Motion and geometric distortions of EPI acquisitions remain challenging as motion during an fMRI scan affects the stability of the time series in two ways: Rigid motion displaces voxels by “moving” the spins. A change in head orientation and the complex interplay of external and internal susceptibility differences lead to a change in the off-resonance field. In combination with an EPI readout this change in off-resonance field leads to an “apparent” voxel displacement. Here, we propose the simultaneous acquisition of a blip-down navigator with the original blip-up host-EPI sequence which is used to derive a snapshot of dynamic off-resonance changes.
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