DIFFUSION-WEIGHTED IMAGING DIFFERENTIATES ISCHEMIC TISSUE FROM TRAUMATIZED TISSUE

Background and Purpose Diffusion-weighted magnetic resonance imaging ( MRI) has been shown to be particularly effective in detecting early (0 to 4 hours) pathophysiological changes in localized brain regions aft er cerebral ischemia. The present study sought to establish whether di ffusion-weighted MRI would be similarly effective in predicting outcom e after traumatic brain injury. Methods Diffusion-weighted MRI images and T2-weighted MRI images were obtained over 4 hours after either mod erate fluid percussion-induced traumatic brain injury or unilateral ca rotid ligation in rats. Results Diffusion-weighted MRI images of traum atic brain injury demonstrated focal regions of image hypointensity as early as 1 hour after trauma. The relative diffusion coefficient in t hese hypointense regions was significantly increased (P<.005) by 4 hou rs after trauma compared with the noninjured hemisphere, but only in t he transverse plane in the x direction. In contrast, induction of diff use, nonfocal ischemia by unilateral carotid ligation resulted in scat tered regions of hyperintensity with a significant (P<.001) decrease i n relative diffusion coefficient as early as 1 hour after ligation com pared with the noninjured hemisphere. This decrease exhibited no direc tionality. Conclusions We conclude that traumatic brain injury results in an increased water diffusion distance with the directionality indi cative of bulk flow of extracellular fluid toward the lateral ventricl es (vasogenic edema). In contrast, the decreased water diffusion dista nce with no apparent directionality observed in ischemia is most likel y indicative of cytotoxic edema. Diffusion-weighted MRI therefore has the potential to differentiate cases of traumatic brain injury with no focal ischemia from those instances of traumatic brain injury in whic h focal ischemia is a complication.