Mechanical index

Mechanical index is a unitless ultrasound metric originally developed by Robert Apfel and Christy Holland. It is defined as
where
MI is measured with a calibrated hydrophone in a tank of degassed water. The pulse pressure amplitudes are measured along the central axis of the ultrasound beam. The P_r is calculated by reducing it using an attenuation coefficient of 0.3 dB/cm/MHz.
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 Food and [Drug Administration|FDA] stipulates that diagnostic ultrasound scanners cleared using the Federal Food, Drug, and [Cosmetic Act#Premarket_notification_(510(k),_PMN)|510(k)] pathway
cannot exceed a mechanical index of 1.9.