Mechanical index

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

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).

MI is measured with a calibrated hydrophone in a tank of water. Derated (0.3 dB/cm/MHz) Pulse Pressure Squared Integral (ppsi) values are measured along the axis of the ultrasound beam. (ppsi is similar to intensity.) The PNP is measured at the position where the derated ppsi is at a maximum.

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

  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)
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