Spatial location and strength of BOLD activation in high‐spatial‐resolution fMRI of the motor cortex: a comparison of spin echo and gradient echo fMRI at 7 T

Abstract

The increased blood oxygenation level-dependent contrast-to-noise ratio at ultrahigh field (7 T) has been exploited in a comparison of the spatial location and strength of activation in high-resolution (1.5 mm isotropic) gradient echo (GE) and spin echo (SE), echo planar imaging data acquired during the execution of a simple motor task in five subjects. SE data were acquired at six echo times from 30 to 55 ms. Excellent fat suppression was achieved in the SE echo planar images using slice-selective gradient reversal. Threshold-free cluster enhancement was used to define regions of interest (ROIs) containing voxels showing significant stimulus-locked signal changes from the GE and average SE data. These were used to compare the signal changes and spatial locations of activated regions in SE and GE data. T(2) and T(2)* values were measured, with means of 48.3 ± 1.1 ms and 36.5 ± 3.4 ms in the SE ROI. In addition, we identified a dark band in SE images of the motor cortex corresponding to a region in which T(2) and T(2)* were significantly lower than in the surrounding grey matter. The fractional SE signal change in the ROI was found to vary linearly as a function of TE, with a slope that was dependent on the particular ROI assessed: the mean ΔR(2) value was found to be 0.85 ± 0.11 s(-1) for the SE ROI and -0.37 ± 0.05 s(-1) for the GE ROI. The fractional signal change relative to the shortest TE revealed that the largest signal change occurred at a TE of 45 ms outside of the dark band. At this TE, the ratio of the fractional signal change in GE and SE data was found to be 0.48 ± 0.05. Phase maps produced from high-resolution GE images spanning the right motor cortex were used to identify veins. The GE ROI was found to contain 18% more voxels overlying the venous mask than the SE ROI.