Brain aneurysm. X-ray of a section through the brain of a 45-year-old male patient with an aneurysm (ballooning, lower right) affecting the left internal carotid artery. Aneurysms are an abnormal ballooning of a blood vessel 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.