The ultrasound parameter most closely associated with cavitation risk is

• A. Mechanical Index
• B. Thermal Index
• C. Snell's Law
• D. Avogadro's Number

Answer: (A)

Cavitation risk arises from the large pressure fluctuations ultrasound can create, which can cause microbubbles to form and violently collapse. The Mechanical Index captures this potential by combining the peak rarefactional pressure with the square root of the frequency, roughly indicating how likely inertial cavitation is under a given exposure. A higher MI means stronger tensile pressures at a given frequency, and since lowering the frequency increases MI for the same pressure amplitude, cavitation is more likely at lower frequencies. This is why MI is the parameter most closely tied to cavitation risk and is actively monitored to keep exposure within safe limits, especially during Doppler and contrast-enhanced studies.

Thermal Index relates to tissue heating from ultrasound and informs thermal safety, but not cavitation risk. Snell's Law describes how the beam refracts at interfaces, not bubble formation. Avogadro's number is a chemistry constant unrelated to ultrasound physics.