Sound scatters when the tissue interface is _______.

• A. Large
• B. Rough
• C. Thin
• D. Small

Answer: (D)

Sound scatters when many tiny features are present in the tissue that are small compared to, or on the order of, the ultrasound wavelength. Each tiny feature acts as a separate scatterer, sending waves in multiple directions. When you have a small interface, you effectively have multiple small scattering centers, so the energy is dispersed rather than reflected in a single direction. In contrast, large, smooth interfaces tend to reflect most of the energy in one direction (specular reflection), and roughness alone can diffuse the beam but the prevailing factor for widespread scattering is the presence of small features. So, an interface that is small behaves as multiple point scatterers and causes scattering of the sound.