Diverticular Disease
In the Western world, diverticular disease is the most common disease of the large bowel, affecting 10%–30% of the population aged 50 years and 30%–60% of 80-year-olds. Diverticular disease usually remains asymptomatic. Predisposing factors include older age together with a low-fiber diet as commonly seen in industrialized nations. Additional etiological factors that are assumed to be related to the disease include a diet rich in red meat, fat, and salt.
Pathogenesis
Prediverticulosis. In early-stage disease, known as “prediverticular disease,” there is characteristic thickening of the muscular layer, shortening of the taeniae, and narrowing of the colonic lumen. As the disease progresses, irregularities in the caliber of the colon and the haustral folds appear. This leads to global and uniform thickening of the bowel wall by more than 4 mm. The semilunar folds are prominent, close together, and there is a shortening of the interhaustral segments, producing a “concertina” appearance (Fig. 4.19). The distensibility of the colon is also diminished. At this stage no diverticula are yet present, but small (1–2 mm) transient outpouchings of the wall exist.
Diverticular disease. With the progression of disease, diverticula develop. Anatomically, these outpouchings are by definition “pseudodiverticula,” as they do not include all layers of the colonic wall, but only the mucosa and submucosa. They involve herniation of the mucosa and submucosa through the muscularis propria at weak spots in the intestinal wall, such as the places where supplying arteries pass through the muscularis propria. In rare instances, true diverticula can develop (usually in the proximal colon). These are characterized by an outpouching of all layers of the wall (mucosa, submucosa, and muscularis propria). The two variants cannot be distinguished from each other radiologically.
Morphology at CT Colonography
Diverticula
On 2D CT images diverticula present as circumscribed, sharply delimited air-filled outpouchings of the intestinal wall. Some diverticula may contain residual fluid, and if a contrast medium has been administered orally (e.g., in fluid tagging), the fluid will be labeled. On 3D endoluminal views, diverticula have a well-defined, usually complete dark outer ring, whereas polyps usually exhibit a poorly defined and incomplete outer margin (Fig. 4.20). This typical 3D sign of a diverticulum is also known as the “complete rim sign,” and with sufficient experience it enables diverticula to be easily distinguished from polyps. On 2D views, diverticula and polyps are easy to distinguish on the basis of morphology (see “Polypoid Lesions of the Colon,” p. 91).
Inverted Diverticulum
Very rarely, a diverticulum may invert and therefore protrude into the colonic lumen, mimicking a pseudopolypoid lesion on 2D and 3D images. The corresponding virtual endoscopic 3D view is nonspecific and shows a polypoid-like lesion. Two-dimensional images are essential for correct diagnosis: Unlike polyps, inverted diverticula with a pseudopolypoid form demonstrate fat attenuation due to a central indentation of pericolic fat tissue in the inverted part of the diverticulum.
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