Real-time observation of transient focal ischemia and hyperemia in cat brain.

Gradient-recalled echo-planar (T2*-weighted) imaging was used to noninvasively monitor regional blood oxygenation state changes in real time during transient episodes of focal ischemia in cat brain. Varying ischemic intervals (12 s to 30 min) were caused by middle cerebral artery occlusion. A rapid signal drop was noted upon occlusion, due to deoxygenation of static blood in the ischemic tissues. Upon successful reperfusion, the signal intensity recovered immediately and increased above (overshot) the baseline level before slowly returning to normal. The "overshoot" response was strongly dependent on the duration of the ischemic interval and is thought to reflect reactive hyperemia.