Fuzzy ripple artifact in high resolution fMRI: identification, cause, and mitigation

Area of Science:

• Neuroimaging
• Magnetic Resonance Imaging (MRI)

Background:

• High-resolution functional MRI (fMRI) aims to map brain activity across cortical layers.
• Existing protocols are limited by low spatial frequency artifacts known as Fuzzy Ripples.
• These artifacts hinder higher resolution, faster acquisition, and imaging of lower brain structures.

Purpose of the Study:

• Characterize Fuzzy Ripple artifacts in common fMRI sequences.
• Differentiate Fuzzy Ripples from other EPI artifacts like Nyquist ghosts and GRAPPA.
• Investigate the origin of Fuzzy Ripples using dual polarity readouts.

Main Methods:

• Characterization of Fuzzy Ripples across standard fMRI sequences.
• Distinguishing Fuzzy Ripples from Nyquist ghosts, off-resonance effects, and GRAPPA artifacts.
• Utilizing dual polarity readouts to investigate artifact origins.

Main Results:

• Fuzzy Ripples stem from readout imperfections in k-space trajectories.
• Artifacts are amplified by inductive coupling between third-order shims and readout gradients.
• Mitigation is possible via complex-valued averaging of dual polarity EPI or by disconnecting third-order shims.

Conclusions:

• Developed strategies overcome limitations in layer-fMRI.
• Achieved resolutions beyond 0.8mm, including 0.53mm voxel mapping at 3T.
• Enabled sub-millimeter accelerated fMRI with sub-second TRs and imaging in lower brain areas like the cerebellum.