Acoustic Shadowing: Dark posterior stripe on ultrasound caused by near-complete reflection or absorption of sound by a dense structure such as a gallstone, renal calculus, or bone.
Acoustic shadowing is the absence of echoes posterior to a highly reflective or attenuating structure, appearing as a dark stripe behind the object on ultrasound. It is the cardinal sonographic sign of gallstones, renal calculi, and calcified structures. The shadow arises because sound energy is either reflected (specular shadow) or absorbed (attenuating shadow) before it can reach deeper tissues.