Coloured magnetic resonance angiogram (MRA) scan of an aneurysm (light area) in the basilar artery of the brain. 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, which is often fatal.