Brain aneurysm. Magnetic resonance imaging (MRI) scan of an axial section through the brain of a 38-year-old patient, showing a large signal intensity (pale area, centre) in the posterior cranial fossa, suggestive of a large aneurysm of the termination of the basilar artery. 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.