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

Purpose: To quantitatively assess the effects of high degree and order (1^st -4^th+ ) relative to 1^st -2^nd degree B₀ shimming at 7 Tesla (T) on gradient-echo echo planar imaging (GE-EPI) and blood-oxygen-level dependent (BOLD) activation.

Methods: Simulations and GE-EPI were performed at (2mm)³ and (3mm)³ resolution, evaluating the temporal signal-to-noise ratio (tSNR), transverse relaxivity ( R2*), BOLD % signal change and activated pixel counts in a breath-hold task.

Results: Comparing the 1^st -4^th+ degree with 1^st -2^nd degree shimmed B₀ maps generated spatially varying regions of Δ|B0|=|B01-2|-|B01-4+|. As binned in 10-Hz intervals, the two center Δ|B₀ | (±10 Hz) bins maintained the B₀ offset of 48.6% of gray-matter pixels. In the positive Δ|B₀ | bins greater than 10 Hz, the 1^st -4^th+ degree shimming improved the B₀ offset in 41.1%; in negative Δ|B₀ | bins less than -10 Hz, the offset worsened in 10.2% of the pixels. In the positive Δ|B₀ | bins, we found variable but significant increases in BOLD sensitivity; the negative Δ|B₀ | bins showed significant decreases. In the breath-hold studies, positive bins showed significantly increased activated pixel numbers (+5-29%), whereas negative bins showed -18 to 0% decline.

Conclusion: 1^st -4^th+ degree shimming maintained B₀ homogeneity over central brain regions while improving most of the other regions, including the inferior frontal lobe. Magn Reson Med 78:1734-1745, 2017. © 2016 International Society for Magnetic Resonance in Medicine.