Automated shimming of B0 for spectroscopic imaging

Template-based field map prediction for rapid whole brain B0 shimming

Purpose

In typical MRI protocols, time is spent acquiring a field map to calculate the shim settings for best image quality. We propose a fast template‐based field map prediction method that yields near‐optimal shims without measuring the field.

Methods

The template‐based prediction method uses prior knowledge of the B₀ distribution in the human brain, based on a large database of field maps acquired from different subjects, together with subject‐specific structural information from a quick localizer scan. The shimming performance of using the template‐based prediction is evaluated in comparison to a range of potential fast shimming methods.

Results

Static B₀ shimming based on predicted field maps performed almost as well as shimming based on individually measured field maps. In experimental evaluations at 7 T, the proposed approach yielded a residual field standard deviation in the brain of on average 59 Hz, compared with 50 Hz using measured field maps and 176 Hz using no subject‐specific shim.

Conclusions

This work demonstrates that shimming based on predicted field maps is feasible. The field map prediction accuracy could potentially be further improved by generating the template from a subset of subjects, based on parameters such as head rotation and body mass index. Magn Reson Med 80:171–180, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Effect of degenerate spherical harmonics and a method for automatic shimming of oblique slices

Several spherical harmonics degenerate on an oblique plane. Two-dimensional field mapping of an oblique slice causes ambiguity in determination of the degenerate spherical harmonics terms. Therefore, automatic shimming of oblique slices may lead to erroneous adjustment of shims corresponding to the degenerate spherical harmonics. In this paper an automatic slice shimming technique derived from the FASTMAP method (Gruetter R. Magn. Reson. Med. 1993; 29, 804-811) is proposed. This new method unequivocally determines and corrects all of the first- and the second-order shims for slices of arbitrary orientation.