Which tissue feature typically produces Rayleigh scattering in ultrasound imaging?

• A. Red blood cells.
• B. Vessel walls.
• C. Large fibrous tissue.
• D. Smooth organ surfaces.

Answer: (A)

Rayleigh scattering happens when the individual scatterers are much smaller than the ultrasound wavelength, so each one scatters sound nearly uniformly in all directions and many of them within a single voxel add up to produce diffuse backscatter. Red blood cells fit this size: only a few micrometers across, well below the wavelength used in typical diagnostic ultrasound (a few hundred micrometers). With so many RBCs in a resolution cell, their collective backscatter is diffuse and creates the granular speckle texture seen in ultrasound images. In contrast, vessel walls and large fibrous tissue are larger than the wavelength and tend to produce strong, directional (specular) reflections from boundaries, while smooth organ surfaces also give coherent reflections rather than Rayleigh-type diffuse backscatter.