B0 field mapping inaccuracies at 7T and their effect on shim estimation and EPI distortion

Motivation: Inconsistencies in B0-maps may lead to bias in shimming and acquisition, yet their extent is unclear. Goal(s): Determine the repeatability of standard dual-echo B0-mapping sequences and the impact of bias fields at 7T. Approach: 2D and 3D B0-maps were acquired in vivo and the bias field introduced by frequency-encoding, phase-encoding and slice-select gradients were measured using gradient flipping. The impact on B0 shimming was calculated and demonstrated for EPI. Results: 2D was more robust than 3D B0-mapping. Averaging gradient flipped acquisitions created an 'unbiased' B0-map, removing eddy current fields which were shown to generate compensatory shim terms that led to distortions in EPI. Impact: Inconsistencies in B0-maps lead to poor shimming at 7T, increased linewidths in MRS and distortion artefacts in MRI. This work recommends use of a 2D B0-mapping protocol along with the acquisition of an 'unbiased' B0-map for image-based shimming.