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

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.

B0 field homogeneity recommendations, specifications, and measurement units for MRI in radiation therapy

“…MRI systems with excellent field homogeneity specifications are desirable but not sufficient if the MRI system lacks the capabilities to adequately shim the field in the presence of a human body 54 . Regions of high susceptibility like tissue/air interfaces or metal implants will produce significant field inhomogeneities and may require effective shimming including active shims with high current capacities and higher order terms at high field strengths 55 . Dynamic shimming (e.g., Siemens BioMatrix CoilShim and Slice Adjust) now permits localized shimming based on slice or voxel 56,57 .…”

“…54 Regions of high susceptibility like tissue/air interfaces or metal implants will produce significant field inhomogeneities and may require effective shimming including active shims with high current capacities and higher order terms at high field strengths. 55 Dynamic shimming (e.g., Siemens BioMatrix CoilShim and Slice Adjust) now permits localized shimming based on slice or voxel. 56,57 The aim of dynamic shimming is to address inhomogeneities over single slices or regions that cannot be adequately minimized by conventional field shimming.…”

Distortion‐free imaging: A double encoding method (DIADEM) combined with multiband imaging for rapid distortion‐free high‐resolution diffusion imaging on a compact 3T with high‐performance gradients

“…The image‐processing steps using the FSL toolkit (http://fsl.fmrib.ox.ac.uk/fsl/) proposed in a previous study was followed in this study in order to investigate the frequency range from the field map measured within the brain, which included phase unwrapping, brain extraction, and alignment to the MNI (Montreal Neurological Institute) template in a standard space . After converting the phase map to the frequency map, a search was conducted for the range of frequency values for each individual and for the entire group within the masked brain volume.…”

Influence of respiration‐induced B0 variations in real‐time phase‐contrast echo planar imaging of the cervical cerebrospinal fluid

“…Methods to correct for dynamic respiration-induced B 0 variations include real-time shimming approaches [20][21][22] or dynamic acquired and extrapolated B 0 maps. [23][24][25] To date, these approaches have been solely applied for spatial distortion correction.…”