Red blood cells are considered _______ because they are smaller than the wavelength of the sound beam.

• A. Specular reflectors
• B. Rayleigh scatterers
• C. Mie scatterers
• D. Absorbers

Answer: (B)

When a scatterer is much smaller than the ultrasound wavelength, it scatters sound like a tiny point into many directions in a way described by Rayleigh scattering. Red blood cells are about 6–8 micrometers in diameter, while the wavelength in tissue at diagnostic frequencies (roughly 2–15 MHz) is around 0.2–0.8 millimeters. That means the cells are far smaller than the wavelength, placing them in the Rayleigh regime. In this regime, the scattered intensity depends strongly on the size and the frequency, which is why blood cells contribute noticeable backscatter even though each cell is small. If the scatterer were large compared to the wavelength, you’d get specular reflections from interfaces; if the scatterer were about the same size as the wavelength, you'd be in the Mie scattering region with a different angular pattern; absorbers, meanwhile, are about converting sound energy to heat and aren’t defined by small size relative to wavelength.