Acute appendicitis is the most common abdominal surgical emergency, affecting 250,000 individuals in the United States annually. The lifetime risk of developing acute appendicitis is 8.6% for men and 6.7% for women. Radiologists play a critical role in evaluating patients with suspected appendicitis and minimizing complications by confirming or excluding the diagnosis in atypical cases. They can also reduce the number of misdiag-noses and negative laparotomies, provide a correct alternate diagnosis, and manage appendiceal abscesses and post-operative complications. Contrast-enhanced helical CT has a sensitivity, specificity, and accuracy over 95% for the diagnosis of acute appendicitis [8-10].
On CT, the abnormal appendix presents as a slightly distended, fluid-filled, or collapsed structure, approxi mately 0.5-2 cm in diameter. Inflammatory hyperemia causes the wall of the diseased appendix to show homogenous enhancement during the arterial phase of contrast administration. The wall is circumferentially and asymmetrically thickened (usually 1-3 mm). Periap-pendiceal inflammation, the hallmark of appendicitis, is characterized by increased hazy density or linear stranding of adjacent mesenteric fat, by fluid-containing abscesses, and by ill-defined, heterogeneous soft-tissue densities representing a phlegmon [8-10]. There may be secondary inflammatory and edematous changes, with thickening of the wall of the adjacent ileum and cecum, which may mimic primary ileocolic inflammatory disease (Fig. 3).
On non-contrast CT scans, the diagnosis of acute appendicitis requires the detection of a thickened appendix (diameter exceeding 6 mm) with associated inflammatory changes in the periappendiceal fat, or abnormal thickening of the right lateroconal fascia, with or without a calcified appendicolith. The detection of an appendicolith confirms a specific diagnosis of appendicitis in the appropriate clinical setting. An appendicolith can be visualized on CT in approximately 28% of adult patients (compared with 10% for plain films), reflecting the higher sensitivity of CT in detecting small intra-abdominal calcifications [8-10].
The combination of right lower quadrant inflammation, a phlegmon, and an abscess adjacent to the cecum is suggestive but not diagnostic of appendicitis. Indeed, if an abnormal appendix or an appendicolith is not shown, the differential diagnosis must also include Crohn's disease, cecal diverticulitis, ileal diverticulitis, perforated cecal carcinoma, and pelvic inflammatory disease. A barium enema is required to visualize the appendix and evaluate the colon and terminal ileum for primary intestinal disease. Abscesses may be found in locations distant from the ce-cum because of the length and position of the appendix and the patterns of fluid migration in the peritoneal cavity.
The majority (60-70%) of patients referred for cross-sectional imaging with suspected appendicitis do not have this disease. Although most patients have benign, self-limited gastrointestinal disorders such as viral gastroenteritis, CT and ultrasound (US) can often suggest a specific alternate diagnosis [8-10].
Adnexal cysts, masses, salpingitis, and tubo-ovarian abscesses can be readily shown on US. Ureteral calculi and pyelonephritis can be detected on CT and US. Enlarged lymph nodes in the right lower quadrant suggest mesenteric adenitis or infectious ileitis; mural thickening of the terminal ileum can be seen in Crohn's disease or infectious ileitis.
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