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

Laurentius Renzo Huber¹, Rüdiger Stirnberg², A Tyler Morgan¹

¹NIMH, NIH, Bethesda, Maryland, USA.

Magnetic Resonance in Medicine

|April 2, 2025

Summary

This summary is machine-generated.

Researchers identified and mitigated "Fuzzy Ripples," artifacts in high-resolution functional MRI (fMRI) caused by k-space trajectory imperfections. These solutions enable sub-millimeter laminar fMRI, even in challenging brain regions.

Keywords:

7 T acquisition Fuzzy Ripples layer‐fMRI ventral brain

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Area of Science:

Neuroimaging
Magnetic Resonance Imaging (MRI)
Functional MRI (fMRI)

Background:

High-resolution fMRI aims to capture functional signal changes across cortical layers.
Low spatial frequency EPI artifacts, termed Fuzzy Ripples, limit current fMRI acquisition protocols.
These artifacts hinder higher spatial resolution, faster acquisition speeds, and imaging of inferior brain regions.

Purpose of the Study:

To characterize Fuzzy Ripple artifacts in high-resolution fMRI.
To distinguish Fuzzy Ripples from conventional EPI Nyquist ghosts and off-resonance effects.
To investigate the origin of Fuzzy Ripples and develop mitigation strategies.

Main Methods:

Characterized Fuzzy Ripple artifacts across common fMRI sequences.
Employed dual-polarity readouts to investigate artifact origins.
Distinguished Fuzzy Ripples from other EPI artifacts like Nyquist ghosts and off-resonance effects.

Main Results:

Fuzzy Ripples originate from readout imperfections in k-space trajectories.
Artifacts are exacerbated by short-term eddy currents and inductive coupling with third-order shims.
Mitigation achieved through complex-valued averaging of dual-polarity EPI or disconnecting third-order shim coils.

Conclusions:

Proposed mitigation strategies overcome limitations in laminar fMRI protocols.
Sub-millimeter resolution (0.53 mm) fMRI is feasible, enabling detailed functional connectivity mapping.
Sub-millimeter fMRI is now achievable in inferior brain areas, including the cerebellum, with accelerated acquisition.