Abdominal aortic aneurysm. Ultrasound scan of the abdomen of a 83 year old female patient with an abdominal aortic aneurysm (AAA, dark area across top). An aneurysm is a blood-filled dilation caused by a weakening of the vessel wall. The aorta is the main artery of the body. Aneurysms are most common in men over 65 years of age. If left untreated they rupture and are often fatal. Treatment is with repair or replacement of the blood vessel.