Which statement defines frequency in ultrasound?

• A. The number of compression/rarefaction cycles per second, expressed in hertz.
• B. The rate of tissue heating.
• C. The duration of the ultrasound pulse.
• D. The amplitude of the wave.

Answer: (A)

Frequency is the number of compression/rarefaction cycles that occur each second, expressed in hertz. This tells you how often the ultrasound wave repeats over time, not how much energy it carries or how long it lasts. In therapy, higher frequency means shallower penetration with better resolution, while lower frequency penetrates deeper but with less detail. The rate at which tissue heats, the duration of the ultrasound pulse, and the wave’s strength are determined by other parameters (intensity/duty cycle, pulse duration, and amplitude), so they are not defining frequency.