Abdominal aortic aneurysm with crescent sign

These images show a large infrarenal aortic aneurysm in a 77-year-old male. The lumen of the aorta can be appreciated in panel A (arrow) and is surrounded by mural thrombus. A hyperdense crescent can be seen within the thrombus in panel B (arrowhead) suggesting acute hemorrhage within the thrombus. This finding suggests an unstable AAA with impending rupture. Narrow windows (C and D) are used to accentuate the high density crescent sign (arrowhead in D).