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Radio Frequency Detector Arrays for High-Field Magnetic Resonance Imaging Open Access


Other title
Magnetic Resonance Imaging
RF coils
high field MRI
Type of item
Degree grantor
University of Alberta
Author or creator
Kordzadeh, Atefeh
Supervisor and department
Dr. Abdulhakem Elezzabi, Electrical and Computer Engineering
Dr Nicola DeZanche, division of Medical Physics, Department of Oncology
Examining committee member and department
Dr. Atiyah Yahya, division of Medical Physics, Department of Oncology
Dr. Alan Wilman, Biomedical Engineering
Dr. Roger Zemp, Electrical and Computer Engineering
Dr. Scott King, NSERC
Department of Biomedical Engineering
Department of Electrical and Computer Engineering

Date accepted
Graduation date
2017-06:Spring 2017
Doctor of Philosophy
Degree level
In magnetic resonance imaging (MRI), radio frequency (RF) detectors or “coils” are used to excite and receive signal from the nuclear spins. This thesis focuses on the design and development of RF coils and methods to achieve the best possible image quality at high static magnetic field strengths (B0). The coils are designed to operate at 200 MHz (4.7 tesla) for the human MRI system located in the Peter S. Allen MR Research Centre at the University of Alberta. This intermediate field strength is beneficial for body imaging because it takes advantage of MRI at fields that are higher than those of standard clinical scanners (1.5 T and 3 T) while avoiding the severe challenges of imaging deep targets in the body (e.g., torso, abdomen and pelvis) at ultra-high fields (7 T and 9.4 T). The image quality benefits of high-field MRI include higher signal-to-noise ratio (SNR), while the challenges are due to the short wavelength which introduces bright and dark areas in the images (respectively, constructive and destructive interference of RF fields). Moreover, due to the RF losses inside tissues, the penetration depth of the RF fields is significantly lower at higher B0 fields. The objective of this thesis is to address and overcome these issues and technical challenges to achieve high image quality (resolution, SNR and uniformity) while keeping the RF power deposition (SAR) values within acceptable limits. High dielectric constant (HDC) pads or liners have been proposed in the literature to increase RF field homogeneity in the imaging region. The effects of HDC pads on the transmission and safety performance of an array are discussed in chapter 2, where it is found that there is an optimal value for the dielectric constant of the pads which is much lower than that currently used in the literature. Using HDC pads increases the magnetic field however it can affect SAR, matching and mutual coupling adversely. Therefore, a wise choice of dielectric constant as well as geometry of these pads is a necessity. A method (described in the Appendix) was also developed specifically to measure the dielectric constant of powders, liquids and suspensions used in HDC pads. Using transmit coil arrays provides the ability to change amplitude and phase of each element which helps to achieve more homogenous RF magnetic field and also provides more control over the SAR. However, arrays introduce the challenge of maintaining mutual coupling within acceptable levels. Coupling between elements in an array at high frequencies is quite sensitive to the presence of HDC liners. Therefore, in chapter 3, mutual impedance between elements in an eight channel array is investigated, and an appropriate method to mitigate coupling in the presence of an HDC liner is found. Results show that both real and imaginary parts of the mutual impedance can be removed using capacitive bridges with minimal degradation of the transmit performance. The final aim of this thesis is to design a body array for 4.7 T. In chapter 4 a transverse electromagnetic horn antenna is designed to image deep targets in the body. This element achieves better efficiency than dipoles that have been recently proposed as alternatives to traditional loop coils, especially for deep targets. In all chapters of this thesis the theory is described and simulations are performed to assist with design and investigate the concepts. Structures are fabricated and tested on the bench before imaging using appropriate phantoms to verify the design and simulations.
This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for the purpose of private, scholarly or scientific research. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
Citation for previous publication
Chapter 2: A. Kordzadeh, N. De Zanche, “Optimal-permittivity dielectric liners for a 4.7T MRI transceiver array”, submitted to Physics in Medicine and Biology. Chapter 3: A. Kordzadeh, N. De Zanche, "Control of Mutual Coupling in High-Field MRI Transmit Arrays in the Presence of High-Permittivity Liners," in IEEE Transactions on Microwave Theory and Techniques , doi: 10.1109/TMTT.2017.2668406 Appendix A: A. Kordzadeh, N. De Zanche, "Permittivity Measurement of Liquids, Powders, and Suspensions Using a Parallel-Plate Cell", Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering, 46, 1; 19-24; 2016, DOI: 10.1002/cmr.b.21318.

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