All intensities have units of _____.

• A. W/cm2
• B. dB
• C. W/m2
• D. mW/cm2

Answer: (A)

Intensity is power per unit area, so in the ultrasound context the common way to express it is as watts per square centimeter. This W/cm^2 convention is used because beam energies are routinely discussed over tissue areas measured in square centimeters on the transducer face. While the SI base unit for radiant power per area is W/m^2, the field often adopts W/cm^2 for practicality, since 1 W/cm^2 equals 10,000 W/m^2. Decibels are for relative levels, not absolute units, and milliwatts per square centimeter is simply a smaller scale of the same concept. That’s why W/cm^2 is the standard unit highlighted in this context.