Motion-robust pulse design for parallel transmission excitation at ultra-high field MRI

Watkins, Luke 2023. Motion-robust pulse design for parallel transmission excitation at ultra-high field MRI. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Ultra-high field (UHF) MRI offers higher signal-to-noise and contrast-to-noise ratios compared to lower B0-field strengths, which can be leveraged for improved spatial or temporal resolution, but suffers from B1+-field inhomogeneity, which can lead to artificial signal and contrast variations in an image. Parallel transmission excitation (pTx) systems allow non-identical radiofrequency (RF) pulses to be simultaneously applied, reducing excitation pulse duration, and increasing global excitation homogeneity, which can reduce image artifacts related to B1+-field inhomogeneity at UHF. Within-scan patient head motion can lead to imaging artifacts such as blurring or aliasing. Variation in image contrast caused by patient motion is related to the RF excitation, therefore requires prospective motion correction to recover contrast homogeneity. The added complexity of pulse design for pTx increases computation times past what is feasible for typical prospective motion correction, therefore an alternative method is required. In this thesis, a method was demonstrated in-silico for reducing the effects of patient head motion on radiofrequency field homogeneity for pTx. This was achieved by designing motion robust pulses (MRPs), which were optimised over the centred head position, and off-centre positions during initial pulse design, using B1+-maps simulated within a generic 8-channel pTx transmit array. The MRPs required multiple input B1+-maps which would increase scan complexity. Recent literature has been published demonstrating how a deep learning neural network could be used to estimate off-centre B1+-maps from a single B1+-map, collected at the centred position. This could improve the practicality of the MRPs, and would allow any desired combination of input B1+-maps. MRPs were designed using off-centre B1+-maps estimated from the centred position, and evaluated over simulated positions. Finally, a pseudo in-vivo investigation of the MRP design was performed using ten B1+-maps collected in-vivo using an 8-32-channel pTx/Rx coil (Nova Medical, MA, USA) within a Siemens 7T Magnetom scanner (Siemens Healthcare, Erlangen, Germany).

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Physics and Astronomy
Subjects:	B Philosophy. Psychology. Religion > BF Psychology Q Science > QC Physics
Uncontrolled Keywords:	MRI, patient motion, parallel transmission excitation, parallel transmit, pTx, B1 inhomogeneity, ultra-high field, RF pulse design
Funders:	EPSRC 50%, Cardiff University, School of Psychology 25%, Cardiff University, School of Physics 25%
Date of First Compliant Deposit:	22 June 2023
Last Modified:	23 Jun 2023 08:43
URI:	https://orca.cardiff.ac.uk/id/eprint/160533

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