Which statement best describes the difference between specular and Rayleigh scattering in tissue, and what is a typical ultrasound example of each?

• A. Specular scattering results from many small scatterers; example: red blood cells. Rayleigh scattering comes from smooth vessel walls.
• B. Specular scattering results from smooth surfaces larger than the wavelength; example: vessel wall. Rayleigh scattering comes from many small scatterers smaller than the wavelength; example: red blood cells.
• C. Specular scattering: rough tissue; Rayleigh: smooth bone surface.
• D. Specular scattering: single-point target; Rayleigh: diffuse reflector.

Answer: (B)

Scattering in ultrasound depends on how the size of the structures compares to the wavelength. When the boundary is smooth and its dimensions exceed the wavelength, the wave reflects in a single preferred direction—this is specular reflection. Vessel walls act like smooth interfaces that are larger than the ultrasound wavelength, so you see a clear, specular echo from them.

When there are many tiny particles much smaller than the wavelength, each scatters in various directions and the backscatter is diffuse. This Rayleigh-type scattering from numerous small scatterers builds up the speckle you see in the image. Red blood cells are only a few micrometers across, far smaller than typical tissue wavelengths, so they produce this diffuse backscatter.

So the best description is: specular scattering from smooth surfaces larger than the wavelength (vessel wall), and Rayleigh scattering from many small scatterers smaller than the wavelength (red blood cells).