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RF Pulse Design for Parallel Transmission in Ultra High Field Magnetic Resonance Imaging

Zheng, Hai (2012) RF Pulse Design for Parallel Transmission in Ultra High Field Magnetic Resonance Imaging. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Magnetic Resonance Imaging (MRI) plays an important role in visualizing the structure and function of the human body. In recent years, ultra high magnetic field (UHF) MRI has emerged as an attractive means to achieve significant improvements in both signal-to-noise ratio (SNR) and contrast. However, in vivo imaging at UHF is hampered by the presence of severe B1 and B0 inhomogeneities. B1 inhomogeneity leads to spatial non-uniformity excitation in MR images. B0 inhomogeneity, on the other hand, produces blurring, distortions and signal loss at tissue/air interfaces. Both of them greatly limit the applications of UHF MRI. Thus mitigating B1 and B0 inhomogeneities is central in making UHF MRI practical for clinical use.
Tailored RF pulse design has been demonstrated as a feasible means to mitigate the effects of B1 and B0 inhomogeneities. However, the primary limitation of such tailored pulses is that the pulse duration is too long for practical clinical applications. With the introduction of parallel transmission technology, one can shorten the pulse duration without sacrificing excitation performance. Prior reports in parallel transmission were formulated using linear, small-tip-angle approximation algorithms, which are violated in the regime of nonlinear large-tip-angle excitation.
The overall goal of this dissertation is to develop effective and fast algorithms for parallel transmission UHF RF pulses design. The key contributions of this work include 1) a novel large-tip-angle RF pulse design method to achieve significant improvements compared with previous algorithms; 2) implementing a model-based eddy current correction method to compensate eddy current field induced on RF shield for parallel transmission and leading to improved excitation and time efficiency; 3) developing new RF pulse design strategy to restore the lost signal over the whole brain and increase BOLD contrast to brain activation in T2*-weighted fMRI at UHF.
For testing and validation, these algorithms were implement on a Siemens 7T MRI scanner equipped with a parallel transmission system and their capabilities for ultra high field MRI demonstrated, first by phantom experiments and later by in vivo human imaging studies. The contributions presented here will be of importance to bring parallel transmission technology to clinical applications in UHF MRI.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Zheng, Haihaz17@pitt.eduHAZ17
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairBoada,
Committee MemberNoll,
Committee MemberTamer, Ibrahimtsi2@pitt.eduTSI2
Committee MemberStetten, Georgestetten@pitt.eduSTETTEN
Date: 4 June 2012
Date Type: Publication
Defense Date: 23 January 2012
Approval Date: 4 June 2012
Submission Date: 12 March 2012
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 158
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Bioengineering
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: transmit SENSE;multidimensional pulse;high field;eddy current;susceptibility artifacts;functional MRI;signal recovery;perturbation analysis;T2* weighted;B1 mapping
Date Deposited: 04 Jun 2012 18:54
Last Modified: 15 Nov 2016 13:55


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