Coloured localised frontal view angiogram of the right internal carotid of a 55-year-old patient, as part of an assessment for a subarachnoid hemorrhage. The angiogram shows an aneurysm of the anterior cerebral artery with the anuerysmal sac measuring 8 millimetres in diameter. Aneurysms are abnormal balloonings of blood vessels caused by a localised weak spot in the blood vessel wall. If left untreated, a brain aneurysm can burst leading to a stroke due to a loss of the blood supply to the brain.