A comparative FLASH and EPI study of repetitive and sustained visual activation

Functional responses to either brief repetitive or sustained activation of the human visual cortex (movie presentation) were monitored using both fast low angle shot and echo planar imaging sequences. To allow for proper comparisons, native image contrasts were equally sensitized to changes in cerebral blood oxygenation with other experimental conditions matched as much as possible. Putative influences of receiver bandwidth and absolute voxel size were specifically addressed. In all cases resulting correlation maps and regional signal intensity time courses showed excellent spatial and temporal congruence, respectively. In particular, for a 6 min protocol of sustained activation, both FLASH and EPI yielded an initial signal increase (oxygenation overshoot), a subsequent signal decrease during ongoing stimulation, and a marked signal drop (oxygenation undershoot) after the end of stimulation. These findings exclude technical differences between FLASH and EPI as the source of previous contradictory observations more likely to be explained by differences in stimulus design.