Figure 16b. Primary and secondary aortoduodenal fistulas. (a) Primary aortoduodenal fistula. Transverse nonenhanced CT scan in an 80-year-old woman with GI bleeding shows a calcified abdominal aortic aneurysm (A) with intraluminal gas (arrow). Massive high-attenuation retroperitoneal hemorrhage (H) surrounds the aorta. (b) Secondary aortoduodenal fistula. Contiguous transverse contrast-enhanced CT scans in a 71-year-old man with GI bleeding and history of aortic repair shows air (black arrow) in the lumen of the aortic graft and tethering (white arrow) of overlying duodenum associated with periaortic inflammatory changes.