Rayleigh scattering occurs when scatterers are smaller than wavelength.

• A. Larger than wavelength
• B. Smaller than wavelength
• C. About the same as wavelength
• D. Variable depending on tissue type

Answer: (B)

Rayleigh scattering depends on the size of the scatterers relative to the wavelength. When the scatterers are much smaller than the wavelength, they act as tiny point sources and scatter energy uniformly in all directions. This regime is exactly what Rayleigh scattering describes. In tissue, many microscopic structures are smaller than the ultrasound wavelength, so their scattering falls into this category, contributing to the characteristic, more isotropic backscatter pattern. If scatterers were larger or about the same size as the wavelength, other scattering regimes (like Mie or geometric) would dominate, with different angular distribution and wavelength dependence. Therefore, the statement that Rayleigh scattering occurs when scatterers are smaller than the wavelength is the correct description.