High‐temporal‐resolution studies of the human primary visual cortex at 4 T: Teasing out the oxygenation contribution in FMRI

We examined the EPI time course in functional MR imaging at 4 T from cortical gray matter, local draining veins, and venous sinuses. We found a small temporal shift between the initial hyperoxygenation blood oxygen level‐dependent (BOLD) response in tissue and the larger draining veins. However, the fluctuations in the tissue time course showed strong similarity with the fluctuations in the local veins when shifted by the appropriate lag, suggesting that a large amount of this apparent “noise” is hemodynamic in nature and not related to motion. The tissue BOLD time course was observed to display a biphasic time dependence (first negative, then positive) in response to photic stimulation when substantial intersubject averaging was performed. Other areas, including those where draining veins are visible, showed only a monophasic (positive) signal change, reaching a maximum of about 6% after the onset of the visual stimulus. These time constants are in very good agreement with published data obtained with intrinsic optical mapping techniques, in which a deoxygenation phase has been shown to occur in functionally specific cortical columns, followed by an increase in blood volume which is more distributed in nature. We can use the time lag between oxygen demand and flow increase to estimate stimulus induced local tissue oxygen extraction increases of about 11%.