Which of the following is associated with cavitation?

• A. thermal index
• B. Avogadro's number
• C. Snell's law
• D. mechanical index

Answer: (D)

Cavitation in ultrasound is driven by the negative pressure portion of the sound wave that can cause gas pockets to expand and, if strong enough, collapse violently. The mechanical index is the metric used to gauge how likely these mechanical bioeffects are during a given ultrasound exposure. It is defined as the peak negative pressure in megapascals divided by the square root of the frequency in megahertz. A higher mechanical index means a higher risk of cavitation, especially at lower frequencies where the same pressure yields more pronounced rarefaction effects. In contrast, the thermal index estimates tissue heating, Avogadro's number is a physical constant with no direct relation to ultrasound bioeffects, and Snell's law governs refraction at interfaces rather than microbubble dynamics. Therefore, the mechanical index best fits cavitation association.