What are causes of small bowel wall thickening (> 3 mm) on MDCT?
Smooth and concentric bowel wall thickening is typical for nonmalignant disease (e.g., Crohn’s; ulcerative colitis; and ischemic, infectious, or radiation enteritis). Extraintestinal findings are important. In acute Crohn’s, MDCT is the best initial examination to evaluate for associated abscesses, fibrofatty proliferation, fistulas, mesenteric inflammation, and engorged vasa recta (“comb sign”). However, small bowel follow-through remains more sensitive for subtle mucosal changes and should be performed if suspicion for Crohn’s remains after a normal MDCT. MRI is excellent at diagnosing perianal disease, including fistulas.
Eccentric and irregular bowel wall thickening of more than 2 cm, especially if confined to a short segment, is suspicious for malignancy. Carcinoid is the most common primary malignant small bowel tumor. Carcinoid is typically located in the ileum; however, the actual tumor is often small and not visible on CT. A surrounding desmoplastic reaction with spiculated, often calcified mesenteric lymph nodes suggests the diagnosis. Adenocarcinoma is the most common primary malignant proximal small bowel tumor. It often presents as a mass or annular stricture that may obstruct. B-cell lymphoma occurs in the distal small bowel (⅔) and T-cell lymphoma in the proximal small bowel (⅓). Massive mesenteric or retroperitoneal adenopathy is often present. Lipomas are easily recognized by their low attenuation (≈ –100 HU). The most common metastatic tumors to the small bowel include lung and melanoma.