Acoustic shadowing occurs when an ultrasound beam is strongly reflected or absorbed by a structure — such as a gallstone, renal calculus, calcified plaque, or gas pocket — leaving a dark shadow in the tissue behind it. The shadow forms because little or no sound energy passes through to create echoes beyond the obstructing structure. Shadowing is a key diagnostic feature: clean shadowing from a calcification, for example, is a hallmark finding for gallstones and kidney stones.