Calibration-free regional RF shims for MRS

Area of Science:

• Magnetic Resonance Imaging
• Spectroscopy
• Radio-Frequency Engineering

Background:

• Achieving precise radio-frequency (RF) transmit fields is crucial for single-voxel magnetic resonance spectroscopy (MRS) at ultrahigh fields.
• Current RF shimming methods using parallel transmission necessitate individual RF field mapping and optimization, limiting practical application.

Purpose of the Study:

• To develop calibration-free RF shims for predefined regions of interest applicable to any participant.
• To generate desired absolute RF transmit field magnitudes (|B1+|) without per-participant calibration.

Main Methods:

• RF shims were optimized offline using a database of B1+ maps from 11 subjects across various regions (occipital cortex, hippocampus, posterior cingulate, whole brain).
• The performance of calibration-free shims was compared to a tailored shimming approach using MR spectra acquired from 4 participants.
• Specific absorption rate (SAR) deposition was estimated using Duke and Ella dielectric models.

Main Results:

• No significant difference was found in the mean |B1+| produced by calibration-free versus tailored RF shimming in targeted regions (occipital cortex, hippocampus, posterior cingulate).
• MR spectra acquired with calibration-free shims demonstrated comparable signal-to-noise ratio (SNR) and low residual water signals.
• SAR deposition was lower with calibration-free shims compared to quadrature mode, with approximately 35% less total head SAR for the occipital cortex.

Conclusions:

• Static, offline-optimized RF shims eliminate the need for per-participant B1+ mapping and optimization for specific regions of interest.
• Calibration-free RF shimming allows for potentially increased power settings, enhancing the utility of RF shimming techniques.
• This approach simplifies and improves the efficiency of ultrahigh-field MRS procedures.