Rayleigh scattering in ultrasound occurs when scatterers are much smaller than the wavelength.

• A. True
• B. False
• C. Only at high frequencies
• D. Only in bone

Answer: (A)

Rayleigh scattering describes how ultrasound echoes come from structures that are much smaller than the acoustic wavelength. When a scatterer’s size is far less than λ, the scattered field acts like a tiny dipole and the backscattered intensity grows rapidly with frequency (roughly as f^4). This helps explain why small elements like red blood cells produce diffuse, speckled backscatter in soft tissues, especially at higher frequencies. The statement is correct because the defining factor is the size relationship to the wavelength, not frequency alone or location in bone.