Carotid stenosis. Doppler ultrasound scan showing stenosis (narrowing) of the carotid artery in the neck. The carotid artery (red) is the main vessel supplying blood to the brain. Its normal width is seen at the lower left of the scan. The red colour represents normal smooth blood flow. At centre, a grey intrusion is seen into the artery's width. Blood flow in this area is shown in a variety of colours, indicating abnormal blood flow. The reduced width means that blood cannot reach the brain in sufficient volume, which may lead to a stroke. The narrowing may be due to a fatty deposit in the artery (atheroma) or a blood clot (thrombus). Surgery may help widen the artery.