Narrowed coronary artery. Coloured coronary angiogram (X-ray) of a 53 year old patient with severe stenosis (narrowing) in the circumflex coronary artery (centre left). One such narrowing can be seen at upper left. The coronary arteries are found on the surface of the heart, and supply blood to the heart muscle. Narrowing of these arteries is usually caused by fatty deposits called atheroma. This heart currently has sufficient blood flow, but if left untreated, the atheroma can block the arteries and cause a heart attack (myocardial infarction, MI). Treatment is by coronary angioplasty to widen and strengthen the arteries, or a coronary artery bypass operation to graft new arteries on to the heart.