Effect of the reperfusion after cerebral ischemia in neonatal rats using MRI monitoring.

Cerebral hypoxia-ischemia is an important cause of brain injury in the newborn infant. Our purpose was to study magnetic resonance (MR) imaging changes in P7 rat brains submitted to permanent or reversible ischemia. Ischemia was induced by permanent electro-cauterization of the middle cerebral artery combined with a permanent or a transient (50 min) common carotid artery occlusion. The early events during ischemia and reperfusion were investigated by T2-weighted images (T2WI) at 1 and 3 h and by serial diffusion-weighted images (DWI) during 3 h in a 7 T magnet with a standard weighted diffusion sequence (b=1282.04 s mm(-2)) and a SEMS sequence. Within the first hour after MCA occlusion, the T2WI areas of contrast enhancement increased to a mean volume of 12.9+/-6.4%, a steady state still detected at 3 h after the ischemic onset (10.5+/-2.5%). Contrast enhancement in DWI increased as soon as 15 min of ischemia in all animals up to 50 min after CCA occlusion. In permanent ischemia, DWI abnormalities volume then increased more slowly from 50 min to 3 h after CCA occlusion (+25%, n=5). In reversible ischemia, the DWI abnormalities volume either moderately decreased and reached a plateau (-8.4%, n=4) or dramatically decreased (-53.0%, n=3). Both T2WI and DWI evidenced a "patchy" pattern of recovery as also shown on cresyl violet-stained sections. In contrast to the adult, early ischemic injury in P7 rat brains is detected as an increase in hyper-intensities both in T2WI and DWI. Our data indicate that reperfusion is able to block edema evolution after neonatal stroke and that early T2WI and more accurately DWI allow to distinguish between different patterns of injury in reversible ischemia.