Carotid aneurysm surgery. Surgeon repairing an aneurysm in a patient's carotid artery. A 3D scan of the artery is seen on the screen at upper right. The internal carotid artery carries blood to the head. The aneurysm, a blood- filled dilation in a blood vessel, is just before the internal carotid artery branches into the middle cerebral artery. Aneurysms are caused by a weakening of the vessel wall, which may be caused by hardening of the artery (atherosclerosis), high blood pressure or injury. If left untreated aneurysms rupture and are often fatal.