What are the common units for attenuation in ultrasound?

• A. Pascal per second
• B. Watts per square meter
• C. Hertz per meter
• D. Decibels per centimeter (dB/cm), often expressed per MHz (dB/cm/MHz)

Answer: (D)

Attenuation is how much the ultrasound signal weakens as it travels through tissue. In practice, we quantify this weakening as a loss of signal per unit length, and we also account for the fact that higher frequencies attenuate more. That’s why the standard units are decibels per centimeter, with an additional frequency term when needed: decibels per centimeter per megahertz (dB/cm/MHz).

Using dB/cm makes the measurement intuitive: it tells you how many decibels of attenuation occur over every centimeter of travel. Adding the per-MHz factor (dB/cm/MHz) reflects the proportional increase in attenuation with frequency, which is a fundamental property of biological tissues. For soft tissue, typical values are around 0.5 to 1.0 dB/(cm·MHz), meaning at 1 MHz you’d expect roughly 0.5–1.0 dB of loss per centimeter, and higher frequencies suffer proportionally more loss per centimeter.

The other units listed correspond to different physical quantities: Pascal per second relates to pressure change over time, watts per square meter to acoustic intensity, and hertz per meter to spatial frequency. None of these describe how much the signal weakens as it propagates, which is exactly what attenuation quantifies.