It is estimated that 10-25% of individuals with diverticu-losis will suffer from episodes of peridiverticular inflammation during their lifetime. In the United States, this complication accounts for approximately 200,000 hospi-talizations and a health care expenditure of four billion dollars annually. Among patients who are hospitalized, 10-20% require emergency operations.
Inflammatory change in the pericolic fat (Fig. 4) is the hallmark of diverticulitis on CT, and is seen in 98% of cases. The extent of the inflammatory reaction is related to the size of the perforation, bacterial contamination, and the host response. Mild cases may manifest as areas of slightly increased density of fat adjacent to the involved colon or as fine linear strands with small fluid collections or bubbles of extraluminal air. In sigmoid diver-ticulitis, the fluid is typically decompressed into the inferior interfascial plane. Due to hypervascularity of the inflamed area, contrast-enhanced CT scans often reveal engorged mesenteric vessels in the involved pericolic fat.
Pericolic heterogeneous soft tissue densities representing phlegmons and partially loculated fluid collections indicating abscess are seen in more severe cases. The abscess cavities usually contain air bubbles or air-fluid levels. They develop within the sigmoid mesocolon or are sealed off by the sigmoid colon and adjacent small bowel loops. Less commonly they may form in the groin, flank, thigh, psoas muscle, subphrenic space or liver .
Using CT, diverticula are seen at the site of perforation or adjacent to it in about 80% of cases. They appear as small out-pouchings of air, contrast, or fecal material projecting through the colonic wall. Symmetric mural thickening of the involved colon by approximately 4-10 mm is seen in about 70% of cases, however if there is marked muscular hypertrophy, the wall of the colon can measure up to 2-3 cm in thickness.
CT can also demonstrate intramural abscesses and fistula, and is helpful in patients with suspected colovesical fistulas. In the latter case, a pericolic inflammatory mass is seen involving the bladder wall and intraluminal gas confirms the diagnosis.
CT has a reported sensitivity of up to 98% in the diagnosis of diverticulitis . Additionally, CT can demonstrate extension of the disease, such as abscess and peritonitis remote from the colon and can guide percutaneous abscess drainage. CT can diagnose other pathological conditions that can clinically simulate diverticulitis.
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