Coloured computed tomography (CT) scan of the brain of a 58 year old patient that has had an aneurysm (orange) in the middle cerebral artery. An aneurysm is a blood-filled dilation in a blood vessel. 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. Haemorrhaging of an aneurysm can be fatal.