appendicitis (mean diameter of 8.3 mm). 15 Visualization of the normal appendix in its entire length, including its tip, definitively excludes appendicitis. 16 However, an appendiceal US in which the appendix is not visualized and no inflammatory findings are present in the RLQ has been shown to have a high negative predictive value. 7 Acute Appendicitis Appendix thickening, maximal tenderness over the thickened appendix, noncompressibility, (large) appendicolith, and hyperemia (Figure 6) are the primary US indicators of appendicitis. 16 While 6 mm is the conventional cut‐off for appendiceal diameter for US, it has been suggested that specificity may be improved with a cut‐off of 7 mm. 17 Note that this cut‐off value does not apply to CT or MRI. Secondary signs of appendicitis include RLQ mesenteric fat stranding, the presence of a complex fluid collection, mesenteric lymphadenopathy, and/or periappendiceal fluid. 16 Fat stranding appears as mesenteric thickening and hyperechogenicity (Figure 7). Hyperechoic, thickened fat is highly specific for inflammatory disease in the RLQ. 18,19 An appendicolith has been variably associated with acute appendicitis. 20 An additional concerning finding is the presence of an excessive volume of free abdominopelvic fluid. A small amount of simple free fluid has low specificity for appendicitis in both boys and girls, but a moderate to large amount has been reported to be highly Figure 6. Primary signs of appendicitis (A). Calipers are placed on the outer walls of the dilated, fluid-filled, hyperemic appendix. Note the displacement of the appendix away from adjacent structures by the thickened, echogenic surrounding fat stranding, a secondary sign of appendicitis. Free fluid (*) is also present. Color Doppler (B) shows hyperemia within the appendiceal wall, a primary sign of appendicitis. Free fluid is also demonstrated. A B Figure 7. Grayscale US image (A) of the right lower quadrant (RLQ) shows a dilated, inflamed appendix with surrounding fat stranding and an adjacent complex fluid collection (*) in a patient with a perforated appendicitis and abscess. Color Doppler image (B) of the RLQ shows complex free fluid (*), hyperemia, and fat stranding. A B Pediatric Appendicitis US: Practical Considerations November / December 2024 Applied Radiology 9