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

${\displaystyle {\text{MI}}={\frac {P_{r}}{\sqrt {f_{c}}}},}$

where

• P_r is the peak rarefaction pressure of the ultrasound wave (MPa), derated by an attenuation factor to account for in-tissue acoustic attenuation
• f_c is the center frequency of the ultrasound pulse (MHz).

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

References

1. Thomas Szabo (2004). Diagnostic Ultrasound Imaging: Inside Out (Biomedical Engineering). ISBN 978-0-12-680145-3
2. Postema M (2011). Fundamentals of Medical Ultrasonics. London: CRC Press. doi:10.1201/9781482266641. ISBN 978-0-203-86350-3.
3. http://www.accessdata.fda.gov/cdrh_docs/pdf9/K091970.pdf (accessed July 2010)

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