Ultrasound of a 55-year-old man with jaundice shows dilated intrahepatic and extrahepatic bile ducts with a hyperechoic focus in the common bile duct casting an acoustic shadow posteriorly. CT confirms a calculus at the distal common bile duct. What is this acoustic shadow phenomenon called and what does it indicate?

• A Posterior acoustic enhancement; fluid-filled structure
• B Ring-down artifact; gas-containing structure
• C Mirror artifact; reflective interface
• D Posterior acoustic shadowing; calcified or dense structure ✓

Explanation

Posterior acoustic shadowing is produced by highly attenuating structures such as gallstones, calcifications, and bone that absorb or reflect the ultrasound beam, preventing visualization of tissues posterior to the structure. The shadowing appears as a dark cone-shaped area behind the echogenic focus. This is the key sonographic sign of cholelithiasis and choledocholithiasis. Posterior acoustic enhancement occurs behind fluid-filled cysts. Ring-down artifact is seen with gas, producing comet-tail reverberation artifacts. Mirror artifact creates a false duplication across a strong reflector.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.