Which of the following conditions yields the highest mechanical index?

• A. High frequency and low power
• B. High intensity and high frequency
• C. Low frequency and high signal amplitude
• D. Low intensity and low frequency

Answer: (C)

Mechanical Index gauges the likelihood of cavitation and rises with stronger negative pressure and lower frequency. It’s roughly proportional to p_neg divided by the square root of frequency (MI ≈ p_neg / √f). So, the highest MI occurs when the ultrasound has a large peak negative pressure (high signal amplitude) and a low frequency, which maximizing p_neg and keeping √f small. The option with low frequency and high signal amplitude fits that best, producing the largest MI. The other scenarios either lower the peak negative pressure or raise the frequency, which reduces MI even if other factors like intensity or power are high.