Impedance is defined as the product of propagation speed and density. Which pair of properties is involved?

• A. Density and Attenuation
• B. Density and Frequency
• C. Propagation speed and Density
• D. Propagation speed and Wavelength

Answer: (C)

Impedance in ultrasound is defined as Z = ρ c, where ρ is density and c is the speed of sound (propagation speed) in the medium. This means the pair of properties involved is density together with propagation speed. The product ρ c characterizes how the medium resists the passage of the wave; higher impedance contrast at a boundary leads to more of the wave being reflected rather than transmitted. Frequency or wavelength do not form the impedance, though they influence other aspects of imaging. Attenuation describes energy loss, not the intrinsic impedance.