DIFFUSION AND PERFUSION MR-IMAGING OF CEREBRAL-ISCHEMIA

Over the last few years, diffusion and perfusion magnetic resonance (M R) Imaging methods have found increasing use for monitoring the effect s of cerebral ischemia under clinical and experimental conditions. Blo od perfusion can be visualized by studying the patency of the cerebrov ascular bed (MR angiography), by recording exchange of diffusible trac ers between blood and brain ([H-2]water or [F-19]trifluoromethane clea rance), or by measuring the volume and transit time of the circulating blood (bolus track or spin-tagging imaging). In addition, changes in blood oxygenation level can be visualized by taking advantage of the s usceptibility changes of the magnetic field homogeneity (functional or blood-oxygenation-level-dependent imaging). Diffusion imaging is base d on the modulation of signal intensity by brain water diffusion. Reco rding a series of diffusion-weighted images allows calculation of the apparent diffusion coefficient (ADC) and the reconstruction of quantit ative ADC images. Brain ADC changes are a function of intra-extracellu lar water homeostasis and therefore are a sensitive marker of ionic eq uilibrium. Since disturbances of ion and water homeostasis are among t he first pathological alterations induced by brain ischemia, diffusion imaging is able to detect the incipient injury within minutes. Conver sely, the reversal of these alterations is an early and reliable predi ctor of postischemic recovery. Applications of perfusion and diffusion imaging are reviewed in relation to the pathophysiology, the pathobio chemistry, and the therapy of evolving brain infarct after focal ische mia and the manifestation and reversal of ischemic injury during and a fter global ischemia.