Which change would increase the mechanical index?

• A. Increase frequency
• B. Increase transmitted power
• C. Decrease peak rarefaction pressure
• D. Increase imaging depth

Answer: (B)

Mechanical Index reflects cavitation potential and is determined by how strong the peak rarefaction pressure is relative to the square root of the frequency. Since MI = peak negative pressure divided by sqrt(frequency), increasing the transmitted power raises the peak negative pressure, which directly increases the MI.

Increasing frequency would lower the MI because the denominator sqrt(frequency) grows. Decreasing the peak rarefaction pressure lowers the MI, and imaging depth by itself doesn’t set MI—the pressure amplitude and frequency do. Therefore, increasing transmitted power is the change that increases the mechanical index.