Understanding signal specificity in fMRI: bSSFP vs. GRE-EPI signal dependence on cortical orientation to B0 at 9.4 Tesla

Motivation: GRE-EPI, the most widely used sequence for BOLD fMRI, is highly biased towards large draining veins that follow the cortical curvature and influence the surrounding magnetic field in an orientation-dependent manner increasing with field strength. Goal(s): This work aims to investigate large vein biases resulting in cortical orientation-dependent signal variations in GRE-EPI and bSSFP resting-state fMRI signals. Approach: We compared 2D and 3D GRE-EPI with 3D bSSFP rs-fMRI signal fluctuations in their dependence on the cortical orientation to B0 in five subjects at 9.4 Tesla. Results: Unlike GRE-EPI, intra- and inter-subject comparisons revealed no dependence of bSSFP on the cortical orientation to B0. Impact: Fluctuations in the GRE-EPI signal are highly dependent on the cortical orientation and depth. This was not observed with bSSFP, demonstrating the potentially higher specificity of bSSFP for smaller veins, closer to brain activation at field strengths ≥ 7 Tesla.