Appendix on Ultrasound

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Features of a normal appendix:

• Usually a blind-ending tubular structure arising from the cecum/ileocaecal junction. 

• Diameter (outer wall to outer wall) ≤ 6 mm is generally accepted as normal. 

• Thin wall, typically less than ~3 mm in thickness. 

• Compressible under graded probe pressure (in many cases). 

• No significant surrounding fat-echogenicity, free fluid, or hyperaemia on Doppler. 

• Often fairly difficult to visualise completely; non-visualisation does not exclude a normal appendix. 

Technique tips:

• Use high-frequency linear transducer for superficial RLQ imaging.

• Start at the area of tenderness (e.g., right lower quadrant/McBurney’s point) then trace the colon to the cecum, and follow any blind-ending tubular structure. 

• Apply graded compression to displace bowel gas and assess compressibility.

• Visualise the appendix in both transverse and longitudinal planes.

🚨 Abnormal Appendix (Appendicitis) on Ultrasound

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Key ultrasound features suggestive of appendicitis:

• Diameter > 6 mm (outer wall to outer wall) in cross-section is a common cutoff, though there is overlap with normal values. 

• Non-compressible tubular structure (when graded compression is applied) arising from the cecum. 

• Blind-ending, often with “target sign” or “bull’s-eye” appearance in transverse view (hypoechoic centre + echogenic wall). 

• Appendicolith (echogenic focus with posterior acoustic shadowing) may be present. 

• Hyperechoic surrounding fat (indicating inflammation/edema of adjacent fat) or pericecal fluid. 

• Increased vascularity/hyperemia in wall on colour Doppler in many cases (“ring of fire” sign). 

• Secondary signs: loss of normal wall layering, thickened wall, possibly perforation signs (fluid collection, abscess, phlegmon). 

Important caveats:

• Despite the classic cutoff of >6 mm, there is overlap: some normal appendices may measure more than 6 mm, and some inflamed ones less. 

• Visualization of the appendix may fail (especially retro-cecal or in obese patients) — non-visualization does not exclude appendicitis. 

🧷 Summary Table: Normal vs Appendicitis

📌 Key Take-Away Tips for Practice

• Always include both transverse and longitudinal scans of the appendix.

• Use graded compression to try to displace bowel gas and assess compressibility.

• Measure diameter carefully (outer wall to outer wall) and document if >6 mm.

• Look for ancillary signs (fat stranding, fluid, appendicolith, Doppler hyperemia) rather than relying solely on diameter.

• If you cannot visualise the appendix and suspicion remains high clinically, further imaging (CT or MRI) may be justified.