Mechanical index

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

{\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 wave (MHz).

MI is measured with a calibrated hydrophone in a tank of water. The Pulse Pressure Squared Integral (ppsi) values are measured along the axis of the ultrasound beam. The P_r is calculated by applying an attenuation coefficient to the maximum negative ppsi.

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.[2]

References

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

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[1] Thomas Szabo (2004). Diagnostic Ultrasound Imaging: Inside Out (Biomedical Engineering). ISBN 978-0-12-680145-3

[2] ttp://www.accessdata.fda.gov/cdrh_docs/pdf9/K091970.pdf (accessed July 2010)