Mechanical index (MI) is a unitless
ultrasound metric. It is defined as
:
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.
[http://www.accessdata.fda.gov/cdrh_docs/pdf9/K091970.pdf (accessed July 2010)]
References
{{Reflist
Acoustics
Ultrasound
Medical physics
Medical ultrasonography