Screen Printed MRI Receive Coils Joseph Corea Electrical Engineering and Computer Sciences University of California at Berkeley Technical Report No. UCB/EECS-2018-147 http://www2.eecs.berkeley.edu/Pubs/TechRpts/2018/EECS-2018-147.html December 1, 2018 Copyright © 2018, by the author(s). All rights reserved. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission. Screen Printed MRI Receive Coils by Joseph Russell Corea A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Engineering – Electrical Engineering and Computer Sciences in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Ana Claudia Arias, Chair Professor Michael Lustig Professor Ali Niknejad Professor Jack Gallant Fall 2016 Screen Printed MRI Receive Coils Copyright 2016 by Joseph Russell Corea 1 Abstract Screen Printed MRI Receive Coils by Joseph Russell Corea Doctor of Philosophy in Engineering - Electrical Engineering and Computer Sciences University of California, Berkeley Professors Ana Claudia Arias and Michael Lustig The widespread use of MRI has created a demand for high quality images for current and emerging applications. High quality images are currently acquired by using application specific receive coils that fit close to the patient’s body to produce high signal to noise ratio. These coils are manufactured using traditional printed circuit board fabrication technologies that produce large heavy coils that do not suit some applications very well. In particular, two areas that have not been able to take advantage of the current receive coils are pediatric imaging and MR guided high intensity focused ultrasound therapy due to size, weight, and thickness constraints. These two areas would benefit greatly from the high quality imaging provided by purpose-built lightweight and thin receive coil arrays. One way to achieve a very lightweight and thin coil is to fabricate it from solution using printed electronics. Here for the first time, advances in solution- processed fabrication techniques have allowed lightweight, thin, and flexible receive coil arrays to be made for these applications. In this thesis the development of printed MRI receive coils is discussed, covering fabrication, characterization, and implementation. An entirely printed approach is used to create single element receive coils that are characterized and tested on 1.5 T and 3 T clinical systems. The materials used to fabricate the coil components are identified as a main avenue for improvement. A fully printed proof-of-concept array is made to demonstrate feasibility and is used to image a volunteer on a 3 T clinical system. Coils are optimized with components made from high quality flexible substrates to make better performing printed coils and arrays. These printed arrays are compared to commercially available arrays on several phantoms as well as on a volunteer on a 3 T scanner. In addition to creating coils for standard clinical imaging, several coils and arrays are optimized for use in an high intensity focused ultrasound interventional MRI. Coil materials are evaluated for acoustic transparency, stability in water, safety, and electrical quality. The optimized coils are used to evaluate image quality as well as to characterize the system level of acoustic transparency. An 8-channel coil array is used to characterize image quality on a volunteer. To show a system level proof-of-concept on an interventional MR system, optimized arrays are used to track ultrasonic heating inside phantoms and ex-vivo tissue. Overall, the characteristics of the printed coils described in this thesis enable a new generation of coils design for both traditional and emerging applications. i Contents 1. Introduction and Background ......................................................................................... 1 1.1 Introduction ........................................................................................................... 1 1.2 Thesis Outline ........................................................................................................ 2 1.3 Basics of MRI ........................................................................................................ 2 1.3.1 Signal Generation and Detection .............................................................. 2 1.3.2 Spatial Encoding ...................................................................................... 4 1.3.3 Coils in an MRI scanner ........................................................................... 5 1.4 Receive Coil .......................................................................................................... 6 1.4.1 Coil Circuit ............................................................................................... 6 1.4.2 Coil Matching ........................................................................................... 7 1.4.3 Blocking Circuit ....................................................................................... 7 1.4.4 Coil Decoupling ....................................................................................... 8 1.4.5 Coil Cabling and Baluns ........................................................................ 10 1.4.6 Body Noise Dominance ......................................................................... 11 1.4.7 Coil Efficiency ....................................................................................... 12 1.4.8 Coil Sensitivity and Positioning ............................................................. 13 1.4.9 Traditional Flexible Coils Fabrication ................................................... 17 1.4.10 Solution Processed Flexible Coils Fabrication ....................................... 19 1.5 References ........................................................................................................... 21 2. MRI Receive Coil Characterization .............................................................................. 26 2.1 Introduction ......................................................................................................... 26 2.2 Phantoms for Characterization ............................................................................ 26 2.3 Electrical Characterization .................................................................................. 27 2.3.1 Scattering and Impedance Parameters .................................................... 27 2.3.2 Quality Factor Measurement .................................................................. 29 2.3.3 Input Impedance Measurement .............................................................. 31 2.3.4 Blocking Impedance Measurement ........................................................ 32 2.3.5 Balun Impedance Measurement ............................................................. 33 2.4 Scanning .............................................................................................................. 36 2.4.1 Connecting to Coils to a Scanner ............................................................ 36 2.4.2 Scanner Setup ......................................................................................... 38 2.4.3 Image SNR .............................................................................................. 39 2.4.4 Location of preamplifiers ....................................................................... 41 2.4.5 Coil Coupling ......................................................................................... 42 2.4.6 Safety Considerations for Human Imaging ............................................ 42 2.5 References ........................................................................................................... 44 3. Fabrication of Printed MRI Receive Coils ................................................................... 47 3.1 Introduction ......................................................................................................... 47 3.2 Fabrication of Printed Coils for 1.5 T and 3 T .................................................... 50 3.2.1 Calculating Coil Size .............................................................................. 51 3.2.2 Printed Dielectric .................................................................................... 53 3.2.3 Substrate Dielectric ................................................................................ 53 3.3 Printed Capacitor Characterization ..................................................................... 53 3.4 Printed Coil Characterization .............................................................................. 55 3.4.1 Quality Factor Measurements ................................................................ 55 3.4.2 SNR at 1.5T and 3T ................................................................................ 56 ii 3.4.3 Offset Imaging ........................................................................................ 58 3.4.4 Curved Phantoms ................................................................................... 59 3.5 Volunteer Imaging with a printed array .............................................................