Impedance is the product of which two tissue properties?

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

Answer: (C)

Acoustic impedance describes how much a tissue resists the passage of a ultrasound wave. It is defined as the product of the tissue’s density and the speed of sound in that tissue, Z = ρc. This means both how heavy the tissue is (density) and how fast sound travels through it (speed) determine impedance. Interfaces with large differences in impedance reflect more sound, which is why boundary contrasts are so important in imaging. The other properties don’t define impedance: attenuation describes energy loss, not resistance to wave flow; frequency affects the wave and its attenuation, not the impedance; and wavelength is derived from speed and frequency, not a direct factor in impedance. So the impedance depends on density and propagation speed.