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Table 2 Direct and indirect (secondary) signs of acute appendicitis in graded-compression, real-time US, colour Doppler and contrast-enhanced US (CEUS; adopted according to references 7, 9, 20 and 21)

From: How to diagnose acute appendicitis: ultrasound first

Real-time US signs of acute appendicitis

Direct signs

Indirect signs

Non-compressibility of the appendix

 Perforation: appendix might be compressible

Free fluid surrounding appendix

Diameter of the appendix > 6 mm

Local abscess formation

Single wall thickness ≥ 3 mm

Increased echogenicity of local mesenteric fat

Target sign:

 Hypoechoic fluid-filled lumen

 Hyperechoic mucosa/submucosa

 Hypoechoic muscularis layer

Enlarged local mesenteric lymph nodes

Appendicolith: hyperechoic with posterior shadowing

Thickening of the peritoneum

Colour Doppler and contrast-enhanced US:

 Hypervascularity in early stages of AA

 Hypo- to avascularity in abscess and necrosis

Signs of secondary small bowel obstruction