Carotid aneurysm. 3D coloured digital angiogram of an aneurysm (orange ball) in the left internal carotid artery. 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. Treatment is with repair or replacement of the vessel. Angiograms use a contrast dye to highlight blood vessels.