Define the safety indices Mechanical Index (MI) and Thermal Index (TI).

• A. MI estimates tissue heating; TI estimates mechanical bioeffects.
• B. MI estimates peak rarefaction pressure relative to the square root of frequency; TI estimates potential tissue heating.
• C. MI estimates acoustic impedance; TI estimates acoustic power.
• D. MI and TI both measure tissue elasticity.

Answer: (B)

The concept being tested is how ultrasound safety indices relate to different biological effects. The Mechanical Index measures the potential for mechanical bioeffects, such as cavitation, by combining the peak rarefaction pressure with the frequency. Specifically, it uses peak negative pressure divided by the square root of frequency, so higher pressures and lower frequencies raise the index and indicate greater risk of mechanical effects.

The Thermal Index, on the other hand, estimates the potential for tissue heating. It serves as a proxy for how much the tissue temperature could rise during exposure, based on the acoustic output, duration, and tissue characteristics. A higher Thermal Index suggests a greater heating risk.

This description matches the statement that the Mechanical Index estimates peak rarefaction pressure relative to the square root of frequency, while the Thermal Index estimates potential tissue heating. The other options mix up these concepts or reference unrelated properties like impedance, power, or elasticity, which is why they’re not correct.