Give typical speeds of sound for fat and bone and explain how these differences affect propagation.

• A. Fat ≈ 1450 m/s; bone ≈ 3500–4000 m/s; large speed contrasts cause refraction, strong reflections, and shadowing
• B. Fat ≈ 1540 m/s; bone ≈ 1000 m/s; speed contrasts cause less reflection
• C. Fat ≈ 1000 m/s; bone ≈ 5000 m/s; reduces refraction
• D. Fat ≈ 1700 m/s; bone ≈ 3000 m/s; speed contrasts have no effect

Answer: (A)

The main idea is how differences in sound speed at tissue interfaces control what the ultrasound wave does as it meets boundaries. Fat has a speed of about 1450 m/s, while bone is much faster, around 3500–4000 m/s. That big difference creates a large acoustic impedance mismatch between fat (or soft tissue) and bone. When the wave encounters such a boundary, a large portion of the energy is reflected back toward the transducer, producing strong echoes at the interface. The remaining energy may refract, changing direction according to Snell’s law, but the high-speed, high-impedance bone also attenuates and absorbs a lot of energy, leading to little transmission beyond the bone and resulting shadowing behind it. These effects—strong reflections and shadowing—are characteristic of the large speed (and impedance) contrasts between bone and surrounding tissues. The other options use speeds that don’t match typical tissue values or misstate the consequence of speed differences, which would still include reflections and refraction.