Mechanical index is an ultrasound

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 metric. It is defined as


where:

• PNP is the peak negative pressure of the ultrasound wave (MPa)
• 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

Food and Drug Administration

The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...

stipulates that diagnostic ultrasound scanners cannot exceed a mechanical index of 1.9.