Berry aneurysm. Magnetic resonance imaging (MRI) scan of an axial section through the brain of a patient with an aneurysm in a cerebral artery. This type of aneurysm is known as a berry, or saccular, aneurysm. 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.