If the propagation speed is unchanged at the interface, which sound-tissue interaction is most likely when the ultrasound wave strikes?

• A. Specular reflection
• B. Scattering
• C. Diffraction
• D. Refraction

Answer: (B)

When ultrasound enters tissue and the speed of sound remains the same across the interface, there is no bending of the wave path (no refraction). However, soft tissue is full of tiny structures and inhomogeneities whose sizes are comparable to the ultrasound wavelength. These small features disrupt the wave and cause energy to scatter in many directions rather than reflecting cleanly off a smooth boundary. That diffuse scattering is the dominant interaction in such a scenario, producing backscatter and the characteristic speckle pattern seen in ultrasound images. Specular reflection would require a smooth, well-defined boundary with a significant impedance difference, which isn’t the general case here, and diffraction is mainly about waves bending around obstacles rather than scattering from numerous microscopic inhomogeneities.