Computational approaches to parallel transmission MRI

Mirfin, Christopher (2020) Computational approaches to parallel transmission MRI. PhD thesis, University of Nottingham.

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This thesis is concerned with the design of spatially-selective RF pulses accelerated by parallel transmission. These pulses perturb the magnetisation in an object within a 2D or 3D sub-volume, whilst leaving magnetisation outside of these regions unaffected. As such, they are shown to be useful in certain dynamic field-shimming strategies.

The first chapter reviews the equation of motion for a magnetisation vector and alternative compact formulations; electromagnetic fields in tissue at 7 T; requisite imaging sequences; and some numerical and computational methods.

The second chapter reviews some existing design strategies and capabilities of parallel transmission (PTx). It also includes a derivation of the adiabatic condition, which is critical for the design of frequency-modulated pulses.

In Chapter 3 the design of short PTx pulses with optimised k-space trajectories to excite an arbitrary 3d volume is presented. As part of this, the parameterisation of the excitation k-space trajectory is considered under alternative linear transformations subject to realistic gradient performance. A GPU is used to reduce computational time of an existing local and a novel second global optimisation algorithm.

In Chapter 4 a novel acquisition strategy is developed and implemented to improve the overall static field homogeneity by utilising reduced field of excitation with acceleration via parallel transmission in conjunction with dynamic first-order local shimming.

In the preceding chapters the use and design of spatially-selective RF pulses was strictly limited to amplitude-modulated waveforms. In Chapter 5 a spatially-selective FM pulse that is resilient to a level of transmit-field inhomogeneity is implemented. An optimisation scheme is sought that exploits the adiabatic condition to relax the spatial transmit-field magnitude and phase patterns.

Chapter 6 discusses the work presented in this thesis, and highlights possible directions for future projects.

An Appendix is appended to this thesis, highlighting a software application that was developed to ameliorate some of the experimental workflow challenges associated with pTx.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Bowtell, Richard
Glover, Paul
Keywords: MRI, Parallel transmission
Subjects: Q Science > QC Physics > QC501 Electricity and magnetism
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 60871
Depositing User: Mirfin, Christopher
Date Deposited: 18 Aug 2020 13:54
Last Modified: 18 Aug 2020 14:00

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