Calibration-free regional RF shims for MR spectroscopy

Purpose: Sufficient 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. Methods: RF 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. Results: Calibration-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. Conclusion: This 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.

Robust 3D Bloch-Siegert based B1+ mapping using Multi-Echo General Linear Modelling

PurposeQuantitative MRI applications, such as mapping the T1 time of tissue, puts high demands on the accuracy and precision of transmit field (B1+) estimation. A candidate approach to satisfy these requirements exploits the difference in phase induced by the Bloch-Siegert Shift (BSS) of two acquisitions with opposite off-resonance frequency RF pulses. Interleaving these RF pulses ensures robustness to motion and scanner drifts, however, here we demonstrate that doing so also introduces a bias in the B1+ estimates.MethodsWe show via simulation and experiments that the amplitude of the error depends on MR pulse sequence parameters, such as TR and RF spoiling increment, but more problematically, on the intrinsic properties, T1 and T2, of the investigated tissue. To solve these problems, we present a new approach to BSS-based B1+ estimation that uses a multi-echo acquisition and a general linear model (GLM) to estimate the correct BSS-induced phase.ResultsIn line with simulations, phantom and in-vivo experiments confirmed that the GLM-based method removed the dependency on tissue properties and pulse sequence settings. It also showed greater robustness to hardware imperfections.ConclusionThe GLM-based method is recommended as a more accurate approach to BSS-based B1+ mapping.