Stroke. Coloured magnetic resonance imaging (MRI) scan of the brain of a patient after a cerebrovascular accident (CVA, or stroke). A large infarct (area of dead tissue, red) is seen in the sylvian fissure of the right hemisphere (seen here on the left as the brain is viewed from below). The infarct is a result of ischaemia, a lack of blood flow to that area of the brain, caused by a blockage in a blood vessel. The blockage may be due to a thrombus (blood clot) or embolism (air bubble or loose blood clot). Strokes can be instantly fatal, or may pass unnoticed. Symptoms can include muscle weakness, behavioural changes or paralysis.