Calibration-free regional RF shims for MRS.

Abstract

Achieving a desired RF transmit field ( ) in small regions of interest is critical for single-voxel MRS at ultrahigh field. Radio-frequency (RF) shimming, using parallel transmission, requires mapping and optimization, which limits its ease of use. This work aimed to generate calibration-free RF shims for predefined target regions of interest, which can be applied to any participant, to produce a desired absolute magnitude (| |). The RF shims were found offline by joint optimization on a database comprising maps from 11 subjects, considering regions of interest in occipital cortex, hippocampus and posterior cingulate, as well as whole brain. The | | achieved was compared with 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 deposition were estimated using Duke and Ella dielectric models. There was no difference in the mean | | produced using calibration-free versus tailored RF shimming in the occipital cortex (p=.15), hippocampus (p=.5), or posterior cingulate (p=.98), although differences were observed in the RMS error | |. Spectra acquired using calibration-free shims had similar SNR and low residual water signal. Under identical power settings, specific-absorption-rate deposition was lower compared with operating in quadrature mode. For example, the total head specific absorption rate was around 35% less for the occipital cortex. This work demonstrates that static RF shims, optimized offline for small regions, avoid the need for mapping and optimization for each region of interest and participant. Furthermore, power settings may be increased when using calibration-free shims, to better take advantage of RF shimming.