Magnetic resonance imaging during cerebral hypoxia-ischemia: T₂ increases in 2-week-old but not 4-week-old rats

We investigated whether the changes detectable with magnetic resonance imaging techniques during and after an episode of cerebral hypoxia-ischemia differ in immature and older brain. Diffusion weighted (DW) and T₂-weighted (T₂W) images were repeatedly acquired before, during, and after an episode of cerebral hypoxia-ischemia (unilateral carotid artery occlusion plus hypoxia) in 2- and 4-wk-old rats lightly anesthetized with isoflurane. Areas of increased brightness were detected in DW images from both 2- and 4-wk-old rats by 10-20 min after the start of hypoxia. These hyperintense areas increased during hypoxia, comprising 60.8 ± 4.9% and 30.5 ± 2.7% of the brain image at the level of the thalamus in 2-wk-old and 4-wk-old animals, respectively (p < 0.003). Hyperintense areas (e.g. 27.0 ± 8.3%) also appeared in T₂W images during hypoxia-ischemia in 2-wk-old animals, but these did not occur in 4-wk-old animals (p < 0.02). This observation was reflected in T₂, which increased during hypoxia-ischemia in the 2-wk-old but not the 4-wk-old group. By 60 min after the termination of hypoxia-ischemia in either age group, areas of hyperintensity resolved and then reappeared 24 h later on both DW and T₂W images. Thus, irrespective of age, magnetic resonance imaging changes during transient hypoxia-ischemia generally recover with a delayed or secondary increase in DW and T₂W hyperintensity hours later. Immature brain differs from older brain primarily with respect to some combination of hypoxic/ischemic cellular or biochemical changes, that are detectable as increases in T₂ within 2-wk-old but not 4-wk-old animals.