Magnetic resonance imaging during cerebral hypoxia-ischemia: T-2 increasesin 2-week-old but not 4-week-old rats

We investigated whether the changes detectable with magnetic resonance imag ing techniques during and after an episode of cerebral hypoxia-ischemia dif fer in immature and older brain. Diffusion weighted (DW) and T-2-weighted ( T2W) images were repeatedly acquired before, during, and after an episode o f cerebral hypoxia-ischemia (unilateral carotid artery occlusion plus hypox ia) 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 r ats by 10-20 min after the start of hypoxia. These hyperintense areas incre ased during hypoxia, comprising 60.8 +/- 4.9% and 30.5 +/- 2.7% of the brai n image at the level of the thalamus in 2-wk-old and 4-wk-old animals, resp ectively (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 n ot occur in 4-wk-old animals (p < 0.02). This observation was reflected in T-2, 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 i maging changes during transient hypoxia-ischemia generally recover with a d elayed or secondary increase in DW and T2W hyperintensity hours later. Imma ture brain differs from older brain primarily with respect to some combinat ion of hypoxic/ischemic cellular or biochemical changes, that are detectabl e as increases in T-2 within 2-wk-old but not 4-wk-old animals.