Acoustic impedance increases with which of the following factors?

• A. Increased tissue density
• B. Increased propagation speed
• C. Increased transducer frequency
• D. A and B

Answer: (D)

Acoustic impedance is defined by Z = ρ c, where ρ is tissue density and c is the speed of sound in that tissue. If either density or the propagation speed increases, the impedance increases because it’s the product of those two properties. Frequency, on the other hand, does not appear in this equation, so changing the transducer frequency doesn’t change impedance (it mainly affects attenuation and resolution). So impedance rises with increased tissue density and with increased propagation speed, but not with frequency. That’s why the correct factor combination is both density and propagation speed.