What are the units of intensity used in ultrasound?

• A. Watts
• B. W/cm^2
• C. Joules per square centimeter
• D. Decibels

Answer: (B)

In ultrasound, the quantity called intensity represents how much acoustic power flows through a given area each second. That is power per unit area, so the units reflect both the strength of the beam and how that power is spread across the beam cross-section. The standard way to express this in clinical and physics contexts is watts per square centimeter (W/cm^2). Watts alone would be total power and ignore how the beam is distributed over area, while joules per square centimeter is energy per area (not a rate of energy transfer). Decibels measure a relative level on a logarithmic scale, not an absolute unit of intensity. So the appropriate unit for ultrasound intensity is W/cm^2.