Inflammatory Bowel Disease
Of the inflammatory bowel diseases, diverticulitis and chronic inflammatory bowel diseases are the most common causes of stenotic changes to the bowel wall.
CT colonography is generally contraindicated in patients with acute inflammatory bowel disease, because of the high risk of perforation and because it is doubtful what added diagnostic value it provides.
Quite apart from those considerations, CT colonography is also not indicated for other inflammatory diseases of the colon such as infectious colitis, graft-versus-host disease, radiation colitis, and ischemic colitis.
In the large bowel, inflammatory reactions generally lead initially to thickening of the wall (Fig. 4.85), which is associated with increased perfusion and hence increased contrast enhancement. The various types of colitis differ with regard to their primary location, pattern of involvement, and course of extracolonic involvement and potential complications.
Examination technique. Limited data are available on the usefulness of fecal tagging in inflammatory bowel disease. However, since it generally helps to differentiate residual feces from colonic lesions, it is expected to be useful. Because of the increased risk of perforation, tagging protocols using barium should be avoided and bowel insufflation should be performed with caution. In unclear cases an unenhanced low-dose scan can be performed before insufflation is started to provide information about the presence of acute inflammatory changes which would preclude any distention. Intravenous administration of a contrast medium is recommended for the evaluation of inflammatory changes in the colonic wall and the pericolonic tissues. The primary evaluation of inflammatory bowel disease is done on the 2D views. These allow the bowel wall to be evaluated together with the accompanying pericolic inflammation with involvement of the fatty tissue, vessels, and lymph nodes. Endoluminal 3D views play a lesser role, but they can enable visualization of typical mucosal changes such as a cobblestone pattern or inflammatory pseudopolyps. Global 3D views are suitable for depicting the extent and distribution of morphological changes in the colon such as stenoses or loss of haustration.
Contraindications and indications. Inflammation of one or more diverticula leads to symptomatic diverticulitis, which in two-thirds of patients is localized to the sigmoid colon. Conventional abdominal CT, without distension of the colon, is the most commonly used imaging modality for the diagnosis of acute diverticulitis. CT colonography does not add substantial diagnostic benefits in its evaluation; the acute colonic changes of the bowel wall may be readily observed without colonic distension and in 2D views. Moreover, distending the colon can cause it to perforate, because of the acute inflammatory changes to the wall (Fig. 4.86). CT colonography is thus contraindicated for diagnosing acute diverticulitis.
However, in chronic diverticulitis without any clinical signs of acute inflammation (at least 4–6 weeks after an acute episode), CT colonography may be performed for the assessment of postinflammatory stenotic changes and evaluation of the prestenotic colon.
Morphology. Significant 2D findings in chronic diverticulitis include mild to moderate thickening and cone-shaped stenosis of the wall affecting a relatively long bowel segment (usually >10 cm) which transitions gradually—that is, without shoulder formation—to the level of the unaffected bowel wall (Fig. 4.87a, b). Air- or fluid-filled diverticula are often detected in the affected and adjacent unaffected bowel segments (Fig. 4.87a, c). After administration of an intravenous contrast agent, the thickened bowel wall demonstrates increased enhancement. Depending on the degree of inflammatory activity, there may also be pericolic fat stranding and fluid at the root of the mesentery. The intraluminal appearance is nonspecific. Usually, along with diverticula, there are annular symmetrical or asymmetrical stenotic changes. Possible complications include pericolic abscess, contained perforation, hemorrhage, fistula formation, and postinflammatory stenosis (Fig. 4.88).
Differential diagnosis. The most important differential diagnosis is colorectal carcinoma. Unlike diverticulitis, colorectal carcinoma exhibits marked focal thickening of the wall, usually involving a relatively short segment of the colon (less than 5 cm), with shoulder formation at the transition to the normal colonic wall, pericolic fat infiltration, and locoregional lymphadenopathy (see Fig. 4.78).
Focal diverticulitis. If only a single diverticulum or a small number of diverticula are affected, this is referred to as focal diverticulitis or inflammatory pseudotumor (Fig. 4.89). In focal diverticulitis there is circumscribed, moderately edematous thickening of the wall with marked enhancement after intravenous administration of a contrast agent. Isolated stool-filled or contrast-filled diverticula are also often found in the center of the affected region. Fat stranding occurs in the surrounding fat tissue, and endoluminal images show a nonspecific mass-like endoluminal filling defect.
Diagnostic Criteria at CT Colonography
2D: Mild thickening of the bowel wall gradually transitioning to the unaffected bowel wall
2D, 3D: Presence of diverticula
2D, 3D: Cone-shaped stenosis affecting a relatively long segment of the bowel, typically without proximal/distal abrupt shoulder formation (overhanging edges at the transition to normal bowel)
2D: Contrast enhancement of the affected colonic segment
2D: Fluid collections at the mesenteric root
2D: Pericolic fat stranding
3D: Nonspecific symmetrical or asymmetrical stenosis
Important differential diagnosis: colorectal carcinoma
Chronic Inflammatory Bowel Disease
Indications. Ulcerative colitis and Crohn disease are the most common and most important forms of chronic inflammatory bowel disease. The role of CT colonography in the evaluation of these entities is currently not well established. CT colonography, however, is less helpful in the detection and diagnosis of inflammatory bowel disease than for the evaluation of chronic changes occurring over the course of disease, such as stenoses that cannot be passed with an endoscope. In addition, the 2D views in CT colonography allow assessment of the extracolonic extent of inflammatory bowel disease as well as potential complications.
Ulcerative colitis is restricted to the mucosa and submucosa of the colon. Beginning in the rectum, the inflammation spreads contiguously to the more proximal bowel segments. The disease can affect part or all of the colon (pancolitis). Backwash ileitis, in which there is involvement of the terminal ileum, occurs in 10%–40% of patients. Only limited data are available on the usefulness of CT colonography in the evaluation of ulcerative colitis; only a few centers perform CT colonography in patients with chronic inflammatory bowel disease.
Early-stage disease. The subtle inflammatory mucosal changes seen on barium enema studies in the early stages of ulcerative colitis, such as granular wall changes or small punctate ulcerations, are barely visible on CT colonography. Despite the excellent spatial resolution of CT colonography, it is still inadequate for reliable detection of such subtle mucosal changes.
Advanced disease. In advanced stages there is hyperemia and submucosal edema of the bowel wall. In acute disease, in particular, computed tomography shows thickening and stratification (visualization of the layers) of the bowel wall, known as the “target sign” (caused by edematous swelling of the submucosa with depiction of the muscularis propria). Stratification is nonspecific, however, and is also found in other forms of colitis. Two-dimensional views show continuous thickening of the bowel wall, usually affecting a relatively long segment (Fig. 4.90). If an intravenous contrast agent is administered, there is marked mucosal enhancement of the bowel wall. There is usually a continuous transition from the affected colonic wall to unaffected segments without any abrupt step-offs. Because of the bowel distension, the bowel wall thickening and stratification appear milder on CT colonography than on standard abdominal CT scans, where the colon is not distended. Wall thickening can lead to a tubular “lead pipe appearance” with loss of the semilunar folds and narrowing of the colon. As disease progresses, endoluminal 3D views show a diffuse, coarsely granular wall contour, which should not be confused with image noise typical of a low-dose scan (see Fig. 4.9). There are also ulcerations of varying severity, and inflammatory pseudopolyps develop. On 2D views, there may be a relatively smooth outer margin of the affected colonic wall since the disease is limited to the mucosa and submucosa. Typically, the changes affecting the bowel wall in ulcerative colitis are associated with increased paracolic vascularity and mild locoregional lymph node enlargement. Unlike in Crohn disease, abscesses and fistulous tracts are rare.
Complications. In acute stages of ulcerative colitis, CT colonography is generally contraindicated because of the increased risk of complications. Acute ulcerative colitis is a known predisposing factor for colonic perforation. Distension of the colon can lead to intramural laceration or perforation of the bowel wall. In patients with ulcerative colitis, the decision to perform CT colonography should therefore be extremely carefully weighed in terms of the potential benefit and associated risks (e.g., if conventional colonoscopy is incomplete or inconclusive and the evaluation of the colon is deemed necessary). In uncertain cases, an unenhanced low-dose scan performed before insufflation can provide information about the extent and severity of inflammatory changes. Caution should be exercised during bowel distension.
The most severe acute complication of ulcerative colitis is toxic megacolon, which affects 5% of patients and involves a risk of perforation and peritonitis (Fig. 4.91).
Because of the high risk of perforation, acute ulcerative colitis and toxic megacolon are absolute contraindications for insufflation with either air or carbon dioxide.