How is ultrasound intensity defined?

• A. Power (watts)/ERA (cm2)
• B. Energy (joules)/Area
• C. Voltage (V)/Current (A)
• D. Power (watts) x ERA (cm2)

Answer: (A)

Ultrasound intensity is the power delivered per unit area. It is defined as power divided by the effective radiating area (ERA) of the transducer: I = P / ERA. This gives units of watts per square centimeter (W/cm^2). The ERA represents the area over which the energy is spread at the tissue surface.

Understanding why: power is energy per unit time, and spreading that power over a specific area defines how much energy is arriving at each square centimeter each second. Energy per area (J/cm^2) would describe energy density, not intensity. Electrical quantities like voltage or current don’t directly describe the acoustic energy delivery, and multiplying power by area would not yield the correct rate per area. Duty cycle can affect the average intensity over time, but the basic definition remains Power divided by ERA.