Why do calcifications or bones create shadowing?

• A. They reflect all energy.
• B. They strongly attenuate ultrasound, preventing detection of structures behind them.
• C. They reflect energy forward
• D. They produce posterior enhancement

Answer: (B)

When a dense, highly attenuating structure like calcifications or bone lies in the ultrasound beam, most of the energy is not transmitted beyond it. The wave is strongly attenuated through absorption and scattering, and a large portion is reflected at the near surface. Because very little energy reaches tissues behind the structure, there are few echoes from beyond, so that region appears dark on the image—the shadow. This is different from posterior enhancement, which happens when a beam passes through a low-attenuation material (like fluid) and more energy reaches deeper tissues, increasing echoes there. The bright reflection at the surface is common, but the shadow is due to the lack of transmitted energy after the dense structure.