Diverticulitis is the result of perforation of a diverticulum. The perforation is generally localized by the pericolic fat so that free perforation is unusual. The inflammation may evolve into a pericolic abscess or may spread and give rise to a more remote abscess. Fistula formation, most commonly to the bladder, may also complicate diverticulitis. Diverticulitis occurs most frequently in the sigmoid colon, but can occur at other sites (see Film .4).

Mild cases of diverticulitis are usually treated medically without carrying out any imaging study for diagnostic confirmation. In more severe cases, a contrast enema may be performed. A water-soluble contrast agent is used instead of barium if there is any evidence of peritonitis. The examination is usually carried out using single contrast technique and careful fluoroscopic observation to identify any contrast extravasation.

The most common radiographic finding is an area of eccentric luminal narrowing caused by the pericolic inflammation, together with thickening and distortion of underlying mucosal folds. Adjacent diverticula may also be deformed. Infrequently, as in Film .1, a gas-containing pericolic abscess is shown. The most direct evidence of diverticulitis is local extravasation of contrast material, but this is uncommonly seen. Film .2 illustrates extravasation of contrast into a rounded abscess cavity (arrow) inferior to a segment of sigmoid diverticulitis. Note the grossly thickened folds in the involved segment. Uncommonly, a longitudinal sinus tract forms due to pericolic or intramural dissection parallel to the colon. This gives rise to an appearance of "double-tracking," as indicated in Film .3 by the arrows.

Differentiation of diverticulitis from a primary carcinoma, particularly a perforated carcinoma, can sometimes be difficult. Contrast filling of a pericolic abscess, and thickened but intact mucosa, favor the diagnosis of diverticulitis. An example of gross fold thickening and luminal narrowing due to diverticulitis of the hepatic flexure is shown in Film .4. Film .5 demonstrates thickened and distorted mucosal folds in a segment of diverticulitis; carcinoma can be excluded because these folds, though abnormal, have not been destroyed. Metastatic disease to the colon may also mimic diverticulitis.

Computed tomography is now frequently utilized in the initial evaluation of severe diverticulitis. The relative roles of CT and contrast enema remain controversial. The earliest change seen on CT is replacement of the normal pericolic fat by hazy soft-tissue densities. In Film .6, these ill-defined pericolic soft-tissue changes (arrow) can be contrasted with the lower density of intra-abdominal fat elsewhere. More severe diverticulitis may give rise to an obvious fluid- or gas-containing pericolic abscess, as indicated by the arrow in Film .7. An additional benefit of CT is that it may be utilized for percutaneous drainage of a peridiverticular abscess, thereby permitting a single surgical resection rather than a two-stage procedure. In Film .8, there are two communicating gas-containing pericolic abscesses (arrows) that were successfully drained using CT guidance (Film .9). In the latter film, the abscess cavities have been opacified with contrast material.