As the signal to noise ratio (SNR) in magnetic resonance imaging (MRI) improves with increasing static magnetic field strength, there is a strong incentive to develop technology to acquire images at higher fields. While magnet technology has made it possible to generate static magnetic fields of 10 T and more, limitations of the radio frequency (RF) hardware prevent the acquisition of high quality images over full regions of interest in clinical applications. Electromagnetic analysis and the design of high frequency RF coils for magnetic resonance are presented in this thesis to attempt to mitigate the limitations of the RF hardware. Analytical and numerical methods are used in this work to analyze the field and frequency behaviors/limits for various RF volume coils in MRI. As the coilsÂ’ size approach wavelength of operation dimensions, the performance of conventional lumped element design and traditional excitation strategies for these coils, encounter several problems. Field and frequency limits for the traditional RF volume coils are studied to obtain a better understanding of high frequency RF coil designs. Numerical modeling of the excised, fixed human head at the extreme field strength of 11 T is made, which helps to understand the underlying mechanisms for the severe distortions in the MR images in 11T MRI and confirms experimental observations. Results show that the rotating transverse magnetic field (B1) exhibits good homogeneity in air, but strong in-homogeneity with biological loads at 11T. The simulated signal intensity (SI) distribution within the human head confirms that distortions in the experimental images are mainly caused by the field/tissue interactions.. To rapidly simulate RF field behaviour in the human head for MRI applications, Dyadic GreenÂ’s function (DGF)/Method of Moments (DGF/MOM) - based solutions of the electromagnetic field (EMFs) inside a head-sized, stratified sphere are presented. Operating profiles are studied with various RF head coils loaded with various head models/phantoms. The RF field behaviour and tissue/field interactions are also obtained by case studies at different frequencies for different coils. New methodologies and innovations on high frequency RF coil design are next investigated. An inverse method with pre-emphasized B1 field is proposed herein, and the method of pre-emphasizing the target field with the consideration of the dielectric materials proved to be encouraging in preliminary designs. The results demonstrate that inverse-method designed coils with pre-emphases of target fields can help in decreasing the notorious bright regions caused by wavelength effects in the human head images at 4T. An 8-element phased array head coil operating at 4T is designed based on a combined idea from reciprocity theorem and inverse method. Using this new method, either circularly or linearly polarized head coils can be designed. The simulation results reported herein demonstrate the feasibility and flexibility of the design concept, which shows that improved B1 field homogeneity is achievable. Another new method in ameliorating image distortion at 11T (470 MHz) MR brain imaging applications uses a shielded 4-element transcieve phased array coil and involves performing 2 separate scans of the same slice with each scan using different excitations during transmission. That is, parallel transmission phase cycling. By optimizing the amplitudes and phases for each set of the scan, signal distortion that is antipodal from one another can be obtained and by combining both images together, image distortion can be alleviated several fold. The simulation results reported herein demonstrate the feasibility of the concept where transmission phase cycling of parallel imaging elements with different excitation pulse reconstruction is theoretically achievable. The strategy of improving the transmitting B1 field homogeneity through active control of source profiles is studied. This method tailors the RF amplitudes and phases applied individually to the rungs of a resonator with the use of an optimization scheme. Numerical simulations are used in an attempt to find optimal source profiles for high frequency RF coils. The simulation results demonstrate the strength of the optimal source profiles for different targets in high field RF coil or phased array designs.
| Identifer | oai:union.ndltd.org:ADTP/290221 |
| Creators | Xu, Bin |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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