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Calibration-free regional RF shims for MRS

Berrington, Adam; Pova�an, Michal; Mirfin, Christopher; Bawden, Stephen; Park, Young Woo; Marsh, Daniel C.; Bowtell, Richard; Gowland, Penny A.

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Authors

Michal Pova�an

Christopher Mirfin

Young Woo Park

Daniel C. Marsh



Abstract

Purpose: Achieving a desired RF transmit field (B1+) in small regions-of-interest (ROIs) is critical for single-voxel MR spectroscopy at ultra-high field. RF shimming, using parallel transmission, requires B1+ mapping and optimisation, which limits its ease-of-use. This work aimed to generate calibration-free RF shims for pre-defined target ROIs, which can be applied to any participant, to produce a desired absolute magnitude B1+ (|B1+|).
Methods: RF shims were found offline by joint-optimisation on a database, comprising B1+ maps from 11 subjects, considering ROIs in occipital cortex (OCC), hippocampus (Hippo.) and posterior-cingulate (PCC), as well as whole brain. The |B1+| achieved was compared to a tailored shimming approach, and MR spectra were acquired using tailored and calibration-free shims in 4 participants. Global and local 10g specific absorption rate (SAR) deposition were estimated using Duke and Ella dielectric models.
Results: There was no difference in the mean |B1+| produced using calibration-free vs. tailored RF shimming in OCC (p=0.15), Hippo. (p=0.5) or PCC (p=0.98) although differences were observed in the root-mean-square error (RMSE) |B1+|. Spectra acquired using calibration-free shims had similar SNR and low residual water signal. Under identical power settings, SAR deposition was lower compared to operating in quadrature mode. For example, total head SAR was around 35% less for OCC.
Conclusion: This work demonstrates that static RF shims, optimised offline for small regions, avoid the need for B1+ mapping and optimisation for each ROI and participant. Furthermore, power settings may be increased when using calibration-free shims to better take advantage of RF shimming.

Citation

Berrington, A., Považan, M., Mirfin, C., Bawden, S., Park, Y. W., Marsh, D. C., …Gowland, P. A. (2021). Calibration-free regional RF shims for MRS. Magnetic Resonance in Medicine, 86(2), 611-624. https://doi.org/10.1002/mrm.28749

Journal Article Type Article
Acceptance Date Feb 4, 2021
Online Publication Date Mar 21, 2021
Publication Date 2021-08
Deposit Date Feb 12, 2021
Publicly Available Date May 19, 2021
Journal Magnetic Resonance in Medicine
Print ISSN 0740-3194
Electronic ISSN 1522-2594
Publisher Wiley
Peer Reviewed Peer Reviewed
Volume 86
Issue 2
Pages 611-624
DOI https://doi.org/10.1002/mrm.28749
Keywords MR spectroscopy; parallel transmission; RF shimming; universal; 7 T; B1+
Public URL https://nottingham-repository.worktribe.com/output/5318372
Publisher URL https://onlinelibrary.wiley.com/doi/10.1002/mrm.28749

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