# Mechanical index

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Mechanical index (MI) is an ultrasound metric. It is defined as [1]

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

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
• Fc 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.[2]

## References

1. ^ Thomas Szabo (2004). Diagnostic Ultrasound Imaging: Inside Out (Biomedical Engineering). ISBN 978-0-12-680145-3
2. ^ http://www.accessdata.fda.gov/cdrh_docs/pdf9/K091970.pdf (accessed July 2010)