Development of acute edema following cerebral hypoxia-ischemia in neonatal compared with juvenile rats using magnetic resonance imaging.

We hypothesized that the evolution of cerebral edema accompanying cerebral hypoxia-ischemia is dependent on age and that such differences would be detectable using magnetic resonance imaging methods. Thus we examined in immature and juvenile rats the relationship between hypoxic-ischemic changes in T1 and T2 and the alterations in brain water content, as assessed by differences in tissue wet-dry weights. One- and 4-wk-old rats were anesthetized and subjected to unilateral carotid artery occlusion and subsequent exposure to hypoxia (8% oxygen). T1 and T2 maps were acquired at 9.4 T, and then brain water content was measured in sham controls or in hypoxic-ischemic animals before, during, and 1 or 24 h after hypoxia-ischemia. In sham controls, T1, T2, and proton density decreased with increasing age, corresponding to an ontogenic decrease in water content. In 1-wk-old rats, increases in T1 and T2 were observed during and at 1 and 24 h after hypoxia-ischemia, corresponding to elevations in water content. In 4-wk-old rats, T1 and water content increased during and at 1 and 24 h after hypoxia-ischemia whereas T2 was not increased until 24 h after hypoxia-ischemia. Regression analysis showed that T1 correlated better with total water content than T2. In both immature and older brain, an increase in total brain water develops acutely and persists after an episode of cerebral hypoxia-ischemia, and T1 imaging detects this change better than T2. Hypoxic-ischemic changes in T2 are age dependent, reflecting other physicochemical changes of water in the tissue than water content alone.