Why is pulsed ultrasound intensity higher than continuous intensity?

• A. Because energy is delivered only part of the time.
• B. Because energy travels faster in tissue.
• C. Because the beam area is larger.
• D. Because the frequency is higher.

Answer: (A)

Pulsed ultrasound delivers energy in short bursts with off times in between. During each on-period, energy is concentrated into a brief interval, so the power per unit area (the peak or instantaneous intensity) can be higher than in continuous mode. Because energy isn’t delivered during the off periods, the average intensity over time is reduced by the duty cycle. So the higher peak during a pulse comes from concentrating energy into a brief on-time, while the overall exposure stays governed by how long the pulses last and how often they repeat. Other factors like propagation speed, beam area, or frequency don’t explain the higher instantaneous intensity in pulsed mode.