Mechanical index

Mechanical index (MI) is a unitless ultrasound metric. It is defined as 
where 
MI is measured with a calibrated hydrophone in a tank of water. The Pulse Pressure Squared Integral (ppsi) values are measured along the axis of the ultrasound beam. The P"r" is calculated by applying an attenuation coefficient to the maximum negative ppsi.
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.