Narrowed renal arteries. Coloured magnetic resonance imaging (MRI) scan of the abdomen of a 57-year-old patient with stenosis (narrowing) of both renal arteries. The large blood vessel running from top to bottom is the aorta, the main artery of the body, which starts at the heart. The two renal arteries that serve the kidneys (purple) are branching from the centre of the aorta. They have both become narrowed near the aorta. The most common cause of stenosis is atheroma, fatty deposits that thicken the inner coat of the arteries. Treatment is with either balloon angioplasty, or a stent to re-open the artery.