Figure 10b. Impending aneurysm rupture in a 57-year-old man with a known abdominal aortic aneurysm and increasing abdominal pain. Axial unenhanced (a) and axial contrast-enhanced (b) CT images depict an abdominal aortic aneurysm with a hyperattenuating crescent sign (arrow in a), which represents an acute hematoma within the aneurysm wall.