An abscess is defined as a localized collection of purulent material, often requiring percutaneous or open drainage. Presenting symptoms include fever, pain, and leukocytosis, though steroids can mask symptoms. The differential list of etiologies is long, and includes (in no particular order) post-surgical complications, bowel perforation, pancreatitis, pelvic inflammatory disease, appendicitis, diverticulitis, Crohn’s disease, trauma, cancer, and ischemia.

The classic CT finding is a rounded fluid collection with an enhancing rim. Abscesses are commonly septated and thick-walled, with gas bubbles or an air-fluid level present in 1/3. Early on, prior to liquefactive necrosis, only a phlegmon may be present – an enhancing inflammatory mass without a fluid collection.

Mimics of abscesses are myriad, and include pancreatic pseudocysts, mesenteric cysts, loculated ascites, lymphoceles, bilomas, urinomas, chronic hematomas, large diverticula, necrotic tumors, and unopacified fluid-filled bowel.

Common sequelae of abscesses include obstruction of the colon or urinary tract.

With respect to diverticula, perforation from diverticulitis is typically initially confined to the leaves of the mesocolon. The inflammatory mass will thus be extraluminal and extraperitoneal. These are best evaluated with CT with oral contrast, thus avoiding the potential trauma of an enema. Diverticulitis is usually manifest by thickening of the bowel wall more than 3mm, pericolic inflammatory soft tissue mass (often with air, fluid, contrast), pericolic inflammatory fat stranding, and sinus tracts/fistulae to adjacent organs or the skin.

With respect to Crohn’s disease, the disease is confined to the colon in only 25%, with CT features including wall thickening and fibrofatty proliferation of the surrounding mesenteric fat (“creeping fat”). Vascular engorgement of the supplying mesenteric vessels with prominent spacing gives rise to the “comb sign”. Common complications include sinus tracts, fistulae, bowel obstruction, and abscesses.