This thesis concentrates on the reduction of RF field (or B1) inhomogeneity in high magnetic field MRI. B1 inhomogeneity is one of the major drawbacks in high field MRI. The non-uniformity causes regions of increased and decreased signal intensity in the images. None of existing methods can perfectly correct the non uniformity. This thesis aims to develop new methods that are practical, safe, and required no additional devices.
Specifically, three-dimensional (3D) tailored RF (TRF) pulse were designed and validated in human MRI experiments at 3 Tesla. Two novel designs of 3D TRF pulses for B1 inhomogeneity reduction are presented in this thesis. Both designs are based on the small flip angle approximation. The first design is for a thick slab 3D acquisition. These pulses employ a 3D stack of spirals k-space trajectory simultaneously with B1 inhomogeneity compensated RF pulse waveforms during excitation. Another pulse design uses analytical functions as a compensated B1 inhomogeneity pulse weighting function. The k-space is modeled in the manner such that kx-ky provides compensated spatial weighting function for quadratically varying B1 inhomogeneity patterns. The kz-direction is controlled by fast switching gradients in the fashion similar to Echo planar imaging (EPI). This design is more appropriate for 2D high resolution acquisition images.
The two pulse designs show equal improvement of signal loss of approximately 30%. Long pulse length, 22 ms, and limited peak B1 are the major concern of the first design. The second design, the compensated fast kz pulses provide relatively shot pulse length only 3-5 ms. The primary limitation of this design is that it can be used for only a quadratic pattern of B1 inhomogeneity and may cause resonance shift.
| Identifer | oai:union.ndltd.org:PITT/oai:PITTETD:etd-12032004-102907 |
| Date | 31 January 2005 |
| Creators | Saekho, Suwit |
| Contributors | V Andrew Stenger, Fernado E. Boada, George Stetten, Qing X Yang |
| Publisher | University of Pittsburgh |
| Source Sets | University of Pittsburgh |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.library.pitt.edu/ETD/available/etd-12032004-102907/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University of Pittsburgh or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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