Abstract
PurposeIn 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.MethodsThe 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.ResultsStatic 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.ConclusionsThis 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.
Highlights
Shimming is a technique for homogenizing the static magnetic field in MRI [1,2,3] and spectroscopy [4]
In addition to 3D volume-based field mapping methods, projection-based methods [11,12,13,14] are used, which allow rapid shim determination based on the B0 field distribution information along the field map projections
The averaged fixed shim provided a residual standard deviation of 61 Hz on average, comparable to the averaged registered field map. These results suggest that an averaged registered field map offers useful information on the field distribution of the brain for second-order static shimming
Summary
Shimming is a technique for homogenizing the static magnetic field in MRI [1,2,3] and spectroscopy [4]. Balanced steady-state free-precession sequences [10] have been demonstrated to provide B0 field maps with high signal-to-noise ratio on a short time scale This performance is degraded if a wide range of frequencies is present in image voxels. In addition to 3D volume-based field mapping methods, projection-based methods [11,12,13,14] are used, which allow rapid shim determination based on the B0 field distribution information along the field map projections (e.g. the fast, automated, shimming technique by mapping along projections) This method is typically based on the assumption that the shim field can be well described by a sum of spherical harmonics, which is good for local shimming targets, such as spectroscopy. It is not applicable for some other shimming applications, such as slice-wise dynamic shimming, because of limited spatial field distribution information available for determining shim settings for thin slices
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