum, and a 3/8 in dia. stainless steel hex head bolt was used to provide the support for the wheel. The wheel was machined 1.25 in thick so that it would be oversized compared to the 1.05 in thick belt. A bronze sleeve bearing (P/N

6391K179, McMaster-Carr) was inserted into a hole in the bearing to provide the rotational surface. Stainless steel washers placed on the bolt on either side of the upright

74 were used to provide compression surfaces while the tightening the nut for height adjustments, while still allowing rotation of the wheel. Sufficient tension is applied to the slack side of the belt to ensure that the drive belt does not slip, while also ensuring that the tension is not great enough to provide excessive torque on the hydraulic motor. This process is performed experimentally each time the tensioner is reassembled.

5.3.5.4 Shaft Coupler

Figure 5.11: Shaft coupler

A Lovejoy AL050 aluminum, jaw-type coupler was selected to couple the

hydraulic motor shaft to the drive shaft. The requirements were a maximum speed of

2300 RPM and max torque of 10 N-m. The AL050 coupler with Hytrel spider

accommodates 3600 RPM and 25.7 N-m of torque.

5.3.6 Drive System Component Summary

Table 5.8 contains a list of the critical parts contained in the drive system.

Name Part Number Drawing Hydraulic Motor M6, The Water Hydraulic Figure B.1 Col, Ltd Hydraulic Power Pack P6, The Water Hydraulic Figure B.2 Col, Ltd Drive belt 9003K109,McMaster-Carr N/A Shaft Coupler AL050, Lovejoy N/A Drive Bearing 5192K26, McMaster-Carr Figure B.3 Tensioner Bushing 6391K179, McMaster-Carr N/A Tensioner Wheel 1 Figure B.4 Tensioner Body 2 Figure B.5 Flywheel 3 Figure B.6 Motor Shaft 4 Figure B.7 Drive Bearing Mount 5 Figure B.8 Drive Roller 6 Figure B.9 Shaft Collar 7 Figure B.10 Motor Mount 8 Figure B.11 Drive Pulley 9 Figure B.12 Optical Speed sensor 42EFG1JBAF4, Rockwell N/A Automation

Table 5.8: Part list for drive system

5.4 Elevation System

Bruce Treadmill exercise protocol requires the treadmill to attain a maximum

grade of 22% to accommodate patients with a wide range of physical conditions. An

elevation system was developed which uses a nonferromagnetic hydraulic cylinder acting

on a lever arm to raise and lower the treadmill body (Figure 5.2). Details of the elevation

system design follow, and a summary may be found in Table 5.9.

5.4.1 Alternative strategies:

Two alternative strategies were examined to actuate the non-ferromagnetic cylinder

during the development of the elevation system. The first strategy was to use two

cylinders in a master-slave configuration. A conventional cylinder would be located

outside the room and actuated through valves. This cylinder would push fluid through

the wall to a MRI-compatible cylinder located on the treadmill within the MRI

examination room. This configuration is similar to the method used in MRI-compatible

robotic arms [51]. The second design, which was ultimately chosen, is to precharge an

accumulator with enough fluid to run an exercise test. At each stage of the test, fluid will

be discharged from the accumulator and directed towards the cylinder located on the

treadmill.

5.4.2 Lever Arm

Clevis Mount P/N 12

Upper Leg Upper P/N 10 Brace Crossbar P/N 15 P/N 13

Lower Lower Brace Leg P/N 14 P/N 11

Figure 5.12: Schematic of elevation lever arm

Space constraints dictate a cylinder with a 3 in stroke, which pulls on an elevation lever arm in order to lift the treadmill (Figure 5.2). To maximize available space in the front of the treadmill, the 3 in cylinder stroke will act on a 3.4 in lever, which rotates the

78 legs through a 60° rotation (Figure 5.12). Calculations were performed as follows to determine the maximum required height of the front of the treadmill at the highest Bruce

Protocol stage (22% grade).

Rear Pivot Elevation L= 50” Bearing X

Figure 5.13: Side view of treadmill showing critical dimensions for elevation calculations

The length used (50 in) is that from the elevation bearing to the center position of the rear pivot (Figure 5.13). This dimension was chosen rather than using the entire treadmill length because it represents a static point during rotation.

L = 50” 60° at max β elevation Tread mill Lower legs Y X α

Figure 5.14: Relationships used for elevation system calculations

The following properties were used to determine the length of the elevation mechanism legs:

Treadmill effective length (L)=50 in

At max elevation, α = 22% = 12.4°

The required lift height for the treadmill to obtain maximum angle of rotation can be found from:

Y=L sin( αmax ) = 50 sin(12.4°) = 10.7 in 5.12

Therefore, the bearings of the elevation system need to be raised 10.7 in to obtain maximum required treadmill elevation. The heights and corresponding elevation lever rotation angles required for each Bruce Protocol stage may be found in Table 5.9. The length of the elevation legs can be found as follows:

β at maximum rotation = 30°

Y = 10.7 in

The required length of the lower elevation leg for maximum elevation is then:

X= Y / cos β = 10.7in / cos (30°) = 12.4 in 5.13

Therefore, the elevation legs need to be 12.4 in long to achieve an elevation gradient of 22%.

One half inch thick aluminum bar stock was used to create the elevation mechanism in order to maximize strength while minimizing weight. The mechanism was connected to the treadmill body using 3/8 in stainless steel bolts passing through a 3/8 in bronze bearing in an aluminum housing (2820T13, McMaster-Carr, Figure 5.15).

Figure 5.15: Elevation system legs in both flat and extended positions

5.4.3 Cylinder

Elevation sensor (Rockwell Automation P/N: 42DRA540FO)

Clevis Mount (P/N: 12)

Upper Leg Cylinder (P/N: 10) (P/N: 1D-1463, Lehigh Fluid Power) Tunion Mount (P/N: 16 )

Figure 5.16: Front end of the treadmill showing cylinder and upper leg of elevation system

The use of tap water as the hydraulic fluid requires smaller tolerances and tighter seals than a traditional cylinder to prevent leakage due to the low viscosity of water. A cylinder was custom made to these specifications by Lehigh Fluid Power (P/N 1D-1463,

Lehigh Fluid Power). The cylinder was based on their conventional Medium Duty

Stainless Steel Hydraulic Cylinder, but used 316 SS for the tie rods and piston rather than the traditional 17-4 SS traditionally used in their cylinders (Figure 5.16).

Cylinder rod C

3.4” θ

Lower legs X= 12.4”

Figure 5.17: Relationships for determining cylinder stroke length

The cylinder stroke at each stage (C) may be found using Equation 5.14. Results

for each stage can be found in Table 5.9.

C = 3.4 in sin θ 5.14

The effective weight of the treadmill is found by assuming the maximum patient weight (400 lbs) and the treadmill weight (200 lbs) is acting at the center of the treadmill

(Figure 5.18). The effective weight (W eff ) at each stage may be found as follows, and results are found in Table 5.9:

L/2 = 25” α

Figure 5.18: Relationships83 for determining effective weight Weff = 600 lbs / 2cos( α) 5.15

The cylinder force (F cyl )at each stage may then be found using Figure 5.17 and

Equation 5.16.

Fcyl = W eff * (X cos ( θ) / (3.4 cos (90- θ)) 5.16

Results are found in Table 5.9 and it is shown that with the maximum patient weight at maximum load condition (0% elevation), the cylinder must be capable of developing 2200 lbs of pull force.

A stainless steel cylinder containing a 2 in bore (D bore ) with a 1 in diameter piston rod (D rod ) was chosen to actuate the elevation system. Based on these dimensions, the cylinder pressure (P cyl ) at each stage can be calculated by Equation 5.21. Results are found in Table 5.9.

2 2 Pcyl = F cyl / ( π/4*(D bore - Drod )) 5.17

The volume of fluid required in the rod end of the cylinder (V fluid ) at each stage

can be found using the relationship between stroke length and annular area as follows:

2 2 Vfluid = C* π/4*( (D bore - Drod ) 5.18

5.4.4 Accumulator

To provide a controlled volume of fluid to the cylinder, an accumulator

(Pulseguard PiG-SS / 15i / B57x98-EP / 1250p / 1/2”x3/8”FNPT / 316) will be charged

84 before each test with fluid diverted from the hydraulic motor through an ancillary circuit.

Treadbelt rotation will momentarily stop when the accumulator charge valve is activated, and will resume when the valve is deactivated. Because the accumulator will be located with the power pack outside the MRI room, no special provisions for MR compatibility were required.

There will be a pretest stage in which the treadmill is elevated to the grade required for Stage 1 of the Bruce Protocol. To minimize the size of the accumulator, it will be charged initially, the treadmill will raise to the desired starting elevation, and the accumulator will be recharged once the desired height is reached. This operation reduces the required size of the accumulator by almost half (Table 5.9).

The total fluid volume required to be sent by the accumulator at each stage (V stage ) can be found by determining the difference in total required fluid volume for the current stage (V fluid (current stage) ) and the volume required for stage 1 (V fluid (stage 1)).

Vstage = V fluid (current stage) - Vfluid (stage 1) 5.19

The fluid volume to be discharged from the accumulator at each stage (∆Vstage ) may then be found by determining the change in fluid volume at each stage.

∆V stage = V current stage - Vprevious stage 5.20

Results from both previous calculations can be found in Table 5.9. Before beginning stage 1, the accumulator will be charged to a pressure of 1200 psi with 3.7 in 3 of fluid, sufficient to operate the treadmill for a complete patient test session. At each protocol stage, approximately 0.6 in 3 of the stored fluid will be directed to the nonferromagnetic treadmill lift cylinder (Figure 5.16) by way of solenoid valves and conductors (Figure 5.3).

Total Fluid Fluid ∆ Fluid Front Elev. Volume Volume Vol. End Lever Cyl. Eff. Cyl. Cyl. for Rod at each at Each Height Rot. angle Stroke Weight force Press end Stage Stage Stage Deg (in) (deg) (in) (lbs) (lbs) (psi) (in^3) (in^3) (in^3) M1 0.00 0.00 0.0 0.00 300.0 2200 934 0.00 M2 2.86 2.50 11.6 0.68 300.4 1622 688 1.61 1 5.71 4.98 23.7 1.36 301.5 1254 532 3.21 0.00 2 6.84 5.96 28.7 1.63 302.2 1133 481 3.85 0.63 0.63 3 7.97 6.93 34.0 1.90 302.9 1021 433 4.48 1.26 0.63 4 9.09 7.90 39.6 2.17 303.8 913 387 5.10 1.89 0.62 5 10.20 8.86 45.6 2.43 304.8 804 341 5.72 2.51 0.62 6 11.31 9.81 52.3 2.69 305.9 690 293 6.34 3.12 0.61 7 12.41 10.74 60.0 2.95 307.2 560 238 6.94 3.73 0.61

Table 5.9: Summary of elevation system calculations

A summary of the critical components of the elevation system can be found in

Table 5.10.

Component Part Number Drawing Cylinder 1D-1463, Lehigh Fluid Figure B.13 Power Elevation Bearing 2820T13, McMaster-Carr Figure B.14 Upper Leg 10 Figure B.15 Lower Leg 11 Figure B.16 Clevis mount 12 Figure B.17 Cross brace 13 Figure B.18 Lower brace 14 Figure B.19 Upper brace 15 Figure B.20 Trunion mount 16 Figure B.21 Accumulator Pulseguard PiG-SS / 15i N/A Optical elevation sensor P/N: 42DRA540FO, N/A Rockwell Automation

Table 5.10: Summary of Elevation System Components

5.4.5 Other elevation system components

In order to soften the return of the treadmill to a level position at the end of a test session, rubber bumpers were installed at the bottom of the treadmill (Figure 5.15).

Based on the existing geometry of the treadmill frame and elevation arm, 1.5 in dia. stock rubber bumpers were chosen with ¾ in height. The bumpers contained no metallic support washer and used ¼”-20 stainless steel screw for connection to the treadmill. If

87 the lack of support proves problematic over time, a ¼ in I.D. stainless steel washer may be inserted into the bumper to help distribute the force from the fastener. The bumpers were installed using preexisting support screw locations in the frame (Figure 5.15).

Upon installation of the bumpers, steps were taken to level the treadmill during so that it may sit flat in its non-elevated state. The primary points of reference are the 2 in dia. front wheels. In order to level the front elevation arm, a 0.4375 in spacer was machined and inserted between the frame and the 9/16 in center height bearing connecting the elevation arm to the frame. This location positioned the center of the pivot bolt evenly 1 in from both the bottom of the treadmill frame and the floor. The preexisting rear pivot had a height of 1.615 in from the bottom of the treadmill frame. A

0.385 in spacer was machined and added between the pivot and the frame to bring the height of the rear of the treadmill to 2 in from the ground. Inspection using an angle meter (SSY-0185 VAS Spectratilt, SST) confirmed that these alterations allowed the treadmill to sit flat within the 0.5° of error reported by the angle meter manufacturer.

5.4.6 Fluid Control Valves

3 pos. solenoid valve

Relief Valve Pilot operated check valve 2 pos. solenoid valve

Figure 5.19: Section view of Figure 5.3 emphasizing control valves

A series of valves will direct flow to appropriate fluid components during the

duration of an exercise test. A two position solenoid valve is activated to direct fluid

flow into the accumulator. A three position solenoid valve is used to control the flow of

fluid to and from the lift cylinder. This valve is operated in series with a pilot-operated

check valve (activated by accumulator pressure), which will prevent leakage flow from

the cylinder when the valve is in the neutral position. A braking valve serves to control

pressure at the outlet side of the hydraulic motor. When a combination of operator

weight and elevation cause the user to provide power to drive the motor, the braking

89 valve closes to ensure that the hydraulic motor will control the belt speed. This valve is set to restrict the motor outlet pressure to 300 psi. A pressure relief valve was installed at the pump outlet, and is set to open automatically when system pressure reaches 110 bar to prevent unsafe conditions.

5.5 Additional Hydraulic Components

5.5.1 Hydraulic hoses

Figure 5.20: Hydraulic hoses and stainless steel quick connectors

Non-ferromagnetic hoses (Parflex 540N, Parker) will carry the fluid from the

power pack to the hydraulic motor and cylinder. Sizing for the motor hose inner diameter

was performed using the nomogram in Figure 5.21, with our maximum flow rate of

3GPM and a desired maximum velocity of <20 ft/s. As seen in Figure 5.21, three possibilities exist for choosing the ID of the hose, while trying to minimize hose size.

The use of 0.25 in inner diameter hose would have placed the velocity too close to the 20 ft/s cutoff. A hose with an inner diameter (I.D.) of 0.313 inch would give an acceptable velocity, but it was a non-standard size. A 0.375 inch ID was chosen because it also provided an acceptable flow rate in a standard size.

Valve-loaded quick connects were used near the treadmill side of the system to allow the treadmill to be rapidly removed from operation when testing is complete. The hoses will remain in the MRI room; however the treadmill may be stored elsewhere if desired. The couplers were sized and polarized to prevent incorrect connection during equipment setup. The couplers were installed in opposing directions on each of two hose sizes, allowing the hoses to be connected in only one configuration.

To reduce the fluid flow rate to and from the lift cylinder, a fixed orifice was inserted into the system near cylinder port. The orifice was fabricated from a stainless steel set screw by filing a small slot into the threads. Its purpose was to test the flow restriction required by the system before sizing a permanent restriction. It was found that only a very slight opening was required to generate the desired cylinder lift rate. A tradeoff was found between keeping the orifice small enough to control the speed of elevation and large enough to not get clogged with debris.

5.5.2 Power Pack Cart

Figure 5.22: Power pack cart with prototype controls attached

A cart (Figure 5.22) was constructed from aluminum tubing to safely store and

transport the power pack and electronic components. In this case, aluminum was chosen

due to cost and machinability, rather than MR compatibility, since the power pack will be

94 located outside of the MRI examination room. The motor controller and power supply were attached to the vertical mounting plate. Sensors, data acquisition (DAQ) board, and manual switches were fastened to a case which sits atop the cart. The remaining half of the cart provides room for a PC or laptop computer to be used for treadmill control.

Locking casters were mounted on the bottom of the cart for ease of mobility as well as to ensure that the cart will stay in a fixed position once in its final test location.

5.6 Control System

Figure 5.23: Control System schematic for MRI-Compatible treadmill

The treadmill control system is located on a computer in the MRI equipment

room (Figure 5.1). A Labview application controls the speed and grade of the treadmill,

as well as providing constant visual monitoring of the safety features (water level and

temperature). The control program automatically runs the treadmill speed and elevation

96 through the Bruce, or other similar exercise stress protocols, as well as allowing for manual control. Feedback from the speed and elevation sensors ensures the protocol is being followed precisely. An optical sensor was positioned adjacent to the flywheel

(Figure 5.4), which is coupled to the hydraulic motor shaft. The sensor reads a series of pulses from a reflective pattern located on the flywheel to indicate hydraulic motor speed.

The signal feeds back into the motor controller to provide for automatic monitoring and adjustment of motor speed using proportional-integral (PI) control. The signal to change speed at each protocol stage comes from the Labview program. Because treadmill elevation is related to cylinder stroke, a photoelectric sensor capable of sensing changes in distance was mounted on the cylinder body (Figure 5.16). This sensor points towards a reflective surface located on the cylinder head for the purpose of monitoring cylinder stroke length. For safety purposes, an emergency stop button will be located on the treadmill. The program will also allow the user to switch to manual treadmill control if necessary to accommodate patient needs. All connections were made using fiber optic cables to a data acquisition board located outside of the MRI room so that no RF noise will interfere with the imaging process.

CHAPTER 6

PERFORMANCE TESTING AND REDESIGN

Several rounds of design improvements were performed until the system functioned properly and efficiently.

6.1 Initial System Testing

Initial operation of the system revealed several problems, both in the design and in

parts obtained from suppliers.

6.1.1 Drive System Performance

Two problems were observed with the hydraulic motor. It was not able to start on its

own without human assistance and was not generating sufficient pressure and flow to

power the treadbelt. The application of a starting torque by hand allowed the motor to

spin, and it remained turning without further assistance. To test the hydraulic motor

performance, the electric motor was operated at 40 Hz and the water returning from the

motor outlet and from the case drain was diverted to a beaker for flow measurement. The

time to accumulate 1 qt of water was measured and results are shown in Table 6.1. The

98 flow coming through the case drain is 16.7% of the flow through the motor outlet. This number would be expected to be less than 5% typically. The higher the percentage of flow through the case drain, the lower the performance of the hydraulic motor.

Time for 1 Qt. (s) Flow Rate (GPM) Motor Outlet 10 1.50 Case Drain 58 0.25

Table 6.1: Flow rate for examining hydraulic motor leakage

6.1.1.1 Hydraulic Motor Components

The motor was disassembled and inspected (Figure 6.1). Some of the internal pistons were much stiffer than expected, which may have led to decreased motor performance.

The motor was sent back to the manufacturer for further performance testing. It was revealed that in addition to the stiff pistons, during the time between receiving the motor and performance testing some corrosion had built up within the motor. After cleaning and further performance testing to ensure it was meeting specifications, the motor was returned. A discussion of the second round of performance testing, including that performed upon the motor’s return, can be found in Section 6.2.

Pistons

Figure 6.1: Hydraulic motor disassembled for inspection

6.1.1.2 Relief Valve

Another reason for the lower than expected available motor pressures at lower operating speeds was the faulty operation of the system relief valve. The valve was set to open when the system pressure reached 110 bar, however the valve was leaking, creating the effect of partial opening at lower than expected pressures. The net effect of this was that a significant amount of flow was diverted from the intended path, and the desired operating pressures could not be reached at lower flow rates. A test was performed in which the electric motor speed remained constant, and a needle valve was inserted to

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control pump pressure. The hydraulic motor was removed to the circuit, and flow to the

motor and returning from the relief valve was diverted into beakers. The time it took to

fill the 1 Qt beakers from each fluid line can be found in Table 6.2. Based on the pump

performance specs, the system should have been developing 6 cc/rev (2.38 GPM at 1500

RPM). Table 6.2 shows that we never reach a value greater than 93% of ideal. It was

determined that the system was not generating the required pressure and flow based on

these results.

A new relief valve was obtained from the manufacturer and installed in the system.

Results of testing performed with the improved valve are discussed in Section 6.2.

Based on 1500 Ideal rpm both lines (total Flow Vol. performance tank line pump flow) Rate Eff curve Time Time For For Control Pump One One Ev Freq. Speed Pressure P Quart Q Quart Q Q (% of (Hz) (rpm) (psi) (Bar) (s) (gpm) (s) (gpm) Qt (gpm) Ev (gpm) ideal ) 50 1495 0 0 61.00 0.246 6.53 2.297 2.37 97% 50 1488 290 20 28.37 0.529 7.00 2.143 2.36 91% 2.22 93 50 1481 580 40 14.00 1.071 7.15 2.098 2.35 88% 2.13 89 50 1473 870 60 11.91 1.259 7.10 2.113 2.34 89% 2.09 88 50 1465 1160 80 9.06 1.656 7.22 2.078 2.32 87% 1.98 83 50 1458 1450 100 8.85 1.695 7.50 2.000 2.31 84% 1.89 79 50 1449 1700 120 7.56 1.984 7.56 1.984 2.30 83% 1.84 77

Table 6.2: Relief valve testing at 50 Hz. electric motor speed

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6.1.1.3 Shaft Coupler

The hydraulic motor manufacturer suggested that drive system alignment may not be satisfactory, and that a side load was being imparted on the motor shaft, causing a deflection in the motor’s base plate. This deflection caused leakage in the motor and did not allow the system to develop sufficient starting and operating pressures. The manufacturer further suggested that the type of coupler being used was partially to blame for this situation because it does not allow for large amounts of misalignment. The manufacturer additionally suggested a more rigorous alignment protocol to reduce potential side loading on the shaft. Finding a nonferromagnetic coupler with sufficient misalignment, speed and torque characteristics proved to be a challenge. An Oldham style coupler (P/N MOL-32 C, Lovejoy Inc, Downers’ Grove, Il.) was selected (Figure

6.2).

Figure 6.2: Specifications for Lovejoy Oldam Style shaft coupler

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6.1.1.4 Drive System Component Alignment

Following the acquisition of an appropriate shaft coupler, the next step was to precisely align all the components of the drive system. A base plate was fabricated to allow the drive components to function as a subassembly separate from the treadmill base

(Figure 6.3).

Motor Mount Coupler

Bearing 1

Bearing 2

Base Pl ate

Figure 6.3: Image of Treadmill Drive system with new coupler and drive system mount.

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The bearing previously affixed to the side of the treadmill (bearing 1, Figure 6.3) was removed, and a new bearing mount was fabricated to position the center of the bearing at the identical height of the other bearing. Beginning from the bearing 1, each bearing and mount was aligned using a dial indicator and t-square. Spring pins were used to ensure correct alignment was maintained. A surface plate and height indicator were used to ensure that the drive shaft was level. The height of the motor mount was found to be too low, so approximately ¼ in was machined off the motor mount and a shim was installed to allow for accurate alignment. This shim and the motor mount were roughly aligned with the other bearings. Following several unsuccessful attempts using a dial indicator, the most accurate method for aligning the motor shaft to the drive shaft was to use a rigid coupler inserted between the other two shafts to indicate their alignment. An aluminum shaft was center-bored to 9mm to match the corresponding diameters of the motor and drive shafts. The motor was placed in its mount, and the components were manipulated until the alignment shaft could comfortably slide between the motor and drive shafts. Once alignment was complete, spring pins were installed to guarantee position of the components.

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6.1.2 Elevation System Performance

The initial operation of the elevation system brought its own set of problems. The

system operated at a basic level, but leaks and faulty valves prevented successful control

of the system.

6.1.2.1 Hose Leakage

The first problem identified with the elevation system was severe leakage from the hydraulic hoses of the elevation system. It appeared as if there was a series of pinhole leaks along the length of the hoses, and further inspection confirmed that the sections of hose overlapping the crimped-on connectors (Figure 5.20) were ballooning outwards as pressure increased. This was caused by an improper crimping procedure when the connectors were attached. The connectors had been crimped to a larger diameter, so water was allowed to flow into the layers of the hose (Figure 6.4) and high pressures caused it to leak through the outer layer of the hoses. After further crimping to the correct dimensions, the hosed operated without any further leakage.

Figure 6.4: Cross Sectional view of Parker Parflex 540N Hydraulic Hose

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6.1.2.2 Pilot Operated Check Valve

As previously described in Chapter 5, a pilot-operated check valve controls the flow of fluid to the cylinder. Two solenoids are used to control the operation of this valve.

When the elevation system was operated, the pilot-operated check valve allowed flow both to and from the lift cylinder; however a leaky valve prevented the ability to maintain cylinder position. The result was that the treadmill could elevate, and maintain a constant height while elevating, but when operating the valve in the direction that allowed fluid flow away from the treadmill, the pilot operated check valve would not close and the treadmill would lower regardless of the solenoid valve operation. A new pilot operated check valve was obtained from the manufacturer and replaced in the system.

6.2 Second Design Iteration and Testing

Following the modifications described above, the system was tested again for

performance. While this second iteration solved many of the problems, several more

revealed themselves during the process.

6.2.1 Braking Valve

Despite the acquisition of a new system pressure relief valve, the system performance was not as desired. The braking valve was not functioning properly to restrict the backpressure to the motor at 300 psi. Instead, a needle valve was installed,

106 which was controlled manually to restrict the backpressure. This valve requires adjustment dependent on treadmill speed. Figure 6.5 shows the results of preliminary testing at speeds and elevations corresponding to the stages Bruce Protocol. Performance with the new relief valve, smoother motor, and manual back pressure valve is closer to expected. Despite the lack of feedback, the treadbelt speed tracked closely with idea.

Differential pressure remained under 300 psi at all stages with no adjustment of the needle valve.

Figure 6.5: Performance testing of system with new system relief valve, fixed orifice braking and no feedback. “Loaded” curves represent tests performed with a 200 lb subject.

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6.2.2 Noise and Vibration

Figure 6.6: Drive shaft showing evidence of wear with first bearing configuration

When the treadmill was run at an electric motor speed of 1000 RPM and above, the system experienced a great deal of noise and vibration from the drive components.

Following several minutes of treadmill operation, bearing 1 (Figure 6.3) became warm to the touch. The bearing was removed from the motor shaft, and the wear shown in Figure

6.6. The bearing was rated for the speed and radial load required for treadmill operations, so it was determined that most likely cause for this wear is that the bearing could not handle the excessive radial load supplied by the drive belt on the motor shaft. A larger

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bearing, identical to bearing 2 (Figure 6.3), was acquired. The subassembly from Figure

6.3 was outfitted with this larger bearing using a similar bearing mount to that which

supports bearing 2, and all components were realigned.

The vibration was thought to be caused by the loose fit of the flywheel on the

driveshaft. A square aluminum key was used to transfer the rotation form the drive shaft

to the flywheel, however the key was slightly undersized to allow for ease of insertion

into the keyway. This slight gap allowed the flywheel to move radially, causing

vibration. To close this gap, an angled key was created which would force into the slot,

creating a tight fit and preventing motion both radially and axially.

6.2.3 Valve leakage

Spring returns

Pilot-operated check valve

Three position solenoid valve

Figure 6.7: Section view of Figure 5.3 indicating problematic valves

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Despite previous replacement of the pilot-operated check valve, the elevation system still experienced leakage when attempting to raise the treadmill. Operation worked as expected when lowering the treadmill, however a 10 s delay appeared between giving the signal to close the “raise” solenoid and the flow of water to cease. This malfunction resulted in great difficulty controlling the treadmill elevation. The leakage also caused the accumulator to drain almost immediately to the reservoir. The control system could compensate for the delay in valve closing when a continual flow was provided to the accumulator, however the treadmill cannot run in the desired manner in this case because the flow is diverted from the hydraulic motor when filling the accumulator. Discussions with the manufacturer revealed that the performance of the spring return in the solenoid valves was not functioning properly. Because of this, the valves were not able to return to their neutral position following activation, and so leakage was occurring. The valves could be temporarily reset by activating the opposing solenoid momentarily, providing some degree of control.

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6.2.4 Maximum Elevation

Figure 6.8: Bearing crafted from aluminum stock

While preparing the treadmill to bring to the MR environment, one of the elevation bearings broke. A solution was found by fabricating a bearing with aluminum stock (Figure 6.8). It was discovered that after replacing the elevation bearings, the treadmill would only achieve an elevation comparable to stage 5 on the Bruce protocol

(10.2°). While this stage is acceptable for the majority of clinical cases to be performed using the treadmill, is did fall short of the desired maximum elevation. Measurements performed on the location of the elevation system components revealed that the elevation lever pivot point was located one inch too far forward on the treadmill, creating two issues. First, the legs were raising the treadmill at a location farther forward on the treadmill than that in which the calculations were performed. It would then require a longer leg length (by about 0.25 in) to achieve the same maximum elevation. The mislocation of the pivot point also prevented the complete rotation of the upper lever 111 through its entire range due to the length of the cylinder body. While an accurate measurement could not be performed, if the rotation was off by a magnitude as little as

5°, it would add 1 in to the required lower leg length. Other elevation system structures prevented the movement of this pivot point without major reconstruction, so the choice was made to run the treadmill only to stage 5. This situation may easily be rectified in the future by creating longer legs or moving the pivot point.

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CHAPTER 7

ADDITIONAL SYSTEM COMPONENTS

7.1 480V Power

The use of 480V power required special wiring in both the treadmill developmental

area (Room W080 Scott Lab) and in the clinical area of the Ross Heart Hospital Non-

Invasive Imaging wing, where the treadmill will be used. A 480V fuse box already

existed in Scott Lab, so it simply had to be retrofitted with 15 amp fuses and had a

receptacle (#2730 30 amp receptacle, Leviton) added (Figure 7.1). No 480V power

existed in the MRI area of the Ross Heart Hospital, so a line had to be run from the

central power supply. A fuse box and receptacle similar to those used in Scott Lab were

installed in the equipment room adjacent to the Ross Heart Hospital Cardiac MRI

examination room (Figure 5.1).

Figure 7.1: 480V fuse box and receptacle installed in W080 Scott Lab 113

7.2 Penetration Panel

3” x 12” copper waveguide

Hydraulic hoses Penetration panel

Fiber-optic cables

Figure 7.2: Existing penetration panel with new waveguide contained in storage cabinet

To provide access through the RF shielding of the MRI room, a waveguide was installed in the preexisting penetration panel to the rear of the MRI examination room

(Figure 7.2). A waveguide is used to stop the intrusion of RF (radiofrequency noise) into the MRI environment, and consists of a conductive cylinder that is 4 times as long as its

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inner diameter. A penetration panel with an unused slot already existed in the MRI room,

so the blank slot was removed and a waveguide (ETS-Lindgren, St. Louis, MO) was

installed in its place. A 3 in diameter (12 in length) waveguide was large enough to

contain the hydraulic hoses and fiber optic cable while minimizing the overall size.

7.3 Patient Monitoring

Figure 7.3 shows the patient monitoring equipment required for exercise stress CMR.

During the exercise test, continuous 12-Lead ECG monitoring of the patient is required. A supine

baseline ECG is recorded following baseline (resting) imaging with the patient lying on the MRI

table. An upright baseline ECG is recorded immediately prior to commencing treadmill exercise.

The 12-lead ECG is continuously monitored during exercise, and an ECG trace is printed at each

stage of the test. The patient will also be monitored and traces recorded during the recovery

period.

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Figure 7.3: MRI compatible patient monitoring equipment for use with exercise stress CMR.

Traditional stress test patient monitoring equipment is not MR compatible due to conventional hardware configuration. Recently, GE Healthcare released a software version of their widely used CASE stress testing system ( Figure 7.4, GE Cardiosoft, GE Healthcare,

Chalfont St. Giles, United Kingdom). This program was installed on a PC located outside of the

MR environment and user interface and display solutions were developed to minimize the amount

of RF noise passing into the MRI room. 116

Figure 7.4: Screen shot of GE Cardiosoft stress test program

LCD monitors are by nature not affected by the magnetic field of the MRI environment; however they may contain ferromagnetic components. A LCD monitor (Dell Computer Corp.) was found that contained only a small quantity of ferromagnetic fasteners. It was attached to a wall-mounted cabinet located just outside of the 5G line ( Figure 7.3). This location was far enough outside of the strong magnetic field to prevent attraction of the monitor towards the magnet, and the monitor was permanently secured in the event that such an attraction occurred.

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The monitor was connected to the PC located outside of the MRI room via a conventional shielded monitor cable passing through a waveguide. This setup transmits minimal RF noise, however ideally a wireless solution will become available. Computer control was provided through a wireless keyboard and mouse using Bluetooth transmission to reduce the number of cables passing into the room. A Bluetooth transceiver was inserted through the waveguide via a

USB extension cable. The CAM-14 ECG acquisition module ( Figure 7.5) was found to contain only minimal ferromagnetic components and can remain attached to the patient during MRI without safety risk or generation of image artifacts. An extended patient cable was acquired to connect the ECG acquisition module to the monitoring PC through the waveguide (Figure 7.5).

Figure 7.5: 12-lead ECG system will connect patient to PC located outside of magnet room 118

While the patient is inside the MRI magnet bore, the ECG is non-diagnostic due to magnetohydrodynamic artifacts caused by blood flow within the magnetic field [52], but heart rate and rhythm can be monitored via the wireless 3-lead ECG system used for MRI image gating. The patient will be inside the magnet for approximately 2 minutes immediately following exercise. During this brief time, in addition to heart rate and rhythm, the perfusion and real-time cine images themselves will directly demonstrate any ischemic cardiac changes to the interpreting physician. In the ischemic cascade (Figure 7.6), perfusion defects and wall motion abnormalities

are expected to precede any ECG changes, so the visualization of the real-time images will

provide a safe method of monitoring the patient during the time in which the ECG is

undiagnostic.

Figure 7.6: Graphical representation of the ischemic cascade, showing perfusion defect and systolic dysfunction (observable as wall motion abnormality) occurring earlier than ECG changes (Source [53]) 119

MRI-compatible infusion and blood pressure monitoring equipment is already available. All interfaces for ECG monitoring and treadmill control are located in the examination room, using the above described MR-compatible LCD monitor, wireless keyboard and mouse (Figure 7.3). The major benefit of this configuration is that it brings the entire stress testing system and team into the MRI exam room in order to allow communication with the patient for the duration of the test.

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CHAPTER 8

VALIDATION

8.1 MR Compatibility Testing

Before operating the treadmill system within the MR environment, the system first

had to be investigated for safety and compatibility.

8.1.1 MR Safety/Conditionality

In order to ensure safe operation in the MR environment, the treadmill and

hydraulic hoses/connectors were checked for any possible ferromagnetic components. A

small hand magnet was used to go over each part of the system in a preliminary check.

This investigation revealed several areas of concern. Each of the two brass bearings in

aluminum housing on the drive shaft appeared to contain a ferromagnetic element within

the bearing sleeve area. Another area of ferromagnetic attraction was the roller bearings.

The balls in the roller bearings were made from 440 stainless steel, which is

ferromagnetic. Several sources for non-ferromagnetic ceramic bearings have been

located and these will be substituted for the bronze bearings in the drive system and

stainless steel bearings in the rollers. In addition, the tie rods on the cylinder exhibited

strong magnetic attraction. Communication with the manufacturer (Lehigh Fluid Power)

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revealed that the wrong material was used for the tie rods. The specifications called for

300-series stainless steel; however 17-4 stainless steel, which is ferromagnetic, was

mistakenly used. The current tie rods will be swapped for new ones made of the proper

nonferromagnetic material. These changes will eliminate all ferromagnetic components

on the treadmill. The treadmill and hoses were checked again using a 3,200 gauss (.32T)

hand magnet (Next Generation Science, Lafeyette, IN) which is designed for checking

MR compatibility before entering the MR environment. No new problem areas were

discovered during this check.

Despite the discovery of some small ferromagnetic components, the treadmill was deemed safe for use in the 1.5T MR environment. The components containing ferromagnetic materials are securely fastened to the treadmill with redundant connections, and the magnetic attraction generated by the components is not sufficient to overcome the weight of the treadmill and pull it towards the MRI magnet. The ferromagnetic components will be replaced in the future to ensure that they do not work themselves loose over time and create a dangerous situation.

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Wave guide/ penetration panel

Imaging phantom Hydraulic hoses/ fiber -optic cables

MRI patient table

Hydraulic quick- connect couplers

Figure 8.1: Treadmill system placed in MRI room

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The hydraulic hoses were fed through the waveguide and connected to the

treadmill, which was brought into the MRI room and placed within approximately two

feet of the MRI machine (Figure 8.1). Neither the treadmill nor the hydraulic hoses

exhibited signs of attraction towards the magnet at this location. Because the magnetic

field increases dramatically immediately adjacent to the magnet, the decision was made

to not place the treadmill directly against the MRI machine while any ferromagnetic parts

remain. The location in which the treadmill was placed was acceptable for the current

stress testing configuration.

The fiber-optic cables used for speed and elevation feedback exhibited a strong

degree of ferromagnetic attraction, and are considered unsafe in the MRI environment.

The cables were secured to the hydraulic hoses (Figure 8.1), which provided sufficient

inertia to prevent their attraction to the MR machine. These cables require close

supervision while in the MRI room, and will be replaced before any extensive testing

with human subjects occurs within the MR environment.

8.1.2 MR Compatibility

In addition to the safety of the treadmill, it was important to determine the effect

the system may have on the MRI system’s performance and image quality. Despite using

nonferromagnetic materials, metals such as aluminum and stainless steel still have very

high magnetic susceptibility and will warp the magnetic field in their vicinity. MRI

124 requires an extremely homogenous field, with typically on the order of only 1 ppm inhomogeneity over the 50 cm spherical imaging volume at the center of the MRI magnet. Placement of the treadmill with its significant mass of aluminum and stainless steel next to the MRI system has the potential to significantly distort the magnetic field, so the homogeneity of the field within the magnet bore had to be tested. Another potential source of image artifact can arise from electrical current arcing caused by electrostatic discharge between loosely connected conductors such as fasteners on the treadmill. An electrical arc will release energy in the RF range that can interfere with the

MR signal and introduce characteristic “spike” artifacts. Another potential source of RF interference comes from passing the conductive elements (wires, cables, etc) from outside the RF shield into the MRI room. Any conductor can act as an antenna,

“wicking” RF noise into the room from outside sources. The MRI signal at 1.5T (63.5

MHz) falls into the VHF frequency range, so TV signals in the airwaves are one potential source of noise. Additionally, hospital and computer equipment is likely to emit RF noise in this same frequency range. The treadmill system design uses fiber-optic cables to reduce this possibility; however the currently used fiber-optic cables have ferromagnetic, and apparently conductive, shielding. Another concern was the conductive properties of the water in the hydraulic hoses. It is possible that outside RF noise may be picked up by the water and transmitted into the MRI room.

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To test these issues, several system tests were performed using service software

tools built into the MRI software (Siemens Medical Solutions, Malvern, PA). Imaging

was performed using a spherical MRI calibration phantom (Figure 8.1). Tests were

executed without the treadmill inside the MRI room, and with the treadmill positioned

adjacent to the MRI patient table as shown in Figure 8.1. All imaging tests were

performed with the treadmill turned off, as the treadmill will not be running during

imaging procedures; the treadmill will be used by the patient prior to imaging, not

simultaneous with imaging. Raw reports from each test may be found in Appendix A and

the results are summarized in Table 8.1 and Table 8.2.

8.1.2.1 Field Homogeneity Test

The first test examined the magnetic field homogeneity of the MRI system.

Additional coils called “shim” coils are built into the MRI magnet bore to correct for inhomogeneities in the main magnetic field due to imperfections in the MR system hardware or the presence of external materials that affect the magnetic field. The shim test examines whether the field homogeneity along each imaging axis is within specification, or whether adjustment of the currents in the shim coils are required to bring the field homogeneity into specification. The test was first performed without the treadmill in the MRI room, and results showed the shim currents were within the specification range (Appendix A.1). The treadmill was then brought into the MRI room,

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positioned for stress testing as shown in Figure 8.1, and the test repeated. Results

showing the shim remaining within specification (Appendix A.2) indicate that the

introduction of the treadmill to the MRI environment did not significantly affect the

homogeneity of the static magnetic field.

8.1.2.2 RF Spike Test

The second test examined the presence of any RF noise spikes within a test phantom image. Electrostatic discharge can create noise spikes in the MR signal, leading to characteristic stripes or “corduroy” artifacts in the images. Tests were performed with and without the treadmill in the MRI room. The test reports show the results of each test to meet specifications, indicating no significant spike artifacts with or without the treadmill in the room (Appendix A.3, A.4). This is a test that will need to be performed periodically as over time fasteners on the treadmill may work loose, leading to the potential for electrostatic discharge.

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8.1.2.3 Artifact Calculations

The artifact calculation test performs phantom imaging in several orientations to investigate whether any image artifacts exist in the air surrounding the phantom. Tests were performed with and without the treadmill in the room, and results show that the system meets specifications for each test (Appendix A.5, A.6), indicating the treadmill does not introduce any significant image artifacts.

Results Test Without Treadmill With Treadmill Shim + + Spike + + Artifact + +

Table 8.1: Results of MRI system performance testing. (+) indicates that specification has been met.

8.1.2.4 RF Noise Tests

A final series of tests investigated the presence of RF noise with various treadmill

system configurations. During the tests, a power cable was laid on the MRI table to act

as an antenna, attracting RF noise and carrying it into the magnet bore where it can be

detected by the receiver coil. This mimics the presence of a patient in the magnet bore,

whose body would act as an antenna in the same fashion. The hydraulic hoses remained

through the wave guide for each test configuration. The test checks for any significant

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RF signals over a 500 kHz bandwidth about the center frequency of the system (63.5

MHz). Results are provided over 10kHz bandwidth increments to enable the frequency and potential sources of noise to be identified. Tests were conducted with the fiber-optic cables and treadmill both in place and removed from the MR environment. Results for each configuration are found in Appendix A.7 – A.11 and are summarized in Table 8.2.

Configuration Results Hydraulic Hoses + Hydraulic Hoses, Fiber Optic Cable - Hydraulic Hoses, Treadmill +

Table 8.2: Results of RF noise tests. (+) indicates specifications have been met. (-) indicates specifications were not met

The results show that neither the hydraulic hoses nor the treadmill were responsible for RF noise interference. The fiber-optic cables did bring a significant amount of RF noise into the room. Figure 8.2 shows an example image from a positive test, in which the hydraulic hoses and fiber-optic cables were run through the waveguide and the antenna was used. The red square in the image indicates an area of RF noise interference in the image. These fiber-optic cables have been shown to contain ferromagnetic conductive shielding and will be replaced to avoid both noise interference and attraction to the MRI system.

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Region of RF noise inter ference

Figure 8.2 Sample image from RF noise test with hydraulic hose, fiber-optic cable, and antenna

8.1.3 Conclusions

The results of testing show that the treadmill may be considered MR conditional for

1.5T, with the condition that it is not pushed directly against the magnet. With the

current components, the treadmill experiences no magnetic attraction when placed at least

two feet from the MRI machine. Following replacement of the bearings and cylinder tie

rods, the treadmill is expected to be completely safe in a 1.5T MRI environment. System

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tests also indicate that there will be no adverse affect on imaging due to the hydraulic

hoses or treadmill. Replacement of the fiber optic cables will be necessary due to their

ferromagnetic attraction and influence on the MRI imaging. Once all ferromagnetic

components and conductive fiber optic cables have been replaced, these tests will be

repeated in order to certify the treadmill as unconditionally MRI safe and compatible.

Ultimately, these tests will also be repeated with a 3 Tesla MRI magnet to verify safety

and compatibility at higher field strength.

8.2 Performance testing inside and outside of the MRI room

Tests were performed both inside (Figure 8.1) and outside the MRI environment to determine any affect of the magnetic field on treadmill operation. Eddy currents generated by moving metallic parts may affect treadmill performance or the remaining ferromagnetic components may experience magnetically induced attraction or torque, which may cause misalignment. The treadmill system was set up both in the hallway of the non-invasive imaging area of OSU’s Ross Heart Hospital and also in the configuration shown in Figure 8.1. The system was connected to the 480V power supply, which is located in the equipment room located to the rear of the MRI room

(Figure 5.1). The treadmill was run through speeds corresponding to Bruce Protocol.

Valve malfunction made elevation control cumbersome, so the testing was performed at a constant elevation. No subject was on the treadmill for these tests, so elevation was not

131 expected to affect performance significantly. Pressure gauges were used to determine hydraulic motor inlet and outlet pressures. Flywheel speed was recorded using an optical sensor, with output displayed in Labview and confirmed on the display of the motor controller. A measuring wheel was used to record belt distance in feet over 1 min. intervals and average belt speed was calculated.

Hydraulic Motor Differential Pressure (psi) Belt Speed (MPH) % % Diff. % % Diff. Desired Outside to Diff. Diff. Outside to Speed Outside Inside Inside Outside from Inside from Inside (Mph) Room Room Room Room Des. Room Des. Room 1.7 300 300 0.0 1.60 5.61 1.63 4.08 -1.6 2.5 300 300 0.0 2.45 1.82 2.46 1.41 -0.4 3.4 290 290 0.0 3.33 2.11 3.34 1.70 -0.4 4.2 370 320 13.5 4.14 1.38 4.11 2.14 0.8 5 350 300 14.3 4.89 2.16 4.88 2.36 0.2

Table 8.3: Results of performance testing with no subject inside and outside the MRI room.

Results of the treadmill performance testing inside and outside of the MRI room

(Table 8.3) show that location in the MRI environment does not adversely affect treadmill performance, since the percentage difference from outside to inside the room is no greater than 1.6% at any speed. Actual speed remained within 6% of the desired value at each stage for unloaded conditions both inside and outside of the MRI room. 132

Differential pressure showed a high percentage of difference at higher stages; however this difference may be attributed to interpretation of the pressure gauge reading, since the needle readout was bouncing considerably at high speeds. This reading is also highly dependent on the setting of the backpressure valve. Because it is a manual needle valve, precise control is difficult, and at high flow rates even a slightly different setting will have a large effect on differential pressure. The control parameters have not yet been calibrated, so the deviation from desired belt speed is expected to improve once the controller is tuned. Choosing a fixed orifice backpressure valve will eliminate inconsistencies due to manual adjustment.

While the MRI system is theoretically capable of imaging patients up to 400 lbs in body weight, in practice patients over about 320 lbs. do not fit inside the MRI magnet bore. Newer MRI magnet designs have larger bores (70 cm vs. 60 cm), allowing for larger patients. With this in mind, we sought to test that the treadmill elevation system could in fact lift the largest patient we might encounter. With the treadmill positioned adjacent to the MRI patient table, two subjects with a combined total weight of 385 lbs. stood on the treadmill at one time to test the elevation system at the maximum design capacity. The treadmill was capable of lifting the two subjects through the entire range of the elevation system.

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8.3 Performance testing with human subject inside the MRI room

The treadmill was tested through the range of the Bruce Protocol with human subjects to ensure adequate performance.

8.3.1 Methods and Materials

Figure 8.3 Treadmill set up in room for performance testing

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The treadmill was set up in the configuration shown in Figure 8.3 to test performance inside the MRI room. Speed control and measurement methods were identical to those described in Section 8.2.

Poor performance of the valves required a modified procedure for obtaining desired elevation. The valves would not return to their required deactivated position, and began leaking to the reservoir so that a steady elevation could not be maintained.

Diverting flow from the accumulator back to the drive system causes the accumulator to drain due to the leaky valves, which removes the pilot signal required to operate the elevation check valve. Because water cannot flow through the check valve, pressure is maintained in the cylinder. For the remaining tests, the treadmill was set to raise or lower until the desired elevation was reached, as displayed on the inclinometer. Flow was immediately diverted to the belt, and if the elevation remained at the desired value, the test proceeded as described above. If the elevation was not acceptable, the procedure was repeated until a desired elevation was met.

The treadmill was set for speed and elevation corresponding to the stages of the

Bruce protocol. For tests involving human subjects, the subject was instructed to begin walking on the treadmill, without holding the handrail. Measurements of belt speed and motor inlet and outlet pressures were taken. The system back pressure was adjusted at each stage to be equivalent to the values determined from testing with no load (Table

8.3). 135

8.3.2 Results and Conclusions

Subject Weight(lb) 0 175 195

Hyd. Hyd. Hyd. Motor % Diff Motor % Diff Motor % Diff Des. Des. Act. Diff. Belt From Diff. Belt From Diff. Belt From Speed Elev. Elev. Press. Speed Des. Press. Speed Des. Press. Speed Des. Stage (MPH) (%) (%) (psi) (MPH) Speed (psi) (MPH) Speed (psi) (MPH) Speed 1 1.70 10 10.2 300 1.65 -3.21 300 1.60 6.08 300 1.63 -4.08 2 2.50 12 12.6 300 2.47 -1.32 360 2.46 1.68 360 2.48 -0.91 3 3.40 14 13.9 280 3.19 -6.08 330 3.15 7.25 330 3.24 -4.81 4 4.20 16 15.5 320 4.23 0.70 370 4.27 -1.73 370 4.27 1.62 5 5.00 18 17.5 300 5.05 0.91 300 5.23 -4.50 300 5.20 3.98

Table 8.4: Results from performance testing inside MRI room

The treadmill performed adequately through the range of the Bruce protocol. Lack

of tuning of the control parameters had a greater effect on treadmill speed under load than

with no subject. Subject weight and gait had a pronounced effect on deviation from

desired speed (Table 8.4). Further tuning of the control system will be required before

this system may be put into clinical use; however the results show that the treadmill is

physically capable of reaching the desired speeds.

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8.4 Demonstration of Treadmill Exercise Stress Cardiac MRI with MRI

Compatible Treadmill

Figure 8.4: Volunteer subject performing maximal exercise stress on MR compatible treadmill

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After MR compatibility and treadmill performance in the MR environment were tested, the ability of the MR compatible treadmill to allow healthy volunteers to perform the Bruce Protocol to reach peak stress (Figure 8.4) and to reduce time-to-image over the setup used for preliminary data was investigated.

8.4.1 Methods and Materials

ECG MRI In-room Monitor Monitor

Blood Pressure Wireless keyboard/mouse

Vacuum Mattresses

Power Injector

Figure 8.5: Ross Heart Hospital MRI room setup for cardiac stress MRI with MR compatible treadmill

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The MRI room in OSU’s Ross Heart Hospital was arranged as shown in Figure

8.5. Treadmill control was performed using a PC located near the hydraulic power pack

located in the equipment room adjacent to the MR exam room (Figure 5.1). Because

faulty valves made elevation control difficult, the choice was made to modify the Bruce

Protocol by keeping the elevation constant (16% grade, equivalent to Stage 4), and

varying the speed according to the standard Bruce Protocol. This modification increased

the net workload of the subject over the course of the protocol. Three healthy subjects

performed a peak treadmill exercise stress test with CMR using the prototype MR

compatible treadmill. First, image localization and baseline function imaging (5 short

axis, 1 HLA, 3 VLA slices) were performed. Each subject then performed the treadmill

stress test to at least target heart rate (85% of age-predicted max) using the above-

described modification of the Bruce protocol. Upon passing target heart rate, the

treadmill was stopped and the patient was immediately transferred to the MRI table.

Stress function imaging then commenced, obtaining the same slices as at baseline. Time

to start and complete function imaging was measured using a stopwatch.

8.4.2 Results and Conclusions

All three subjects successfully completed the study, and all obtained target heart rate. Treadmill exercise was stopped upon subject fatigue after reaching the target heart rate. The time each subject performed treadmill stress, and the time to start and end

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function imaging may be found in Table 8.5. Results show that function imaging was

completed in an average of 45 s, which is 25% below the AHA guidelines of completing

imaging in 60 s.

Time to Heart Rate start Time to End Treadmill at Start of % of Age Function Function Time Imaging Pred. Max. Imaging Imaging Subject (min) (BPM) Heart Rate (s) (s) 1 10 160 98 22 42 2 14 160 83 22 42 3 13 161 86 30 50 mean: 25 mean: 45

Table 8.5: Timing results of maximal stress treadmill exercise CMR study

When compared to the system used to collect preliminary data, the new setup shows remarkable improvements. One major advantage is the improved safety conditions inherent from using only MR safe components. The other is the reduction in time to completion of function imaging. In preliminary studies, thirteen healthy subjects were exercised on a treadmill in the configuration shown in Figure 4.3 [47]. The subjects were required to walk to the MRI table and climb two steps before lying on the

MRI table with their head closest to the magnet. For the current tests, the patient orientation was changed to feet first in order to avoid distortion of the ECG signal by the magnetic field. This orientation adds about 8 s to the time required to start the scan when compared to the orientation used to collect preliminary data because the heart is located

140 farther from the magnet’s isocenter and the table must travel farther when returning to isocenter for imaging. Despite this extra time, two of the three subjects maintained a greater than target heart rate at the start of imaging (Table 8.5). Two additional imaging slices were obtained in the new study to account for slice misalignment between rest and stress imaging, which increased the image acquisition time by 5 s. When the values in

Table 8.5 were adjusted to account for this extra time, the results show that imaging could commence 43% faster and be completed 33% faster than possible with the system used to obtain preliminary data (Table 8.6). Despite the additional time for table movement and imaging, the non-adjusted end-of-imaging times were comparable between preliminary data and the new study, showing that we may use this time savings to improve image quality and patient safety. Low sample sizes in both studies and prevent this comparison from being conclusive, however the degree of improvement and the time to complete imaging with the current setup show this method to be extremely promising. Following some design improvements (discussed in Chapter 9), a larger cohort of healthy volunteers will be used to determine the feasibility of the system, followed by tests on cardiac patients.

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Improvement MR Over Preliminary Compatible Adjusted Preliminary Data Treadmill Results System Time to Start Function 30 25 17 43% Imaging (s) Time to Complete 45 45 32 33% Function Imaging (s)

Table 8.6: Comparison of results obtained in preliminary data and with MR compatible treadmill

Figure 8.6 shows a sample set of function images obtained at rest and stress.

Figure 8.6A and Figure 8.6C show a vertical long axis (VLA) slice and a short axis

(SAX) slice, respectively, both at rest. Figure 8.6C and Figure 8.6D show the VLA and

SAX slices under stress conditions. The increased myocardial thickening due to increased cardiac function can be clearly seen in these images.

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CHAPTER 9

CONCLUSIONS AND FUTURE WORK

9.1 Conclusions

The major benefit of this new configuration over the setup developed to acquire

preliminary data, and the system previously described by Rerkpattanapipat et al [11], is

that it brings the entire stress testing system and team into the MRI exam room, and

places the treadmill directly adjacent to the MRI scan table. The connections leading

outside of the room consist of four flexible hydraulic hoses and two fiber-optic cables, all

of which can be routed through a waveguide in the wall of the MRI room. The treadmill

can be positioned wherever it is deemed most effective, regardless of the magnetic field,

and its location can be varied depending on the needs of the patient. This system can be

utilized in a wide variety of MR environments, room sizes and magnet strengths. This

strategy of developing totally MRI-compatible treadmill and monitoring equipment is

needed to make exercise stress MRI a clinical reality. Several important performance

improvements need to occur to make the system ready for clinical testing, and a holistic

redesign will be required before the treadmill will be ready for routine clinical use.

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9.2 Drive and Elevation System Improvements

The two major issues affecting the current design are operation at low speed

conditions and elevation system performance and stability. Under load, the treadmill has

difficulty overcoming system inertia to begin turning the belt on its own. A quick “kick

start” of the belt by the user starts the belt turning, but this is not an acceptable solution

for a clinical environment. Because of the choice to use 480V power and the low current

requirement of the treadmill system, plenty of headroom exists on the power side to

improve performance. The next prototype will use a larger hydraulic motor and pump, as

well as a larger drive ratio, to improve performance at low speed conditions. These

larger sized components will provide greater flow capability at low speeds, increasing the

torque capacity of the motor.

The valve performance on the current system, while known for some time to not be ideal, became unacceptable by the time of final testing. A new valve configuration

(Figure 9.1) has been selected. The new configuration contains two 2-position solenoid valves, in addition to the accumulator valve, with a smaller flow area to direct fluid to and from the lift cylinder. Reduced leakage is expected to occur with these poppet-type valves, and single direction operation will eliminate the current problems of the 2 directions affecting each other.

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Figure 9.1: Proposed hydraulic system redesign

Investigation into the cause of the bouncing in the elevation system under load must be performed. Compliance in the legs of the elevation mechanism contribute to a degree, however a great deal of movement is most likely occurring around the elevation pivot. One possible solution is to custom build a pivot unit from stainless steel and insert a brass bushing. This would be similar construction to the tensioner. The new bearing could be made twice as thick as the current bearing in order to provide a tighter and more stable pivot point for the elevation unit. The legs themselves may also be made from

146

aluminum or stainless steel tubing, rather than flat stock, to increase system stiffness.

Another cause of lack of system stiffness is compliance due to fluid flow in long

hydraulic hoses. Reducing the pressure in the hoses will reduce the compliance that is

causing the lack of stiffness. This may be accomplished by using two cylinders in series,

which would have the effect of reducing the required elevation system pressure by half.

Another method is to reduce the amount of flow at the cylinder end of the system by

using a smaller orifice, thereby reducing the effects of the hose compliance.

9.3 Next-generation Prototype

While the MRI compatible treadmill described in this thesis is acceptable to test the

feasibility of in-room treadmill exercise stress CMR, it is not suitable for clinical use.

The development of an original frame, coupled with improvements in patient comfort and

usability will be required.

9.3.1 Structural Improvements

The drive and elevation systems developed for this project will be able to fit directly

into a frame designed around the power system baseplate (Figure 5.2). In this way, the

plate containing the motor and cylinder units may be placed into the new frame design.

The frame can be made from extruded aluminum bar stock capable of accommodating

the running board and shock absorption systems. A wooden running board will sit atop

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the aluminum outer frame. In order to accommodate patients with a long stride, the

running board will be made longer than on the prototype model. The running surface

should be a minimum of 18 in wide x 54 in long to accommodate a variety of patients.

A simple improvement over the prototype design will come from utilizing a uniform set of fasteners. Fastener selection for the prototype came from both the immediate best choice for the individual components and the availability of stainless steel fasteners from suppliers. A “big picture” reevaluation of fastener requirements and the location of an all-in-one supplier will allow for uniform sizes and fastener head types, which will make assembly quicker and required inventory lower.

9.3.2 Treadmill Height Adjustment

To allow for the easiest transfer of the patient from the treadmill to the MRI table

upon obtaining peak stress, an elevation system may be implemented to raise the

treadmill to an appropriate height which enables to patient to sit down on the MRI table

rather than climbing steps. Several design strategies may be evaluated. The first is to

design the frame such that the running board permanently sits in a more elevated

position. A step may be used for getting the patient up on the treadmill. Although the

simplest design, it will not accommodate a wide variety of patient heights in the most

efficient manner. Another option would be to use a hydraulic circuit to elevate the

treadmill to the appropriate height for each patient. This circuit may be combined with

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the original elevation circuit through the addition of a lift cylinder on the back-end of the

treadmill. In this case, the accumulator would be used to actuate the lift system prior to

the start of each test. A separate elevation system may also be used to control the

treadmill height independently of the main elevation system using a raising platform or

scissoring method.

9.3.3 Control System

To maximize clinical adoptability, treadmill control will be integrated into a PC-

based exercise stress ECG system (example - GE Cardiosoft). This will allow for a

single set of controls for the operator and allow the ECG program to track any changes in

the treadmill protocol for display in a final report. Most clinicians are familiar with the

interface of a standard stress ECG program, and the combination of the ECG and

treadmill control programs would lessen the possibility of operator error during the test.

A proposed method to accomplish this is to feed the signal from the serial port connected

to the ECG software into the DAQ board, where the signal can be captured and fed into

the Labview program. Treadmill control would still come from the Labview program;

however it would act as a slave to the stress ECG program and could run silently in the

background barring the activation of any of the safety warnings. Such an action could

cause the appropriate warning to come to the forefront of the display, prompting a user

response or terminating the treadmill program if appropriate.

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Longer term, the development of a MR-compatible, wireless 12 lead ECG system

would greatly improve the current method of patient monitoring. Valuable time is lost

unfastening the patient cable from the acquisition module on the Cardiosoft system. The

cable is also a potential safety hazard, since in order to be long enough to reach the MRI

table, there must be enough slack that it collects near the patients’ feet while running.

While this is controllable, it does create a potential trip hazard when transferring the

patient from the treadmill to the MRI table. A complete MRI stress test patient

monitoring program could integrate wireless 12 lead diagnostic ECG with treadmill

control and system safety feature monitoring.

9.4 Significance of the work

This new method of cardiac stress testing has the potential to revolutionize cardiac imaging. Over 10,000 imaging centers in the United States currently perform stress imaging tests using nuclear SPECT and echocardiography. The diagnostic limitations of these modalities and potential health hazards due to radiation exposure during nuclear imaging create the opportunity for an improved form of testing.

Early results using the “semi-compatible” treadmill system and the results of the

testing described in Chapter 8 of this thesis show the emergence of an extremely

promising diagnostic imaging modality, which can offer safer, more accurate diagnosis

for patients and the benefits of a familiar examination procedure for clinicians. After the

150

system improvements suggested in Chapter 9 are performed, the system will be ready for

feasibility testing on healthy volunteers. This testing will provide an opportunity to

further examine the performance of the treadmill system and to smooth the workflow of

the examination procedure. Cooperation will come from cardiologists, exercise

physiologists, and imaging technologists to create a safe, effective procedure for the

patient.

Following this development period, the system will be tested using subjects with

known or suspected coronary artery disease. This testing will pave the way for a larger

scale clinical trial to prove the efficacy of the method in multiple MRI environments.

The exposure generated by such a trial could generate large commercial interest in the

cardiac imaging community.

The primary market for an MRI compatible treadmill is MRI centers equipped to perform cardiac MRI studies. In the U.S., this $177M potential market currently consists of 2360 centers and is growing 10% annually [54]; the global market is approximately double the US market. These centers have already made the initial large investment in

MRI equipment and infrastructure. The addition of exercise stress CMR capability will require a relatively minor expense that will enable these centers to expand their patient referral base, increasing utilization of their MRI system, and profitability. Beyond this immediately addressable market are the centers currently performing nuclear and echocardiographic stress imaging procedures. While this much broader market is not 151 immediately available as these centers do not currently own MRI equipment, this does represent a potential growth market. As the clinical indications for CMR continue to expand and CMR reimbursement rates continue to rise, more non-invasive cardiologists are expected to add MRI to their complement of imaging equipment; the MRI-compatible treadmill could eventually become a common device found in many diagnostic cardiology centers.

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APPENDIX A

RAW REPORTS FROM MRI SYSTEM PERFORMANCE TESTING

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A.1 Results of shim test with no treadmill in room

154

155

A.2 Results of shim test with treadmill in room

156

157

A.3 Results of spike test with no treadmill

158

159

A.4 Results of spike test with treadmill in room

160

161

A.5 Results of artifacts calculation – without treadmill

162

163

164

165

166

167

A.6 Artifacts Calculations – With Treadmill

168

169

170

171

172

173

A.7 RF Noise Test with Hydraulic Hoses

174

175

A.8 RF Noise Test with Hydraulic Hoses and Treadmill

176

177

A.9 RF Noise Test with Hydraulic Hoses and Fiber-Optic Cables

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

APPENDIX B

DETAIL DRAWINGS

193

Figure B.1: Hydraulic Motor (supplied by The Water Hydraulics Co, Ltd)

194

Figure B.2: Hydraulic Power Pack (supplied by The Water Hydraulic Co. Ltd.)

195

Figure B.3: Bearing for Drive Shaft (supplied by McMaster-Carr)

196

Figure B.4: Tensioner Wheel

197

Figure B.5: Tensioner Body

198

Figure B.6: Flywheel

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Figure B.7: Motor Shaft

200

Figure B.8 Drive Bearing Mount

201

Figure B.9: Drive Roller

202

Figure B.10: Shaft Collar

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Figure B.11: Motor Mount

204

Figure B.12: Drive Pulley

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Figure B.13: Hydraulic Cylinder (supplied by Lehigh Fluid Power)

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Figure B.14: Bearing for Elevation System (supplied by McMaster-Carr)

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Figure B.15: Upper Leg

208

Figure B.16: Lower Leg

209

Figure B.17: Clevis Mount

210

Figure B.18: Cross Brace

211

Figure B.19: Lower Brace

212

Figure B.20: Upper Brace

213

Figure B.21: Trunion Mount 214

BIBLIOGRAPHY

1. Thom, T., et al., Heart disease and stroke statistics--2006 update: a report from the American Heart Association Statistics Committee and Stroke Statistics Subcommittee. Circulation, 2006. 113 (6): p. e85-151. 2. 2008 June 2008 [cited; Available from: http://www.nhlbi.nih.gov/health/dci/Diseases/Cad/CAD_WhatIs.html . 3. Blomley, M.J., et al., Microbubble contrast agents: a new era in ultrasound. BMJ, 2001. 322 (7296): p. 1222-5. 4. Lee, T.H. and C.A. Boucher, Clinical practice. Noninvasive tests in patients with stable coronary artery disease. N Engl J Med, 2001. 344 (24): p. 1840-5. 5. Lodge MA, B.H., Mahmoud F, Suh J, Englar N, Geyser-Stoops S, Jenkins J, Bacharach SL, Dilsizian V., Developments in nuclear cardiology: transition from single photon emission computed tomography to positron emission tomography- computed tomography. J Invasive Cardiol, 2005. 17 (9): p. 491-6. 6. Thompson, R.C. and S.J. Cullom, Issues regarding radiation dosage of cardiac nuclear and radiography procedures. J Nucl Cardiol, 2006. 13 (1): p. 19-23. 7. Ishida, N., et al., Noninfarcted Myocardium: Correlation between Dynamic First- Pass Contrast-enhanced Myocardial MR Imaging and Quantitative Coronary Angiography. Radiology, 2003. 229 (1): p. 209-216. 8. Bruce, R.A., et al., Exercising Testing in Adult Normal Subjects and Cardiac Patients. Pediatrics, 1963. 32 : p. SUPPL 742-56. 9. Hill, J. and A. Timmis, Exercise tolerance testing. BMJ, 2002. 324 (7345): p. 1084-7. 10. Tavel, M.E., Stress testing in cardiac evaluation: current concepts with emphasis on the ECG. Chest, 2001. 119 (3): p. 907-25. 11. Rerkpattanapipat, P., et al., Feasibility to detect severe coronary artery stenoses with upright treadmill exercise magnetic resonance imaging. Am J Cardiol, 2003. 92 (5): p. 603-6. 12. Gibbons, R.J., et al., ACC/AHA 2002 guideline update for exercise testing: summary article. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee to Update the 1997 Exercise Testing Guidelines). J Am Coll Cardiol, 2002. 40 (8): p. 1531-40. 13. Salustri, A., et al., Relationship between exercise echocardiography and perfusion single-photon emission computed tomography in patients with single-vessel coronary artery disease. Am Heart J, 1992. 124 (1): p. 75-83. 14. Iliceto, S., et al., Comparison of postexercise and transesophageal atrial pacing two-dimensional echocardiography for detection of coronary artery disease. Am J Cardiol, 1986. 57 (8): p. 547-53. 215

15. Dymond, D.S., et al., Peak exercise and immediate postexercise imaging for the detection of left ventricular functional abnormalities in coronary artery disease. Am J Cardiol, 1984. 53 (11): p. 1532-7. 16. Presti, C.F., W.F. Armstrong, and H. Feigenbaum, Comparison of echocardiography at peak exercise and after bicycle exercise in evaluation of patients with known or suspected coronary artery disease. J Am Soc Echocardiogr, 1988. 1(2): p. 119-26. 17. Dagianti, A., et al., Clinical application of exercise stress echocardiography: supine bicycle or treadmill? Am J Cardiol, 1998. 81 (12A): p. 62G-67G. 18. Hecht, H.S., et al., Supine bicycle stress echocardiography: peak exercise imaging is superior to postexercise imaging. J Am Soc Echocardiogr, 1993. 6(3 Pt 1): p. 265-71. 19. Ryan, T., et al., Exercise echocardiography: detection of coronary artery disease in patients with normal left ventricular wall motion at rest. J Am Coll Cardiol, 1988. 11 (5): p. 993-9. 20. Skorton, D., Schelbert, HR, Wolf, GL, Brundage, BH, Marcus Cardiac Imaging . 2 ed. Vol. 1. 1996, Philadelphia: W.B. Saunders Company. 627. 21. Picano, E., et al., Role of myocardial oxygen consumption in dipyridamole- induced ischemia. Am Heart J, 1989. 118 (2): p. 314-9. 22. Homans, D.C., et al., Effect of exercise intensity and duration on regional function during and after exercise-induced ischemia. Circulation, 1991. 83 (6): p. 2029-37. 23. Fujibayashi, Y., et al., Comparative echocardiographic study of recovery of diastolic versus systolic function after brief periods of coronary occlusion: differential effects of intravenous nifedipine administered before and during occlusion. J Am Coll Cardiol, 1985. 6(6): p. 1289-98. 24. Freedman, S.B., et al., Influence of coronary collateral blood flow on the development of exertional ischemia and Q wave infarction in patients with severe single-vessel disease. Circulation, 1985. 71 (4): p. 681-6. 25. Ryan, T., et al., Detection of coronary artery disease with upright bicycle exercise echocardiography. J Am Soc Echocardiogr, 1993. 6(2): p. 186-97. 26. Designation: F2503-05. Standard Practice for Marketing Medical Devices and Other Items for Safety in the Magnetic Resonance Environement . 2005, ASTM International: West Conshocken, PA. 27. Gassert, R., E. Burdet, and K. Chinzei, Opportunities and challenges in MR- compatible robotics: reviewing the history, mechatronic components, and future directions of this technology. IEEE Eng Med Biol Mag, 2008. 27 (3): p. 15-22. 28. Woods, T.O. (2006) MR Safety of Cardiovascular Implants . Endovascular Today Volume , 216

29. Kanal, E., et al., ACR guidance document for safe MR practices: 2007. AJR Am J Roentgenol, 2007. 188 (6): p. 1447-74. 30. C. L. G. Ham, J.M.L.E.G.T.v.d.W.A.M., Peripheral nerve stimulation during MRI: Effects of high gradient amplitudes and switching rates. Journal of Magnetic Resonance Imaging, 1997. 7(5): p. 933-937. 31. United States Food and Drug Administration, C.f.D.a.R.H. (1997) A Primer on Medical Device Interactions with Magnetic Resonance Systems . Volume , 32. Shellock, F.G., Magnetic resonance safety update 2002: implants and devices. J Magn Reson Imaging, 2002. 16 (5): p. 485-96. 33. Shellock, F.G., Reference Manual for Magnetic Resonance Safety Implants and Devices . 2007: Los Angeles, CA. 34. John, F.S., The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds. Medical Physics, 1996. 23 (6): p. 815-850. 35. Elhawary, H., et al., The case for MR-compatible robotics: a review of the state of the art. Int J Med Robot, 2008. 4(2): p. 105-13. 36. Gassert R, Y.A., Chapuis D, Dovat L, Bleuler H, Burdet E, Actuation methods for applications in MR environments. Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering, 2006. 29B (4): p. 191-209. 37. Ganesh, G., et al. Dynamics and control of an MRI compatible master-slave system with hydrostatic transmission . in Robotics and Automation, 2004. Proceedings. ICRA '04. 2004 IEEE International Conference on . 2004. 38. Moser R., G.R., Burdet E., Sache L, Woodtli HR. An MR Compatible Robot Technology . in IEEE Int. Conference on Robotics Automation . 2003. Taipei, Taiwan. 39. Kim D, K.E., Dohi T, Sakuma I, A New, Compact MR-Compatible Surgical Manipulator for Minimally Invasive Liver Surgery , in Lecture Notes in Computer Science 2002, Springer Berlin Heidelberg, Germany. p. 99-106. 40. Mendelowitz, S., Design of an MRI Compatible Robot for Wrist Rehabilitation , in Mechanical Engineering . 2005, Massachusetts Institute of Technology. 41. Tsekos, N.V., et al., Magnetic resonance-compatible robotic and mechatronics systems for image-guided interventions and rehabilitation: a review study. Annu Rev Biomed Eng, 2007. 9: p. 351-87. 42. Muntener, M., et al., Magnetic resonance imaging compatible robotic system for fully automated brachytherapy seed placement. Urology, 2006. 68 (6): p. 1313-7. 43. Harrington, G.S.W., C.T. , Downs III, J. H., A new vibrotactile stimulator for functional MRI. Human Brain Mapping, 2000. 10 (3): p. 140-145.

217

44. Uffmann K.; Abicht C, G.W., Quick H, Ladd M, Design of an MR-compatible piezoelectric actuator for MR elastography. Concepts in Magnetic Resonance, 2002. 15 (4): p. 239-254. 45. Shinsei Corporation . 06/22/07 [cited; Available from: http://www.shinsei- motor.com/English/index.html . 46. Gregory, C.D. Carl's Roost: MRI Artifact Gallery 1997 [cited; Available from: http://chickscope.beckman.uiuc.edu/roosts/carl/artifacts.html#RadioFrequencyInt erference(RFI) . 47. Jekic, M., et al., Cardiac function and myocardial perfusion immediately following maximal treadmill exercise inside the MRI room. J Cardiovasc Magn Reson, 2008. 10 (1): p. 3. 48. CRC Handbook of Chemistry and Physics. . 75th ed. 1994, Florida: Chemical Rubber Co. 49. Stainless Steel - Grade 316 – Properties, Fabrication and Applications . 2008 [cited; Available from: http://www.azom.com/details.asp?ArticleID=863 . 50. Gottschall, J.S. and R. Kram, Mechanical energy fluctuations during hill walking: the effects of slope on inverted pendulum exchange. J Exp Biol, 2006. 209 (Pt 24): p. 4895-900. 51. Gassert, R., E. Burdet, and K. Chinzei, MRI-Compatible Robotics. IEEE Eng Med Biol Mag, 2008. 27 (3): p. 12-4. 52. Fischer, S.E., S.A. Wickline, and C.H. Lorenz, Novel real-time R-wave detection algorithm based on the vectorcardiogram for accurate gated magnetic resonance acquisitions. Magn Reson Med, 1999. 42 (2): p. 361-70. 53. Kaandorp, T.A., et al., Magnetic resonance imaging of coronary arteries, the ischemic cascade, and myocardial infarction. Am Heart J, 2005. 149 (2): p. 200-8. 54. IMV 2006 MRI Market Summary Report . 2006, IMV Medical Information Division: Des Plains, IL.

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