Mechanical Index

Estimate of the acoustic energy of an ultrasound field used in clinical sonography. The mechanical index (MI) is displayed on the screen of modern US scanners. It is defined as the estimated peak rarefractional pressure in vivo, divided by the square root of the center frequency of the beam. The MI was developed to quantify the likelihood that exposure to diagnostic ultrasound will produce an adverse biological effect by a nonthermal mechanism. It is also used to control the amplitude of the US during contrast‐enhanced sonography, since the amplitude used determines the nature and degree of nonlinear bubble behavior (cross reference) and thus the echo response obtained from the microbubble contrast agent.

Contrast Media, Ultrasound, Hepatic