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.

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