Motivation: Eddy currents and field oscillations due to mechanical resonances are known to adversely impact image quality at ultra-high field. These have already been shown to distort applied field gradients. Goal(s): To investigate spatially-constant zero-order field oscillations and their impact on phase-based measurements including B0 mapping. Approach: Measure phase oscillations on two different 7T MR systems and perform B0 mapping using different gradient orientations and echo spacings to assess bias. Results: Similar oscillatory phase behaviour was observed at both 7T sites. Biases in B0 mapping are introduced that depend on sequence parameter choice, most notably readout direction. Impact: Phase oscillations relating to a mechanical resonance lead to variable phase measured in gradient echo scans consistently at two sites. This leads to biases in B0 mapping that depend on the choice of echo spacing and readout axis.
Leitão et al. (Tue,) studied this question.