Continuous Wave Doppler Device

VANDERBILT UNIVERSITY

BME 273

Dr. King

April 21, 2003

Advisor: Dr. Raul Guzman

Abbas Q. Bandukwala

Padmavathy Chunduru

Christopher D. Harvey I Abstract:

Peripheral vascular disease (PVD) is the restriction of blood flow to the limbs due to the narrowing or clogging of arteries. Our project involves peripheral artery testing of the DP and PT flow signals for the diagnosis of PVD in the leg. Our goal is to reduce the incidence of undiagnosed PVD by checking for presence and location through the quick, noninvasive, and inexpensive means of a continuous wave Doppler device. We propose to reduce cost through creating a device that does not require highly trained personnel.

This project could actually be broken down to two parts. First, a computer program recognizes the signals coming from each pair of Doppler crystals and displays the signal with the highest amplitude. The second goal of the device is to develop a continuous wave Doppler crystal array that is capable of working on ninety percent of the population. This device has the potential to have a very large market. Almost every hospital in the country with a vascular division uses Doppler technology to diagnose peripheral vascular disease because it is the easiest and cheapest method. Our design improves upon the existing method and could likely replace the existing Doppler technology in hospitals. Safety is not an important issue with our design. No part provides a significant hazard to either the user (e.g. nurse) or the patient. We were able to complete the first part of the design. We developed a working computer program that can input signals from the output of audio Doppler boxes and select the signal with the highest amplitude. II Introduction

Peripheral vascular disease (PVD) is the restriction of blood flow to the limbs due to the narrowing or clogging of arteries. PVD most commonly affects the arteries in the legs.

Its causes include most vascular diseases, artherosclerosis, aneurysms, and venous thrombosis. Symptoms include Figure 1: Aneurysm http://www.radsci.ucla.edu:800 tightness or squeezing pain in the calf, thigh, or buttock 0/aneurysm/aneurysm2.html during exercise. The pain usually subsides after an adequate amount of rest. High risk patient population include people older than the age of fifty, smokers, diabetics, patients with high blood pressure, and patients with high cholesterol. One in twenty people over the age of fifty or approximately ten million people have PVD in the United States. Seventy five percent of these cases go undiagnosed. It is Figure 2: Venous Thrombosis important to catch PVD at an early stage because http://www.webMD.com patients with PVD are at a higher risk of obtaining heart disease and stroke. Once

discovered, treatment for PVD normally

includes many lifestyle changes to reduce the

patient’s cholesterol, blood pressure, and gain

control over his diabetes. These lifestyle

changes include the adopting of a more active

lifestyle and healthier eating habits. Figure 3: Arthrosclerosis http://www.webMD.com PVD and many vascular diseases such as advanced atherosclerotic (critical internal carotid artery stenosis), aneurysmal (abdominal aortic aneurysm), or life threatening venous (lower limb deep venous thrombosis) diseases require a noninvasive way to study the vascular flow (Figures 1, 2, and 3). These constricted vessels, aneurysms, and blood clots can be detected through ultrasonography, plethysmography, and a simple handheld continuous wave Doppler device.

Ultrasonography measures flow by using high frequency sound waves much like ultrasound that is used in fetal imaging. Plethysmography measures variations in size of the vessel based on the amount of blood flowing though. It tests for blockages by measuring the systolic pressure with blood pressure cuffs in the lower and upper extremities. Doppler is the simplest and least Figure 4: Triphasic, biphasic and expensive device of these and can be used to monophasic vascular signals diagnose the necessity of vascular imaging methods such as an arteriography or

Computed Tomography. Scanning often provides information necessary to see if a patient requires surgical reconstruction of just an endovascular angioplasty. Ace Physiological Bandage phenomena such as Data Acquisition pressure, blood & Analysis flow, velocity, and Piezoelectric Crystals volume flow are Power Supply detected to diagnose

A/ D Converter disease. Calf and

High Pass & Amplifier circuit ankle pressures can

Figure 5: The Desired Prototype be obtained by listening to the dorsalis pedis (DP) and the posterior tibia (PT) arteries. As shown in

Figure 4, normal peripheral arterial flow is a triphasic audible Doppler signal. An obstruction can make the flow look biphasic and severe ischemia manifests into a monophasic signal. The current Doppler device in use resembles a pen with two crystals, one to send a signal and one to receive a signal. The pen needs to be directed to the vessel to observe the flow and so it requires knowledgeable personnel to position and direct the device properly.

Our project involves peripheral artery testing of the DP and PT flow signals for the diagnosis of PVD in the leg. There are approximately 600 screenings done at Vanderbilt

University every year. Our goal is to reduce the incidence of undiagnosed PVD by checking for presence and location through the quick, noninvasive, and inexpensive means of a continuous wave Doppler device. We propose to reduce cost through creating a device that does not require highly trained personnel. Instead of varying the position of the Doppler pen to find the signal, our goal is to create a computer program that picks the best signal when fed multiple inputs. This method increases efficiency by using more than one set of crystals on the DP and PT arteries arranged in a form of an array and reducing the amount of time that goes in to guess and check approach (Figure 5).

Moreover, this method allows the technician to perform other tasks since their hands will not be occupied during data acquisition. III Methods:

This project could actually be broken down to two parts. First, a computer program recognizes the signals coming from each pair of Doppler crystals and displays the signal with the highest amplitude. The second goal of the device is to develop a continuous wave Doppler crystal array that is capable of working on ninety percent of the population. Trying to develop the array was a tedious task because the necessary number of audio Doppler boxes was not provided. The Doppler crystals ordered form Iowa

Doppler products (Figure 1) did not function well with the initial Doppler boxes resulting in a poor signal. The crystal came on a cloth pad which would be placed onto an Ace bandage-like material which would be wound around the ankle. The crystal Figure 6: Model F Sutra Down was tested with a Parks Medical Electronics audio Transducer from Iowa Doppler http://soli.inav.net/~idp/ Doppler box for compatibility. A stereo wire with a quarter inch plug was ordered from Parks Medical Electronics so that the signal from the

Doppler box could be sent to a display screen.

At first the crystals were connected to the Doppler box with wires at the probe input, and the signal was sent to an oscilloscope from the audio Doppler box using stereo wire. However, this setup caused a tremendous amount of noise that can be attributed to the wall power source. To resolve this problem a couple of paths were explored. A high pass filter and amplifier with a gain of 1000 were used to improve the quality of the signal. Secondly, a differential amplifier was also built and tested to reduce the noise in the signal. The filter and amplifier circuits were created using resistors, capacitors and wires. The values for the resistors, capacitors and cutoff frequency were calculated using equation R * C = 1 / fc.

Because the frequency of the Doppler crystals was known and the capacitor value was held constant, the resistor value was the only variable and could be calculated. A signal gain of 1000 was also desired and resistor values needed to produce this gain were calculated using equation Vout = (Rf / R1) * (V2 – V1). Our attempts at removing the noise

and displaying a clear

signal on the

oscilloscope with the

various filters were

unsuccessful.

Therefore, Dr.

Guzman suggested

that the computer

program part of the

project be developed Figure 7: Actual prototype due to the lack of success and proper material necessary for developing the array.

The computer program was made using LabView, a user friendly computer programming language developed by National Instruments. A Doppler pencil probe was connected to an audio Doppler box. The output of the audio Doppler box was connected to an analog-to-digital converter by a stereo wire. The analog-to-digital inputted the

Doppler audio signal into the LabView program. Experiments were first conducted to test the functionality of LabView in displaying a signal with just one Doppler box input. The program used for this purpose was a VI (Virtual Instrument) that is similar to an oscilloscope. The program has an oscilloscope-like display for the Doppler signal. The program also allows for control of the sampling rate and buffer size. The number of input channels that are displayed can also be controlled. In addition, the program displays the number of the channel providing the highest amplitude signal and the actual amplitude of that signal. The signal displayed on the monitor was the Doppler velocity versus time.

The amplitude on the Y- axis represented the strength of the signal. A proper sampling frequency was decided by using the Nyquist Criteria, a theory which states that the sampling frequency should be twice the highest known analog signal value. Because the maximum frequency humans can hear is about 20 KHz, the necessary sampling rate for this program was set to 40 KHz. Also the buffer size needed to be very large because of the large sampling rate. Mr. Ames Brown helped develop this program that is able to receive up to ten Doppler probe inputs and display the signal with the highest amplitude.

The program was tested using two Doppler boxes with the pen style crystals. Doppler

Gel was applied to the skin to better transmit the signal from the surface into the probe.

When this was done to the wrist or neck around the area of a pulse, the LabView program displayed the proper signal. The test confirmed that the program worked successfully. IV Results:

Our ideal solution to the problem of obtaining an accurate and high amplitude

Doppler signal for vascular disease diagnosis in a cheap and fast manner contains several parts. These parts include an array of Doppler crystals that can be placed near the artery of interest and a computer program to select the signal with the highest amplitude. We were largely unable to complete the first part of this design. However, we identified a type of crystal, namely the suture down crystal from Iowa Doppler Products that is suitable for placement in an Ace bandage-type apparatus for attachment around the ankle to measure Doppler velocity waveforms in the dorsal pedis and posterior tibial arteries.

From this crystal, we were able to obtain a weak signal using a high pass filter to remove

60Hz noise and an amplifier. This signal proved to be too weak and too erratic for use.

The signal was too weak in that a change in flow frequency could be detected on the display of the audio Doppler box, but no signal could be seen on an oscilloscope or in

LabView. Part of the problem came from our inability to remove all the noise, resulting in a masking of the signal by the abundant noise. Also, the signal was erratic in that sometimes a signal on the audio Doppler box could easily be detected and other times no signal was detectable. We are unsure why this may have occurred but believe that our circuit containing a filter and amplifier may have saturated after a certain amount of time after the device was turned on, resulting in loss of signal.

We were able to complete the second part of the design. We developed a working computer program that can input signals from the output of audio Doppler boxes and select the signal with the highest amplitude. The experimental setup included: a Doppler pencil probe connected to an audio Doppler box; the audio output of the Doppler box connected to an analog-to-digital converter by a stereo wire; and, the analog-to-digital converter connected to a computer running LabView 6.1 (Figure 7). The setup was made

in duplicate so that two

Doppler probes could

be used. Due to

equipment constraints,

the program was tested

with only two input

signals, but it was able

to select the signal with

higher amplitude. The

program was tested Figure 8: Sample display from LabView program using Doppler pencil probes in two locations: the throat and the wrist. In both cases, one probe was placed directly on the artery and the other probe was placed a short distance (approximately

1cm) away. The program was able to display the signal from each probe (Figure 8). The signal consisted of a period of very low amplitude (approximately 0.05V) noise with the appearance of the Doppler waveform signal at each heart beat. Each heart beat produced a Doppler waveform with amplitude greater than 1V. The amplitude of the signal, however, varied depending on the placement of the probe. In all cases the signal from the probe directly over the artery had higher amplitude, and the LabView program was able to choose correctly the probe providing the stronger signal. The program provides a visual display of each signal and a real-time readout of the signal with the highest amplitude. Therefore, the user of the program has two means of determining the best signal. All tests were performed using a sampling rate of 40kHz and a buffer size of

10000. Since Doppler provides an audible output and the maximum frequency heard by humans is about 20kHz, sampling was performed at 40kHz in accordance with the

Nyquist sampling criteria. The buffer size of 10000 was determined by trial-and-error. It provided the best signal quality and allowed for an adequate sampling time before computer memory ran out. It is important to note that the program has the capacity to display and select from up to ten input signals. Although we did not test the program using ten signals, we have no reason to believe it should function any differently than it does with two signals.

Safety is not an important issue with our design. No part provides a significant hazard to either the user (e.g. nurse) or the patient. The Doppler probes can in no way injure the patient, especially since the small voltages applied to the transmitting piezoelectric crystal is entirely contained within a casing. The audio Doppler box and computer program do not come in contact with the patient or the user. Also, they do not control any functioning parts, so they are not a hazard to either the user or the patient.

One consideration, however, is the placement of the computer monitor, keyboard, and audio Doppler boxes. The audio Doppler boxes should be placed out of the way of the user and patient. The user should be able to position the patient, computer monitor, and keyboard in such a way that he or she can control the program while watching both the patient and the signals displayed on the computer monitor. This problem is an ergonomic issue that can easily be solved by placing the computer on a rolling cart that can be moved about the room in which the patient is being tested. One final safety concern is possible allergic reaction to the gel used to increase the transmittance of sound waves through the skin. Some patients might develop a mild rash after use of this gel. This risk is no different than that of the method currently used. The gel currently used is hypoallergenic, which limits any possible skin reactions.

This device has the potential to have a very large market. Almost every hospital in the country with a vascular division uses Doppler technology to diagnose peripheral vascular disease because it is the easiest and cheapest method. Our design improves upon the existing method and could likely replace the existing Doppler technology in hospitals.

The ability of our Part Price Quantity Total Price device to provide a Doppler Crystal $100 5 $500 Audio Doppler Box $3,000 5 $15,000 better signal and more Analog-Digital Converter $500 1 $500 Computer $1,000 1 $1,000 accurate Table 1: Sample budget for continuous wave Doppler device LabView $1,000 1 $1,000 measurements would help in vascular disease diagnosis. Therefore, the potential market for our design is very large. However, this potential is limited by the cost of the device.

The final device could include the following parts. Other parts, like connector wires, are a nominal cost. This total cost far exceeds that of the current method. For this reason, a hospital would be extremely unlikely to buy our device to replace the current technology.

The main area of cost is the audio Doppler boxes. The current technology, as far as we know, allows for the input of a single Doppler crystal into an audio Doppler box and the output of single signal from that box. Therefore, a separate box is required for each crystal in the array. If a box could be made or purchased that allows for the input and output of multiple signals, the cost would be cut dramatically. Also, it is possible that simpler, cheaper forms of the audio Doppler box without displays of flow frequencies could be purchased, which could help cut down on the cost. If the cost of the audio

Doppler boxes were cut dramatically, then it would be likely that hospitals would invest in our device because of the advantages that it offers. It is important to note that the development costs to date have been rather inexpensive. Existing equipment has been used in all cases, except for the purchase of two piezoelectric crystals at a price of $50 each. Other nominal costs have also arisen, such as purchase of connectors and batteries.

In terms of labor costs, three students have worked on the project for approximately 10 hours a week each for 20 weeks. If a general wage of $15/hour is chosen, then the labor cost is approximately $9000. A significant amount of development still remains to make a final working prototype that features both an array and a computer program. Other developmental costs could include FDA testing and approval once a final prototype is constructed.

One of the advantages our device offers is related to cost. The current technology requires significant training of nurses so that they can use the Doppler probes to obtain an accurate signal. Our device would require minimal training and could help cut the costs of training. Also, it would cut the cost of having specially trained nurses who only work in the vascular division of the hospital. This is an important cost benefit that can help pay for the extra cost of our device in relation to the cost of the current technology. Another advantage of our device is the low maintenance cost. Once the device is purchased, no alterations to the device and no upgrades would need to be performed. All the equipment could function without fail for years. It is possible that individual Doppler crystals may fail over time, but they can easily be replaced at a cheap cost. Therefore, our device will present a significant one-time cost to hospitals in the purchase of the device. Little money will need to be spent on maintenance of the device, and much money can be saved on training nurses. However, our device will only be able to replace the current Doppler technology if the cost of such a device can be dramatically reduced. V Conclusion:

Our project was successful in completing part of the design for the overall device.

We were able to develop the computer program that could receive multiple Doppler inputs and pick the signal with the highest amplitude. The other part of the project, namely the development of an array of Doppler crystals, was not completed. We did identify a crystal from Iowa Doppler Products (the suture down model) that could be used in an array constructed on an Ace bandage-type material. However, this crystal is not sold in pairs of piezoelectric crystals, as is needed for Doppler. We suggest further searching for the availability of a crystal simila4r to the sutra down model from Iowa Doppler that is sold in pairs for the development of the array. For the part of the design that was completed, we met all requirements necessary for the design. The program can receive up to ten input signals, and this number could even be increased if necessary. Each of the signals is clearly displayed on an oscilloscope output, which allows for visualization of the waveform. The other main criterion for our design was that the signal with the highest amplitude could be chosen. We have demonstrated that out of two input signals, the computer program is able to choose the signal with the higher amplitude. This can be done simply by looking at the oscilloscope-like display. However, the program also provides a display that clearly states which signal has the higher amplitude.

One problem has occurred in the development of the computer program portion of the design. This problem revolves around noise in the signal. Due to our available connectors, we needed to connect the audio output wire from the audio Doppler box to the analog-to-digital converter by means of alligator clips. This created a significant amount of noise in the signal. Precise positioning of the audio Doppler boxes and probes helped to eliminate some of this noise. Less noise may be present in a setting with fewer functioning electrical instruments. It is important to note that on most occasions the noise was never so great that the signal could not be detected. In the future, better connections could be made to reduce noise and filters could be designed to help eliminate noise.

Therefore, the computer program is capable of serving its function. It can select the best signal from multiple inputs, as from an array of Doppler crystals. One of the main methods of diagnosis is the determination of at what pressure, on the blood pressure cuff, the signal appears. This is determined by inflating the blood pressure cuff to a high pressure, say 200mmHg, and then slowly releasing the pressure. The pressure at which the signal is first seen is indicative of vascular health. Our program can function in this type of diagnosis. The other type of diagnosis functions around detection of the waveform and its shape. Our program can display the waveform, but on occasion the noise is too great to clearly tell whether the waveform is triphasic or biphasic or monophasic. In almost all instances it is possible to see the waveform, it is just difficult to see the exact shape of the waveform. The reduction of noise is an essential next step in refining the computer program. Therefore, the computer program functions, but needs further refinement before it can meet all the needs required of it. VI Recommendations:

The final goal is to design a device that can be placed around the patient’s ankle.

The device would inflate a cuff around the leg, while the array of crystals below the cuff would pick up the various signals, and finally the best signal from the array of crystals would be displayed on the monitor. Even though the array was not built, the work completed so far can be used to help with the analysis of vascular disease in the lower extremities. By utilizing the program, a technician can use more than one Doppler probe on the patient while having a visual picture the Doppler velocity waveform. Our group only completed one part of the project. To complete the second part of the project, there is a need for at least three Doppler boxes which are compatible with the Doppler crystals being used to create the array. Also the amount noise from the crystals to the monitor must be removed, perhaps with the use of filters. We were unsuccessful using a one pole high pass filter/amplifier and differential amplifier. The Doppler pens did help in testing how well the audio Doppler box and program worked together. In order to develop the array, pairs of crystals must be placed on an Ace bandage-like material need to be found and ordered. These crystals must be able to provide a good signal with minimal amount of noise. A significant amount of time was spent testing out audio Doppler boxes which did not produce a signal. It would be helpful to have equipment that works so more time could be spent working toward the end goal instead of spending time testing equipment and not being able to move forward. It would also be ideal to have an audio Doppler box that can receive input from more than one Doppler crystal and output more than on audio signal. Having to use more than one audio Doppler box presented a problem because it was difficult to have uniformity of signal strength from box to box. With the computer program there was so trouble with receiving a clear signal at all times. Many times there was a great deal of noise. This noise would not always occur when the Doppler pen was placed in the proper location after several seconds. This noise could be removed by filtering the signal using the LabView program. Also the strength of the output signal was not tested. It was never tested how well this program works when the input signal is weak. Not testing the signal on the lower extremities could cause problems if the program does not display a signal. This problem can easily be solved by amplifying the signal.

In conclusion, progress on this device was achieved but it is not complete.

Developing the array for measuring flow in the lower extremities can be done if the proper equipment is available. The computer program would help simplify the task of building the array making the task of testing patterns of crystal pairs easier. VII Bibliography

American Heart Association April 20, 2003

Esses, Glenn E. “Noninvasive Studies of Vascular Disease.”

Galloway, Robert. Personal Interview. February 12, 2002.

Guidant. April 19, 2003

Parks Medical Electronics.

Pugh, Barlow M. Ed. Stedman’s Medical Dictionary. 27th Edition. Lippincott Williams

& Wilkins. Philadelphia: 2000.

Thomas, Ronald E. The Analysis And Design of Linear Circuits. 3rd Edition. John Wiley

and Sons, Inc. New York. 2001. p183.

UCLA. April 21, 2003

Vanderbilt University Vascular Lab Personnel

WebMD. April 21, 2003