What’s the difference between posterior acoustic shadowing versus
enhancement on ultrasound?

Posterior acoustic shadowing and enhancement are opposite artifacts that help you understand what a structure is made of. Shadowing happens when the sound beam can’t pass through something dense, like bone or calcification. The tissue blocks the beam, and you see a dark band underneath because no sound returns from that deeper area. In MSK ultrasound, this is why the cortex of a bone casts a clean, dark shadow.

Enhancement is the opposite. Fluid-filled structures—like cysts—let sound pass through easily, so more energy reaches the tissues behind them. That deeper area looks brighter because the returning echoes are stronger than expected. When you see clean enhancement under a dark, fluid-filled space, it’s a strong clue you’re looking at true fluid, not solid tissue.