Aortic aneurysm. 3D computed tomography (CT) scan of the heart and the aortic arch and thoracic aorta, with an aneurysm (swelling, upper centre) 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.