Which of the following is an example of a Rayleigh scatterer?

• A. Renal capsule
• B. Arterial wall
• C. Red blood cell
• D. Bowel gas

Answer: (C)

Rayleigh scatterers are tiny particles whose size is much smaller than the ultrasound wavelength, so they scatter sound in a simple, predictable way. In diagnostic ultrasound, wavelengths are on the order of a few hundred micrometers. Red blood cells are about 6–8 micrometers in diameter, which is far smaller than the wavelength, so they fit the Rayleigh regime. This small size means many such particles present in blood contribute to the scattered signal, producing the diffuse, texture-like backscatter characteristic of blood flow.

The other structures are comparatively large relative to the wavelength and don’t scatter in the Rayleigh manner; they produce scattering dominated by larger interfaces or by bubbles, which falls into different regimes (Mie or strong specular reflections). Hence, red blood cells are the clear example of a Rayleigh scatterer.