Transient and permanent resolution of ischemic lesions on diffusion-weighted imaging after brief periods of focal ischemia in rats - Correlation withhistopathology

Background and Purpose-The early ischemic lesions demonstrated by diffusion -weighted imaging (DWI) are potentially reversible. The purposes of this st udy were to determine whether resolution of initial DWI lesions is transien t or permanent after different brief periods of focal brain ischemia and to evaluate histological outcomes. Methods-Sixteen rats were subjected to 10 minutes (n=7) or 30 minutes (n=7) of temporary middle cerebral artery occlusion or sham operation (n=2). DWI , perfusion-weighted imaging (PWI), and T-2-weighted imaging (T2WI) were pe rformed during occlusion; immediately after reperfusion; and at 0.5, 1.0, 1 .5, 12, 24, 48, and 72 hours after reperfusion. After the last MRI study, t he brains were fixed, sectioned, stained with hematoxylin and eosin, and ev aluated for neuronal necrosis. Results-No MRI or histological abnormalities were observed in the sham-oper ated rats. In both the 10-minute and 30-minute groups, the perfusion defici ts and DWI hyperintensities that occurred during occlusion disappeared shor tly after reperfusion. The DWI, PWI, and T2WI results remained normal there after in the 10-minute group, whereas secondary DWI hyperintensity and T2WI abnormalities developed at the 12-hour observation point in the 30-minute group. Histological examinations demonstrated neuronal necrosis in both gro ups, but the number of necrotic neurons was significantly higher in the 30- minute group (95+/-4%) than in the 10-minute group (17+/-10%, P<0.0001). Conclusions-Transient or permanent resolution of initial DWI lesions defend s on the duration of ischemia. Transient resolution of DWI lesions is assoc iated with widespread neuronal necrosis; moreover, permanent resolution of DWI lesions does not necessarily indicate complete salvage of brain tissue from ischemic injury.