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Comparative analysis shows that using 3rd order shims negatively impacts fMRI quality, suggesting disconnection.
Motivation: 3rd order B0-shimming at high resolution 7T fMRI can mitigate geometric distortions. But the corresponding shim coils can induce field perturbations when mechanical resonances are excited (Boulant 2024). Goal(s): We aim to investigate the advantages and disadvantages of 3rd order shimming on high resolution fMRI across scanners and sequences. Approach: We compared EPI artifacts (Fuzzy Ripples) and geometric distortions with and without 3rd order shims in N=15 scans. Results: We find that for high-resolution fMRI protocols, it is best to conduct experiments without 3rd order shims. Shims should be physically disconnected. Impact: The results presented here inform the UHF-fMRI community about a simple trick to improve the data quality: disconnecting 3rd order shim coils.
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Huber et al. (2025) studied this question.
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