3 years ago

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

Tiejun Zhao, Yoojin Lee, Tae Kim, Jullie W. Pan, Hoby P. Hetherington
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 International Society for Magnetic Resonance in Medicine.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/mrm.26563

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