What is the average intensity during the pulse repetition period?

• A. Temporal Average
• B. Pulse Average
• C. Spatial Average
• D. In Vivo

Answer: (A)

The concept being tested is how intensity is averaged over time when the ultrasound is delivered in pulses. The average intensity during the pulse repetition period is temporal average intensity. This is because the pulse repetition period includes both the on-time (when energy is being emitted) and the off-time (when no energy is emitted). Temporal average takes into account the energy delivered across the entire period, effectively averaging the beam’s intensity over the full cycle.

If the instantaneous intensity during the pulse is I_p and the pulse duration is t_p with a repetition period T (so the off-time is T − t_p), the duty factor DF = t_p / T. The temporal average intensity equals I_p multiplied by the duty factor (I_p × DF), reflecting how much energy is delivered on average per unit time over the whole cycle.

This differs from pulse-average intensity, which only averages during the pulse itself and ignores the off-time. Spatial average concerns averaging across the beam’s cross-sectional area, not over time. In vivo is a context term, not a type of average.