Which statement about power in ultrasound is true?

• A. It is independent of the sound source
• B. It is energy per unit volume
• C. It is the speed of energy transfer
• D. It is measured in watts, and is adjustable by the user

Answer: (D)

Power in ultrasound is the rate at which energy is delivered to tissue, so it is measured in watts and can be adjusted by the user using the machine’s output control. This means increasing the output control increases how quickly energy reaches the tissue, affecting potential thermal and mechanical effects. The other statements mix up different quantities: energy per unit volume describes energy density, not power; the speed of energy transfer describes the propagation velocity, not the rate of energy delivery; and power is not independent of the sound source—it depends on the transducer and the machine settings.