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Posterior acoustic shadowing
Posterior (acoustic) shadowing is characterized by the apparent lack of signal void deeper to an imaged structure. The transmitted sound wave is being reflected back to the transducer or absorbed by the tissue so the tissue underneath is not visible. It commonly occurs when an area of interest contains a high Z/solid tissue (e.g. calcified gallstone or bone) or at an interface with high acoustic impedance mismatch (e.g. soft tissue/air).