Parallel transmit pulse sequence design for ultra-high field (7T) MRI
Ultra-high field (UHF, B0 ≥ 7 T) magnetic resonance imaging (MRI) scanners allow more sensitive, faster and higher resolution imaging than clinical MRI systems at 1.5 T and 3 T. However, radiofrequency field (B+
- inhomogeneities can cause signal dropouts in UHF MRI. The most flexible solution to improve image homogeneity is parallel transmission (pTx). A pTx-enabled scanner has multiple independently-driven transmit channels, providing additional degrees of freedom for improved spatial and temporal control of the B+1 fields.
This thesis aims to extend the methodology and applications of pTx for neuroimaging at 7 T, specifically on the first clinically-approved model of UHF whole-body 7 T MRI scanner (MAGNETOM Terra).
First, this thesis sets out to understand the pTx framework on the new scanner (Chapter 4) and compares the performance of the vendor-provided single transmission head coil against its pTx counterpart (Chapter 5). Image quality is comparable for both coils, with the pTx coil showing significant increases in B+1 values (between 4.9 – 18.9%) in the caudate, insula, putamen, and temporal lobes. This translates into better spinal cord visualisation in MP2RAGE images and reductions in signal dropouts in the temporal lobes for diffusion tensor imaging. Overall, running standard neuroimaging protocols on the pTx coil is reasonable, which then gives opportunity to add dynamic pTx sequences with minimal disruption.
Additionally, pTx spokes pulses are incorporated into a gradient-echo echo-planar imaging (GRE-EPI) sequence (Chapter 6). A task-based functional-MRI (fMRI) experiment shows that data quality (expressed by temporal SNR) improves by up to 11% in regions across the whole brain for pTx-EPI in healthy volunteers. The data quality improvement leads to higher median z-scores in two functional paradigms. This sequence has been used for applied fMRI studies at the Wolfson Brain Imaging Centre.
Finally, pTx is used to improve 7 T diffusion imaging (Chapter 7). Several aspects are explored: pTx pulses to improve image homogeneity; readout-segmented EPI to reduce T*2 decay and blurring; and various lipid suppression modules are tested to minimise artifacts. Diffusion images with spokes pulses are acquired for single-shot and readout-segmented sequences at 1.5 mm3 isotropic and 0.8 × 0.8 × 3 mm3 resolution, respectively. Furthermore, multi-band 2 readout-segment diffusion images are acquired with B1-shimmed pulses.
Overall, this work shows the improvements from pTx in two crucial neuroimaging sequences. There is growing consensus that pTx will become a core technology in UHF imaging, and hopefully, the methods proposed here will help pTx to transit into mainstream neuroimaging.