Which term represents the fraction of time that an ultrasound pulse is on?

• A. Energy
• B. Duty factor
• C. Dispersion
• D. Fundamental frequency

Answer: (B)

The main idea is how long the ultrasound beam is actively emitting within each cycle. The fraction of time the pulse is on is called the duty factor (also known as duty cycle). It’s defined as the pulse duration divided by the pulse repetition period, and is a unitless value usually expressed as a percentage. For example, if a pulse lasts 2 microseconds and the time between pulse starts is 20 microseconds, the duty factor is 2/20 = 0.1, or 10%. A higher duty factor means more of the time the machine is emitting, leading to higher average intensity and potential heating; continuous-wave ultrasound has a duty factor of 1 (always on). The other terms aren’t about how long the pulse is on: energy is a quantity of work or power over time, dispersion describes frequency spreading, and fundamental frequency is the basic frequency of the waveform.