Mechanical index (MI) is an ultrasound metric. It is defined as
where
MI is measured with a calibrated hydrophone in a tank of water. Derated (0.3 dB/cm/MHz) Pulse Pressure Squared Integral (ppsi) values are measured along the axis of the ultrasound beam. (ppsi is similar to intensity.) The PMP is measured at the position where the derated ppsi is at a maximum.
MI is a unitless number that can be used as an index of cavitation bio-effects; a higher MI value indicates greater exposure. Levels below 0.3 are generally considered to have no detectable effects. Currently the FDA stipulates that diagnostic ultrasound scanners cannot exceed a mechanical index of 1.9.
References
Mechanical index Wikipedia(Text) CC BY-SA