Small voxel sizes reduce extravascular dephasing from large veins in gradient-echo BOLD fMRI at 7T: a simulation study

Motivation: Recent ultra-high-resolution gradient-echo BOLD-fMRI data demonstrate an unexpected reduction in sensitivity to large veins at 7T, suggesting that small voxels may experience less signal loss due to extravascular dephasing. Goal(s): To test whether a reduction in through-plane or in-plane dephasing with small voxels may measurably contribute to reduced large-vein influences in gradient-echo BOLD. Approach: We apply biophysical simulations of simplified vascular architecture, combined with realistic image encoding gradients. Results: While voxels adjacent to veins are still strongly influenced, smaller voxel sizes have reduced extravascular BOLD, which impacts cerebral cortical-depth profiles of activation. The direction of the imaging encoding gradients also affects these profiles. Impact: Rencelty available ultra-high-resolution fMRI improves spatial accuracy, and may also provide an unexpected improvement in neuronal specificity. We demonstrate that small-voxel gradient-echo BOLD achieves reduced unwanted contamination from large veins, potentially providing an fMRI method with high sensitivity AND specificity.