Understanding and overcoming head motion in ultra-high field Magnetic Resonance Imaging with parallel radio-frequency transmission

Plumley, Alix 2022. Understanding and overcoming head motion in ultra-high field Magnetic Resonance Imaging with parallel radio-frequency transmission. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Ultra-high field (UHF) magnetic resonance imaging (MRI) offers more signal-to-noise compared to most clinical systems, but clinical uptake of UHF MRI remains low, partly due to artificial signal contrast variations and a higher risk of undesirable tissue heating at UHF. Parallel RF transmission (pTx) is capable of overcoming both issues, but the implications of patient motion on signal and safety (i.e., specific absorption rate; SAR) when pTx is used are currently not well understood. The work in this thesis aims to better characterise these effects, and presents novel approaches to help overcome them. The study chapters present investigations into firstly, the effects of motion on signal quality when different RF pulse types are used (Chapter 5); secondly, the effects of pTx coil dimensions on motion-related SAR changes (Chapter 6); and thirdly, the inter-subject variability of motion-related SAR changes in pTx (Chapter 7). Following this, two methods are outlined which aim to reduce the sensitivity of signal to head motion in pTx. These comprise firstly, a method which uses composite B1+ maps for pTx pulse design (Chapter 8), and secondly, a deep learning framework which can estimate B1+ maps following head motion (Chapter 9). Finally, the generalisability of the latter approach across coil models is explored (Chapter 10). Simulated data were used for all of the work presented. Electromagnetic field distributions were generated using Sim4Life (Zurich MedTech, Zurich, Switzerland). RF pulse design and evaluation was conducted in MATLAB (The MathWorks Inc., Natick, MA). Findings indicate that systematic differences in the signal and SAR behaviour arise when different RF pulse types and different pTx coils are used, respectively. On the other hand, differences were observed in the SAR sensitivity to motion across different virtual body models, but these were not clearly systematic. These findings indicate that new approaches are needed in order to guarantee good image quality and safety for pTx in cases of subject motion. The two proposed methods reduced the impact of motion on simulated signal profiles.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Cardiff University Brain Research Imaging Centre (CUBRIC) Psychology
Date of First Compliant Deposit:	1 June 2023
Last Modified:	01 Jun 2023 09:35
URI:	https://orca.cardiff.ac.uk/id/eprint/160085

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