Which unit is used for intensity?

Which unit is used for intensity?

• A. Watts per square meter
• B. Joules
• C. Watts per square centimeter
• D. Decibels

Answer: (C)

Intense energy crossing tissue is described by how much power arrives per unit area. That means intensity is power divided by area. In ultrasound practice, this is expressed as watts per square centimeter because clinicians measure the beam over centimeter-scale areas and care about how much power is delivered to each square centimeter of tissue.

Joules is energy, not a rate—so it doesn’t describe how concentrated the beam is at a location. Decibels are a logarithmic ratio used to compare levels, not a direct unit of intensity. Watts per square meter is technically the SI unit for intensity, but the conventional clinical unit used for ultrasound is watts per square centimeter, which is why this option best matches typical practice.

For a quick example: if the probe emits 4 watts over an area of 0.5 cm^2, the intensity is 4 W / 0.5 cm^2 = 8 W/cm^2.