Figure 9a. Impending aneurysm rupture in a 66-year-old man with back pain, who underwent imaging for suspicion of renal colic. Axial unenhanced CT images (a and b are the same section with different window settings) demonstrate an abdominal aortic aneurysm with a hyperattenuating crescent sign (arrow) that represents an acute hematoma within the aneurysm wall.