Posterior acoustic shadowing

Posterior acoustic shadowing is an ultrasound artifact in which a dark (anechoic or hypoechoic) band appears directly behind a highly reflective or highly attenuating structure. This shadow forms because the dense structure absorbs or reflects most of the ultrasound beam, preventing echoes from being generated beyond it. The most classic example is the “clean shadow” seen behind a gallstone — the stone appears as a bright echogenic focus with a dark shadow trailing behind it. Calcifications in any organ, dense bone, and surgical clips can all produce acoustic shadowing. Recognizing this artifact is essential for sonographers because it helps confirm the presence of calcified structures and distinguishes true findings from imaging limitations.