Calibration-free regional RF shims for MR spectroscopy

PurposeSufficient control of the RF transmit field (B1+) in small regions-of-interest (ROIs) is critical for single voxel MR spectroscopy at ultra-high field. Static RF shimming, using parallel transmit (pTx), can improve B1+, but must be calibrated for each participant and ROI, which limits its applicability. Additionally, specific-absorption-rate (SAR) becomes hard to predict. This work aimed to find RF shims, which can be applied to any participant, to produce the desired |B1+| within pre-defined target ROIs. MethodsRF shims were found offline by joint-optimisation on a database, comprising B1+ maps from 11 subjects, considering ROIs in occipital cortex, hippocampus and posterior-cingulate, as well as the whole brain. The B1+ magnitude achieved using calibration-free RF shims was compared to a tailored shimming approach, and MR spectra were acquired using tailored and calibration-free RF shimming in 4 participants. Global and local 10g SAR deposition were modelled. ResultsCalibration-free RF shims resulted in similar |B1+| in small ROIs compared to tailored shimming, in addition to producing spectra of excellent quality and equivalent SNR. Only a small database size was required. SAR deposition was reduced compared to operating in quadrature mode for all ROIs. ConclusionThis work demonstrates that static RF shims, optimised offline for small regions in single voxel MRS, avoid the need for lengthy B1+ mapping and pTx optimisation for each ROI and participant. Furthermore, power settings may be increased when using calibration-free shims to better take advantage of the flexibility provided by RF shimming for regional acquisition at ultra-high field.