Radio Frequency Detector Arrays for High-Field Magnetic Resonance Imaging

• Author / Creator

  Kordzadeh, Atefeh

• 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.

• Subjects / Keywords

  • Magnetic Resonance Imaging
  • RF coils
  • High field MRI

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3HQ3SB0W

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. 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.