Acoustic shadowing occurs when a highly reflective or absorptive structure, such as a gallstone, calcification, or bone, blocks the ultrasound beam from passing through to deeper tissues. The area behind the obstructing structure appears dark or anechoic on the image because no echoes are being returned from that region. This artifact is actually very useful diagnostically because it helps confirm the presence of calcified structures like kidney stones and gallstones. Sonographers learn to look for the characteristic bright structure with a clean dark shadow trailing behind it as a reliable indicator of calcification.
Sonography Term