BOLD BASED FUNCTIONAL MRI AT 4-TESLA INCLUDES A CAPILLARY BED CONTRIBUTION - ECHO-PLANAR IMAGING CORRELATES WITH PREVIOUS OPTICAL IMAGING USING INTRINSIC SIGNALS

We show that the EPI time course in functional MR imaging at 4 Tesla d isplays a multiphasic response in response to photic stimulation. Foca l areas of gray matter display an initial negative change in signal in tensity that reaches a maximum of 1% about 2 s after the onset of phot ic stimulation. This component then changes sign, reaching a positive maximum about 5 s after the onset of the stimulus. Other areas, includ ing those where draining veins are visible, show only a positive signa l change, reaching a maximum of about 6% after the onset of the visual stimulus. These time constants are in very good agreement with publis hed data obtained with intrinsic optical mapping techniques, where a d eoxygenation phase has been shown to occur in functionally specific co rtical columns, followed by an increase in blood volume which is more distributed in nature. Thus, we believe there is strong evidence that a sensitivity to capillary oxygenation state is present in high S/N fu nctional MR images obtained with EPI at 4 T.