Define mechanical index (MI) and describe how it scales with peak negative pressure and frequency.

• A. MI = peak negative pressure / sqrt(frequency); higher MI implies greater potential for bioeffects
• B. MI = peak negative pressure × frequency; higher MI implies greater energy
• C. MI = peak negative pressure / frequency; higher MI implies lower risk
• D. MI = peak negative pressure / frequency; independent of pressure

Answer: (A)

Mechanical Index quantifies the potential for cavitation by tying together how large the negative pressure swings are and how high the ultrasound frequency is. It is defined as the peak negative pressure (in MPa) divided by the square root of the center frequency (in MHz). This means that as the peak negative pressure increases, the MI increases, signaling a higher potential for bioeffects. As the center frequency increases, the MI decreases for a given pressure because you’re dividing by the square root of frequency. The square-root dependence reflects how cavitation thresholds depend on frequency, so a higher MI indicates greater potential for bioeffects; formulations that multiply pressures or divide by frequency without the square root don’t capture the same relationship.