3D computed tomography (CT) angiogram scan of a 64-year-old male patient with the presence of a large fusiform aneurysm of the ascending thoracic aorta. 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.