3 phase Helical CT of the abdomen and pelvis with non-contrast, arterial and portal venous phases.
There is a large volume of free retroperitoneal gas which appears centred on the lower rectum and extends superiorly around the aorta and behind the pancreas. The rectum and sigmoid colon is featureless with a mildly thickened wall and contains fluid in its lumen. Within the lower rectum there is subtle luminal high attenuation on the portal venous phase which may represent a source of haemorrhage. No potential sign of haemorrhage are identified elsewhere within the large bowel. Small bowel is unremarkable.
The right kidney is atrophic. The solid organs are otherwise unremarkable. Incidental bilateral fat containing inguinal herniae. The lung bases are clear. Evidence of sacroiliitis without ankylosis.
Conclusion
Bowel perforation most likely from the lower rectum with a potential source of haemorrhage in the region, on a background of colorectal findings in keeping with the history of ulcerative colitis.