Mechanical index (MI) is a unitless

ultrasound Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies ...

metric. It is defined as :

\text = \frac,

where :*P''_r'' is the peak rarefaction

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

of the ultrasound wave (

MPa MPA or mPa may refer to: Academia Academic degrees * Master of Performing Arts * Master of Professional Accountancy * Master of Public Administration * Master of Public Affairs Schools * Mesa Preparatory Academy * Morgan Park Academy * Mou ...

), derated by an attenuation factor to account for in-tissue acoustic attenuation :*''f_c'' is the

center frequency In electrical engineering and telecommunications, the center frequency of a filter or channel is a measure of a central frequency between the upper and lower cutoff frequencies. It is usually defined as either the arithmetic mean or the geometri ...

of the ultrasound pulse (

MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...

). 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 The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...

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