Aneurysm surgery. Surgical team removing an aneurysm in a patient's brain. They are using X-rays to guide them to the site of the aneurysm. Aneurysms are blood-filled dilations in a blood vessel wall. They are 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 aneurysms rupture and are often fatal.