Abstract #0287

3rd order shim for high resolution fMRI at 7T: It hurts more than it helps

Renzo Huber¹, Nicolas Boulant², A Tyler Morgan¹, Rüdiger Stirnberg³, Govind Bhagavatheeshwaran¹, Yuhui Chai⁴, Benedikt Poser⁵, Federico De Martino⁵, Joelle Sarlls¹, and Peter Bandettini¹

¹NIH, Bethesda, MD, United States, ²University of Paris-Saclay, CEA, CNRS, NeuroSpin, BAOBAB, Gif Sur Yvette Cedex, France, ³DZNE, Bonn, Germany, ⁴University of Illinois Urbana-Champaign, Urbana-Champaign, IL, United States, ⁵Maastricht University, Maastricht, Netherlands

Synopsis

Keywords: fMRI Acquisition, fMRI Acquisition, layer-fMRI, UHF, EPI, sub-millimeter fMRI, 7 tesla

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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