Mechanical index

Mechanical index (MI) is an ultrasound metric. It is defined as

$\text{MI} = \frac{\text{PNP}}{\sqrt{F_c}},$

where

• PNP is the peak negative pressure of the ultrasound wave (MPa), derated by 0.3 dB cm⁻¹ MHz⁻¹ to account for the difference between in-water and in-tissue acoustic attenuation, and
• F_c is the center frequency of the ultrasound wave (MHz).

Mechanical index can be used as an estimate for the degree of bio-effects a given set of ultrasound parameters will induce. A higher mechanical index means a larger bio-effect. Currently the FDA stipulates that diagnostic ultrasound scanners cannot exceed a mechanical index of 1.9.