Functional changes of apparent diffusion coefficient during visual stimulation investigated by diffusion-weighted gradient-echo fMRI.

The signal source of apparent diffusion coefficient (ADC) changes induced by neural activity is not fully understood. To examine this issue, ADC-fMRI in response to a visual stimulus was obtained in isoflurane-anesthetized cats at 9.4 T. A gradient-echo technique was used for minimizing the coupling between diffusion and background field gradients, which was experimentally confirmed. In the small b-value domain (b=5 and 200 s/mm2), a functional ADC increase was detected at the middle of the visual cortex and at the cortical surface, which was caused mainly by an increase in cerebral blood volume (CBV) and inflow. With higher b-values (b=200 and 1000-1200 s/mm2), a functional ADC decrease was observed in the parenchyma and also at the cortical surface. Within the parenchyma, the ADC decrease responded faster than the BOLD signal, but was not well localized to the middle of visual cortex and almost disappeared when the intravascular signal was removed with a susceptibility contrast agent, suggesting that the decrease in ADC without contrast agent was mostly of vascular origin. At the cortical surface, an average ADC decrease of 0.5% remained after injection of the contrast agent, which may have arisen from a functional reduction of the partial volume of cerebrospinal fluid. Overall, a functional ADC change of tissue origin could not be detected under our experimental conditions.