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 (1st–4th+) relative to 1st–2nd 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 1st–4th+ degree with 1st–2nd 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 1st–4th+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 1st–4th+ 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 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.