The Mechanical Index (MI) is related to the likelihood of cavitation, which is the formation of gas bubbles in a liquid due to the low-pressure regions of the ultrasound wave. MI is a parameter that predicts the potential for mechanical bioeffects, including cavitation. A higher MI indicates a greater likelihood of cavitation occurring. It is calculated based on the peak negative pressure and the frequency of the ultrasound wave.
Reference:
ARDMS Sonography Principles and Instrumentation guidelines
Kremkau, F. W. (2015). Diagnostic Ultrasound: Principles and Instruments.