Acute stroke

Diffusion weighted imaging is the most sensitive MR sequence to detect acute stroke. Changes in diffusion weighted images can be seen as early as 30 minutes after insult. At the cellular level, there is random movement and diffusion of water through cellular membranes. In tissues with greater water mobility, there is increased signal loss on diffusion weighted images. In stroke, there is a disruption of cellular membrane ion pumps, which leads to an influx of water into the intracellular space due to osmosis. Intracellular water does not move as freely as extracellular water and this decreased movement leads to increased signal on diffusion weighted images. An increased signal on diffusion weighted images alone is not sufficient to diagnose acute ischemia however, as tumor, infection and trauma can increase intra- and extra-cellular water leading to increased signal on DWI. The apparent diffusion coefficient map diminishes the increased signal associated with increased extracellular water and takes into account only the mobility of water. As the movement of water decreases in ischemic cells, the water diffusion coefficient decreases. This manifests as signal loss on ADC map images. Extracellular fluid on ADC map images will remain bright. Thus, acute ischemia is identified by increased signal on DWI and associated dark signal in the same region on ADC map. The use of DWI and ADC map images is sensitive (88-100%) and specific (86-100%) for acute stroke.


...from Aunt Minnie