Aortic aneurysm. 3D computed tomography (CT) scan of the heart and part of the aorta and ribcage, with an aneurysm (swelling, upper left) of the aortic arch. An aneurysm is a blood-filled dilation in a blood vessel wall. 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. If left untreated an aneurysm can rupture, which is often fatal.