Noninvasive quantification of brain edema and the space-occupying effect in rat stroke models using magnetic resonance imaging

Background and Purpose - Brain edema is a life-threatening consequence of stroke and leads to an extension of the affected tissue. The space-occupying effect due to brain edema can be quantified in rat stroke models with the use of MRI. The present study was performed to test 2 hypotheses: ( 1) Can quantification of the space-occupying effect due to brain edema serve as a noninvasive measure for brain water content? ( 2) Does morphometric assessment of brain swelling allow determination of true infarct size on MRI after correction for the space-occupying effect of edema? Methods - Thirty rats were subjected to permanent suture middle cerebral artery occlusion. MRI was performed after 6 or 24 hours, and hemispheric swelling was assessed morphometrically. Interobserver and intraobserver agreements were determined for MRI measurements. In study I, the space-occupying effect due to brain edema was correlated with the absolute brain water content by the wet/dry method. In study II, lesion volumes corrected and uncorrected for edema were calculated on MRI and on TTC staining and compared. Results - Interobserver and intraobserver agreements for MRI measurements were excellent ( r greater than or equal to 0.97). Brain water content and hemispheric swelling correlated well after 6 and 24 hours ( r greater than or equal to 0.95). Corrected lesion volumes correlated with r = 0.78 between TTC staining and MRI. Without edema correction, lesion volumes were overestimated by 20.3% after 6 hours and by 29.6% after 24 hours of ischemia. Conclusions - Morphometric assessment of hemispheric swelling on MRI can determine the increase in absolute brain water content noninvasively and can also provide ischemic lesion volumes corrected for brain edema.