Crohns Disease

Crohn's disease most commonly affects the terminal ileum and proximal colon. The acute, active phase of Crohn's disease is characterized by focal inflammation, aphthoid ulceration with adjacent cobblestoning, an often transmural inflammatory reaction with lymphoid aggregates and granuloma formation, fissures, fistula and sinus tracts. The chronic and resolving phase of this disorder is associated with fibrosis and stricture formation. When Crohn's disease is limited to the mucosa, the CT scan is often normal. Although inflammatory and post-in flammatory pseudopolyps may be identified on CT, the assessment of the mucosa is best reserved for barium studies and colonoscopy, which are more direct and sensitive. Crohn's disease causes mural thickening of the gut in the range of 1-2 cm. This thickening, which occurs in up to 83% of patients, is most frequently observed in the terminal ileum, but other portions of the small bowel, colon, duodenum, stomach, and esophagus may be similarly affected [1-3].

During the acute, non-cicatrizing phase of Crohn's disease, the small bowel and colon maintain mural stratification and often have a target or double-halo appearance. As in ulcerative colitis, there is a soft tissue density ring (corresponding to mucosa), which is surrounded by a low-density ring. This ring has an attenuation near that of water or fat (corresponding to submucosal edema or fat infiltration, respectively), which in turn is surrounded by a higher density ring (muscularis propria). Inflamed mucosa and serosa may show significant contrast enhancement following bolus intravenous contrast administration, and the intensity of enhancement correlates with the clinical activity of disease [1-3].

The CT demonstration of mural stratification, that is, the ability to visualize distinct mucosal, submucosal, and muscularis propria layers, indicates that transmural fibro-sis has not occurred and that medical therapy may be successful in ameliorating lumen compromise. Additionally, prior to the onset of fibrosis, the edema and inflammation of the bowel wall that cause mural thickening and lumen obstruction are reversible to some extent. A modest decrease in wall thickness often produces a dramatic increase in the cross-sectional area of the lumen and resolution of the patient's obstructive symptoms. Loss of mural stratification is indicative of transmural fibrosis [1-3].

In patients with long-standing Crohn's disease and transmural fibrosis, mural stratification is lost so that the affected bowel wall typically has homogeneous attenuation on CT. Homogeneous attenuation of the thickened bowel wall in the presence of good intravascular contrast medium levels and thin-section scanning suggests irreversible fibrosis so that anti-inflammatory agents may not provide significant reduction in bowel wall thickness. If these segments become sufficiently narrow, surgery or stricturo-plasty will be necessary to relieve the patient's obstruction.

The palpation of an abdominal mass or separation of bowel loops on a barium study in a patient with Crohn's disease evokes an extensive differential diagnosis: abscess, phlegmon, 'creeping fat' or fibrofatty proliferation of the mesentery, bowel wall thickening, and enlarged mesenteric lymph nodes. Each of these disorders has significantly different prognostic and therapeutic implications. This diagnostic dilemma is further complicated by the fact that many patients are receiving immunosuppres-sive therapy that can mask signs and symptoms. CT can readily differentiate the extraluminal manifestations of Crohn's disease.

Fibrofatty proliferation, also known as 'creeping fat' of the mesentery, is the most common cause of separation of bowel loops seen on barium studies in patients with Crohn's disease. On CT, the sharp interface between bowel and mesentery is lost and the attenuation value of the fat is elevated by 20-60 HU due to the influx of inflammatory cells and fluid. Mesenteric adenopathy with lymph nodes ranging in size between 3 and 8 mm may also be present. If these lymph nodes are larger than 1 cm, the presence of lymphoma or carcinoma, both of which occur with greater frequency in Crohn's disease, can be excluded [1-3].

Contrast-enhanced CT scans often show hypervascu-larity of the involved mesentery, manifesting as vascular dilatation, tortuosity, prominence, and wide spacing of the vasa recta. These distinctive vascular changes have been termed the 'comb sign'. Identification of this hy-pervascularity suggests active disease and may be useful in differentiating Crohn's disease from lymphoma or metastases, which tend to be hypovascular lesions.

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