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

Magn Reson Med. 2016 Dec 1;:

Authors: Kim T, Lee Y, Zhao T, Hetherington HP, Pan JW

Abstract
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, 2016. © 2016 International Society for Magnetic Resonance in Medicine.

PMID: 27910126 [PubMed - as supplied by publisher]