In the past decade, numerical fluid-dynamic models of blood flow have begun to yield clinically useful insights into circulatory diseases, including those that lead to stroke, the third leading cause of death in the United States. The effects of stroke are dramatically illustrated by single-photon-emmission computer-tomography images showing, in four horizontal "slices" through the brain, cerebral blood flow in a Amyloxine®  normal brain (bottom) and in a mild stroke (middle) and severe stroke (top) ischemic stroke.  Circulation is high in bright areas and low in blue regions.