Edge shadowing (or edge artifact) appears as thin dark shadows at the lateral margins of round or curved structures such as cysts, vessels, or the gallbladder wall. It is caused by refraction — the bending of the ultrasound beam as it passes through the curved interface — which deflects some energy away from the transducer. Edge shadowing is a helpful secondary sign that confirms the round, smooth-walled, fluid-filled nature of a structure.