Diffusion and perfusion MR imaging of cerebral ischemia

Over the last few years, diffusion and perfusion magnetic resonance (MR) imaging methods have found increasing user for monitoring the effects of cerebral ischemia under clinical and experimental conditions. Blood perfusion can be visualized by studying the patency of the cerebrovascular bed (MR angiography), by recording exchange of diffusible tracers between blood and brain ([2H]water or [19F]trifluoromethane clearance), 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 susceptibility changes of the magnetic field homogeneity (functional or blood-oxygenation-level-dependent imaging). Diffusion imaging is based on the modulation of signal intensity by brain water diffusion. Recording a series of diffusion-weighted images allows calculation of the apparent diffusion coefficient (ADC) and the reconstruction of quantitative ADC images. Brain ADC changes are a function of intra-extracellular water homeostasis and therefore are a sensitive marker of ionic equilibrium. Since disturbances of ion and water homeostasis are among the first pathological alterations induced by brain ischemia, diffusion imaging is able to detect the incipient injury within minutes. Conversely, the reversal of these alterations is able to detect the incipient injury within minutes. Conversely, the reversal of these alterations is an early and reliable predictor of postischemic recovery. Applications of perfusion and diffusion imaging are reviewed in relation to the pathophysiology, the pathobiochemistry, and the therapy of evolving brain infarct after focal ischemia and the manifestation and reversal of ischemic injury during and after global ischemia.