Water compries approximately 70% of the body. Water molecules are in constant random motion called diffusion. Diffusion is a physical property of molecules referring to their ability to move randomly in relation to their thermal energy. Diffusion weighted imaging has made its greatest impact on the imaging evaluation of ischemic stroke. It is a new and efficient method that detects acute ischemic stroke much earlier than conventional T2-weighted MRI and computed tomography. DWI is able to distinguish the location and extent of the ischemic area within one hour following onset of a stroke. By comparison, conventional MRI and CT are sensitive in detecting hemorrhage from infarction and brain ischemia, but not until several hours after onset. Diffusion-weighted imaging detects the slowing of water molecules that occurs during the early stages of a stroke. It measures the net movement of water molecules in the body due to thermal energy (Brownian motion). During a stroke, changes take place in the intra and extracelluar volume. In ischemic areas, a shift in water balance occurs due to a change in the sodium/potassium pump. As a result, sodium and water tend to accumulate in ischemic cells. This accumulation causes the movement of water molecules to slow and the cells to swell, resulting in cytotoxic edema. At a later stage (subacute ischemia), the diffusion coefficient increases well above its normal value. This is associated with vasogenic edema, in which the motion of bulk water takes place. The ischemic area shows up dramatically as a bright area, demonstrating decreased water mobility. One can see the extension of the damaged area with time. Diffusion imaging thus offers the unique opportunity to address noninvasively fundamental issues regarding the response of the brain tissue to CVA at different stages, with potentially important clinical applications. Early detection of CVA, at a stage when tissue damage is sti...
