Inflammatory Disorders Ulcerative Colitis

Ulcerative colitis is characterized pathologically by extensive ulceration and diffuse inflammation of the mucosa. The disease characteristically begins in the rectum and extends proximally to involve part or all of the colon. The pathological changes found in the very early stages of ulcerative colitis are below the spatial resolution of CT. With progressive disease, submucosal edema may be seen, producing a target sign. Severe mucosal ulceration can denude certain portions of the colonic wall, leading to inflammatory pseudopolyps (Fig. 1). When sufficiently large, these pseudopolyps can be visualized on CT.

Fig. 1. Acute ulcerative colitis. CT demonstrates deep ulcerations (arrows) of the fluid-filled rectosig-moid. Inflammatory pseudopolyps appear as residual islands of inflamed mucosa, which protrude above the denuded colonic surface

Mural thickening and lumen narrowing are the CT hallmarks of subacute and chronic ulcerative colitides. Mural thinning, unsuspected perforations, and pneumatosis can be detected on CT in patients with toxic megacolon. In this regard, CT can be quite helpful in determining the urgency of surgery for patients with stable abdominal radiographs yet a deteriorating clinical course [1-3].

In chronic ulcerative colitis, the muscularis mucosae becomes markedly hypertrophied, often by up to a factor of 40. Forceful contraction of this hypertrophied longitudinal muscle may pull the mucosa away from the sub-mucosa, producing diffuse or segmental narrowing of the lumen. The contraction also causes shortening of the colon. The submucosa becomes thickened due to the deposition of fat or, in acute and subacute cases, edema. Submucosal thickening further contributes to lumen narrowing. Additionally, the lamina propria is thickened due to round cell infiltration in both acute and chronic ul-cerative colitides.

Fig. 1. Acute ulcerative colitis. CT demonstrates deep ulcerations (arrows) of the fluid-filled rectosig-moid. Inflammatory pseudopolyps appear as residual islands of inflamed mucosa, which protrude above the denuded colonic surface

On CT, these mural changes produce a target or halo appearance when axially imaged: the lumen is surrounded by a ring of soft tissue density (mucosa, lamina propria, hypertrophied muscularis mucosae). This is surrounded by a low-density ring (fatty infiltration of the submucosa), which in turn is surrounded by a ring of soft tissue density (muscularis propria). This mural stratification is not specific and can also be seen in Crohn's disease, infectious enterocolitis, pseudomembranous colitis, ischemic and radiation enterocolitides, mesenteric venous thrombosis, bowel edema and graft-versus-host disease [1-3].

There are certain CT findings that can help differentiate granulomatous and ulcerative colitis. Mural stratification, that is, the ability to visualize individual layers of bowel wall, is seen in 61% of patients with chronic ulcerative colitis but only in 8% of patients with chronic granulomatous colitis. In addition, mean colon wall thickness in chronic ulcerative colitis is 7.8 mm, significantly smaller than that observed in Crohn's colitis (11 mm). Finally, the outer contour of the thickened colonic wall is smooth and regular in 95% of ulcerative colitis cases, while serosal and outer mural irregularities are present in 80% of patients with granulomatous colitis [1-3].

Rectal narrowing and widening of the presacral space are hallmarks of chronic ulcerative colitis. CT depicts the anatomic alterations that underlie these rather dramatic morphologic changes. The rectal lumen is narrowed due to the previously described mural thickening that attends chronic ulcerative colitis. As a result, the rectum has a target appearance on axial scans, which should not be mistaken for the external anal sphincter, mucosal prolapse, or the levator ani muscles. The increase in the pre-sacral space is caused by proliferation of the perirectal fat. On CT, this fat is characterized by an increased number of nodular and streaky soft tissue densities and an abnormal attenuation value 10-20 HU higher than the normal extraperitoneal or mesenteric fat. These fatty changes relate to a number of factors, including ex vacuo replacement by fat of the void produced by rectal lumen narrowing and lipodystrophy resulting from an influx of inflammatory cells and edema. Edematous adipose tissue and enlarged lymph nodes are often observed in the perirectal region in patients with chronic ulcerative colitis upon abdominoperineal resection [1-3].

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