Stroke. Magnetic resonance imaging (MRI) scan of the brain of a 32 year old woman after a massive cerebrovascular accident (CVA), or stroke. It was caused by a blocked right internal carotid artery and has lead to the swelling of, and areas of ischaemia (lack of blood flow, lighter) in, the right hemisphere (left). 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.