Objective: Ischemic lesions seen on diffusion-weighted imaging (DWI) are reversible if reperfusion is performed within minutes after the onset of ischemia. This study was designed to determine whether acute reversibility of DWI abnormalities is transient following brief temporary focal brain ischemia and to characterize the temporal evolution of in vivo ischemic lesions.

Methods: Eight rats were subjected to 30 minutes of temporary middle cerebral artery occlusion and underwent diffusion-, perfusion-, and T2-weighted MRI during occlusion; immediately after reperfusion; 30, 60, and 90 minutes after reperfusion; and 12, 24, 48, and 72 hours after reperfusion. Average apparent diffusion coefficient (ADC_av) values and the cerebral blood flow index (CBF_i) ratio were calculated in both the lateral caudoputamen and overlying cortex at each time point. The size of the in vivo ischemic abnormalities was calculated from the ADC_av and the T2 maps. Postmortem triphenyltetrazolium chloride (TTC) staining was used to verify ischemic injury.

Results: Both the CBF_i ratio and ADC_av values declined significantly in the two regions during occlusion. The CBF_i ratio recovered immediately after reperfusion and remained unchanged over 72 hours. However, ADC_av values returned to normal at 60 to 90 minutes and secondarily decreased at 12 hours after reperfusion as compared with those in the contralateral hemisphere. The extent of the in vivo ischemic lesions maximized at 48 hours and was highly correlated with TTC-derived lesion size.

Conclusions: Acute recovery of initial ADC_av-defined lesions after reperfusion is transient, and secondary ADC_av-defined lesions develop in a slow and delayed fashion.