Artery stenosis. Coloured angiogram (X-ray) of stenosis (narrowing, seen just to the left of centre) of the carotid artery (brown) in the neck. The backbone is blue/red. Arterial stenosis can be due to atherosclerosis, the build-up of fatty material on an arterial wall, or thrombosis, the formation of a clot in an undamaged artery. It can also result from an embolism, which occurs when a particle or gas bubble that was moving through the artery becomes trapped. The decreased blood flow to the brain may cause a stroke. Angiograms are produced by injecting an X-ray opaque dye into the blood vessel to be studied.