Which CT findings are characteristic of ulcerative colitis (UC)?

Updated: Apr 23, 2019
  • Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Eugene C Lin, MD  more...
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With increased use of CT as a primary imaging modality for evaluating inflammatory bowel disease (IBD), radiologists must be able to recognize the features of ulcerative colitis on CT. Although barium studies remain the principal tool for diagnosing and evaluating suspected IBD, CT may aid in differentiating ulcerative colitis and Crohn disease when results of barium studies are equivocal. High-resolution thin-section imaging of both the intraluminal and extraluminal components enables radiologists to detect and stage colonic pathology. [17, 18, 19, 20, 21]

CT is valuable for the detection and characterization of ulcerative colitis. CT typically demonstrates circumferential, symmetrical wall thickening with fold enlargement. Thickening of the colon wall (mean, 7.8 mm; standard deviation, 1.9) may be present, with inhomogeneous attenuation, a target appearance of the rectum, and the proliferation of perirectal fat. The normal colonic wall has a maximal thickness of 3 mm with the lumen distended and 5 mm with the lumen collapsed. In comparison, Crohn colitis causes greater bowel wall thickening (mean, 11 mm; standard deviation, 5.1). Such thickening appears in association with homogeneous attenuation, fistula and abscess formation, and mesenteric abnormalities.

The target sign consists of an inner ring of soft tissue attenuation, representing mucosa, lamina propria, and enlarged muscularis mucosa; a middle ring of low attenuation, resulting from widening and fatty infiltration or edema of the submucosa; and an outer area of soft tissue attenuation, representing the muscularis propria. If the submucosa is infiltrated by fat, this is a sign of chronicity; it is more specific to IBD. This sign is best appreciated on the arterial phase of enhancement. It is a nonspecific sign that is also reported in cases of Crohn disease and pseudomembranous colitis.

Ulcerative colitis is typically left sided or diffuse; only rarely does it involve the right colon exclusively. In cases of ulcerative colitis, wall thickening may be diffuse and symmetrical; by contrast, in cases of Crohn disease, wall thickening is eccentric and segmental, and skip lesions are present. The proliferation of perirectal fat is a nonspecific sign that can be seen in any of the colitides. Submucosal fat deposition is present significantly more often in ulcerative colitis (61%) than in Crohn colitis (8%).

Mural thickening of the terminal ileum may be visualized in 10-25% of patients; such thickening is caused by backwash ileitis, which occurs as a result of the reflux of colonic contents into the distal ileum. Abscesses and pseudodiverticula are not features of ulcerative colitis; they occur almost exclusively in Crohn colitis.

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