Correlates of asymmetric venous drainage in resting state functional magnetic resonance imaging data.

Functional magnetic resonance imaging (fMRI) is a technique for indirectly measuring cerebral neural activity. It depends on the local cerebral vasculature and perfusion. It has thus been suggested that pathologies and normal variants of the cerebral vasculature can bias conclusions drawn from fMRI. The purpose of this study was to assess whether frequently observed asymmetries of the transverse sinuses are associated with apparent asymmetries of fMRI data timecourses. We re-analysed a publicly available resting state fMRI dataset (n = 135 healthy human subjects included in the main analysis). Voxel-mirrored homotopic connectivity (VMHC) was calculated as a measure of fMRI timecourse symmetry, reflecting the correlation of signal fluctuations in mirroring voxels in both cerebral hemispheres. Transverse sinus (TS) cross-sectional areas were measured in structural MRI data. In the main analysis, we assessed whether VMHC and TS asymmetry exhibited a negative linear association in brain areas near the transverse sinus. This was supplemented by exploratory whole brain analyses, including mapping of subthreshold effects. In the main hypothesis test, VMHC was not significantly associated with TS asymmetry in brain regions close to the transverse sinuses. In the whole brain analyses, VMHC was significantly negatively associated with TS asymmetry in more distant brain areas, and subthreshold effects resembled venous drainage patterns. The results suggest that part of the variance in resting state fMRI signal fluctuations might be explained by asymmetrical venous drainage. Thus, fMRI results might be confounded by the normal variability of the human intracranial venous system.