Mechanical index

Mechanical index (MI) is a unitless ultrasound metric originally developed by Robert Apfel and Christy Holland. ^[1] It is defined as ^[2]

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

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. ^[3]

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 cleared using the 510(k) pathway cannot exceed a mechanical index of 1.9. ^[4]

References

1. Apfel, RE; Holland, CK (1991). "Gauging the likelihood of cavitation from short-pulse, low-duty cycle diagnostic ultrasound". Ultrasound in Medicine and Biology. 17 (2): 179. doi:10.1016/0301-5629(91)90125-g. PMID   2053214.{{cite journal}}: |access-date= requires |url= (help); More than one of |pages= and |page= specified (help)
2. Thomas Szabo (2004). Diagnostic Ultrasound Imaging: Inside Out (Biomedical Engineering). ISBN   978-0-12-680145-3
3. Postema M (2011). Fundamentals of Medical Ultrasonics. London: CRC Press. doi:10.1201/9781482266641. ISBN   978-0-203-86350-3.
4. U.S. Food and Drug Administration. "Marketing Clearance of Diagnostic Ultrasound Systems and Transducers" . Retrieved 22 December 2025.