Correlation of cerebral hypoxic-ischemic T2 changes with tissue alterations in water content and protein extravasation

Background and Purpose-Age-dependent changes in T2-weighted MR images have been reported in cerebral hypoxia-ischemia. However, the biophysical mechan isms responsible for the image changes remain poorly defined. We investigat ed whether cerebral hypoxia-ischemia-induced T2 changes correlate with alte rations in either water content or protein extravasation. Methods-One- and 4-week-old rats were subjected to unilateral carotid arter y occlusion plus hypoxia in 8% oxygen. T2 images were acquired before, duri ng, and 1 or 24 hours after hypoxia-ischemia. Blood-brain barrier disruptio n and brain edema were evaluated by immunohistological detection of IgG ext ravasation and measurement of water content by dry-wet weight and specific gravity methods. Results-In 1-week-old rats, T2 values, areas of hyperintensity on T2-weight ed images, and water content in the ipsilateral hemisphere increased during hypoxia-ischemia, recovered at 1 hour after hypoxia-ischemia, and increase d again at 24 hours after hypoxia-ischemia. Extravasation of IgG occurred d uring hypoxia-ischemia and remained detectable 24 hours after hypoxia-ische mia. In 4-week-old rats, an increase in T2 or extravasation of IgG did not occur until 24 hours after hypoxia-ischemia despite a comparable elevation in water content during and soon after hypoxia-ischemia. Conclusions-T2 imaging appears reliable for detecting edema associated with disruption of the blood-brain barrier but not necessarily an increase in c erebral water or plasma proteins alone. The different hypoxic-ischemic chan ges in T2 in immature and older brain are associated with differences in al terations in water content plus extravasation of protein, consistent with a ge-dependent differences in hypoxic-ischemic alterations in vascular permea bility.