Coloured computed tomography (CT) scan of a female patient with an aneurysm (white bulge, lower centre left) of the right femoral artery (left). An aneurysm is a blood-filled dilation in a blood vessel wall. It is caused by a weakening of the vessel wall, which may be caused by hardening of the vessel (atherosclerosis), high blood pressure or injury. If left untreated an aneurysm can rupture.