CT and US findings of acute appendicitis
US: A distended (> 6 mm), noncompressible appendix with or without an adjacent fluid collection, an appendicolith, peritoneal fluid, abnormal flow in the wall of the appendix and a focal mass representing a phlegmon or abscess.
MDCT: The hallmark findings are a distended (> 6 mm), thick-walled appendix with abnormal enhancement and inflammatory changes in the periappendiceal fat. An appendicolith may be seen in 25% of cases. Additional signs of inflammation include focal thickening of adjacent fascia, focal fluid collections, and adjacent phlegmon or abscess ( Figure 69-24 ). Findings that suggest perforation include (1) abscess, (2) extraluminal air, (3) extraluminal appendicolith, (4) phlegmon, and (5) focal defect in the enhancing wall. If all findings are present, perforation can be diagnosed with 95% sensitivity and specificity.