Gradient-echo EPI using a high-degree shim insert coil at 7 T: Implications for BOLD fMRI

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Neuroimaging
• Biophysics

Background:

• High-field MRI at 7 Tesla (T) offers enhanced sensitivity but faces challenges with B0 inhomogeneity.
• Gradient-echo echo-planar imaging (GE-EPI) is susceptible to B0 field distortions, impacting Blood-Oxygen-Level-Dependent (BOLD) signal.
• Optimizing B0 shimming is crucial for improving image quality and quantitative analysis in fMRI.

Purpose of the Study:

• To quantitatively compare the effects of 1st-4th+ degree B0 shimming versus 1st-2nd degree shimming.
• To assess the impact on GE-EPI and BOLD activation at 7T.
• To evaluate improvements in temporal signal-to-noise ratio (tSNR), R2*, BOLD signal change, and activated pixel counts.

Main Methods:

• Simulations and GE-EPI acquisitions were performed at 2mm³ and 3mm³ resolutions.
• B0 maps were generated for 1st-2nd degree and 1st-4th+ degree shimming.
• Analysis focused on BOLD signal change and activated pixel counts during a breath-hold task.

Main Results:

• Higher-order shimming (1st-4th+) improved B0 homogeneity in most brain regions, particularly the inferior frontal lobe.
• Positive B0 offset regions showed significant increases in BOLD sensitivity (+5-29% activated pixels).
• Negative B0 offset regions exhibited decreased BOLD sensitivity (-18% to 0% activated pixels).

Conclusions:

• 1st-4th+ degree B0 shimming effectively maintains B0 homogeneity in central brain regions at 7T.
• Advanced shimming strategies enhance BOLD signal detection and activated pixel counts in specific brain areas.
• This technique holds promise for improving quantitative fMRI analysis and neuroscientific research.