What is the formula for pulse duration?

• A. Pulse duration equals frequency times depth
• B. Pulse duration equals speed times wavelength
• C. Pulse duration equals number of cycles x period
• D. Pulse duration equals impedance x time

Answer: (C)

Pulse duration is the total time from the start to the end of a single ultrasound pulse. It depends on how many cycles are included in that pulse and how long each cycle lasts. Each cycle has a period T (the time for one cycle), and T is the reciprocal of the frequency (T = 1/f). If the pulse contains n cycles, the duration is n times T, so PD = nT = n/f. This also ties to the idea that a pulse of n cycles lasts longer when the frequency is lower, and shorter when the frequency is higher.

For example, a 2 MHz transducer sending a 4-cycle pulse has a period of 0.5 microseconds, so the pulse duration is 4 × 0.5 μs = 2 μs.

Why the other ways don’t fit: speed times wavelength would give a quantity with the wrong units for time; frequency times depth mixes unrelated quantities. Impedance times time also combines unrelated physical properties and doesn’t yield a valid pulse duration. The correct relation is counting cycles and multiplying by the cycle duration.