Imaging Studies

Imaging studies performed in the workup of possible acute appendicitis include plain radiographs, ultrasound studies, and computed tomography (CT) scans. The utility and role of these studies is controversial.

Plain radiographs of the abdomen are abnormal in 24 to 95 percent of patients with acute appendicitis. 7 Radiographic indicators of possible acute appendicitis include appendiceal fecolith, appendiceal gas, localized paralytic ileus, blurred right psoas muscle, and free air. Since many of these signs are seen in multiple other processes, abdominal radiographs have limited diagnostic value in acute appendicitis.

Graded compression ultrasonography has been widely utilized in the diagnosis of acute appendicitis. This study is safe, noninvasive, and reported to have a sensitivity of approximately 80 to 89 percent.15 The use of this technique is based on the fact that normal bowel loops and a normal appendix can be compressed with moderate pressure while an inflamed appendix cannot be compressed. The diagnostic criteria for acute appendicitis include the visualization of a noncompressible appendix that has a diameter of 6 mm or greater, demonstration of an appendicolith, or demonstration of a periappendiceal abscess. This technique has limitations with a retrocecal appendix, in that the overlying bowel may limit visualization and compression testing. In addition, an early perforation may be missed, since the diameter of the appendix may be normal after perforation.16 In addition, color Doppler ultrasound studies may be helpful when the appendix is well visualized but equivocal in size. With this technique, hyperemia in the wall of the appendix has been found to be a sensitive indicator of inflammation. 17

CT is another adjunctive study used in the diagnosis of acute appendicitis. In one study which directly compared CT to ultrasound in 100 patients, the CT had greater sensitivity (96 versus 76 percent), greater accuracy (94 versus 83 percent), and a greater negative predictive value (95 versus 76 percent). In this study, there was no significant difference when CT was compared to ultrasound with regard to specificity or positive predicitive value. 18 CT findings suggesting acute appendicitis include pericecal inflammation, abscess, periappendiceal phlegmon, or fluid collections. Given its wide availability and usefulness in establishing alternative diagnoses, CT is probably the best choice as initial imaging study, although its radiation exposure limits its application in pregnant patients and children.

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