Narrowed coronary arteries. Coloured coronary angiogram (X-ray) of a 65 year old patient with severe stenosis (narrowing) in the branches of the left coronary artery (orange). The coronary arteries are found on the surface of the heart, and supply blood to the heart muscle. Narrowing of the arteries is usually caused by fatty deposits called atheroma. This 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.