Short-term gradient imperfections in high-resolution EPI lead to Fuzzy Ripple artifacts.

Purpose: High-resolution fMRI is a rapidly growing research field focused on capturing functional signal changes across cortical layers. However, the data acquisition is limited by low spatial frequency EPI artifacts; termed here as Fuzzy Ripples. These artifacts limit the practical applicability of acquisition protocols with higher spatial resolution, faster acquisition speed, and they challenge imaging in inferior regions of the brain.

Methods: We characterize Fuzzy Ripple artifacts across commonly used sequences and distinguish them from conventional EPI Nyquist ghosts and off-resonance effects. To investigate their origin, we employ dual-polarity readouts.

Results: Our findings indicate that Fuzzy Ripples are primarily caused by readout-specific imperfections in k-space trajectories, which can be exacerbated by short-term eddy current, and by inductive coupling between third-order shims and readout gradients. We also find that these artifacts can be mitigated through complex-valued averaging of dual-polarity EPI or by disconnecting the third-order shim coils.

Conclusion: The proposed mitigation strategies allow overcoming current limitations in layer-fMRI protocols: Achieving resolutions beyond 0.8 mm is feasible, and even at 3T, we achieved 0.53 mm voxel functional connectivity mapping. Sub-millimeter sampling acceleration can be increased to allow sub-second TRs and laminar whole brain protocols with up to GRAPPA 8. Sub-millimeter fMRI is achievable in lower brain areas, including the cerebellum.