Section 7‐‐discussion of the mechanical index and other exposure parameters. American Institute of Ultrasound in Medicine.

There have been long‐term efforts to identify a threshold pressure for the onset of inertial cavitation under conditions relevant to ultrasound in medicine. Before the introduction of the output display standard [AIUM/NEMA, 1992a], quantities such as the spatial peak pulse average intensity (I(SPPA)), and, earlier, Im, the spatial peak intensity averaged over the largest half‐cycle, were used to give a measure of the potential of a cavitation‐based bioeffect due to an acoustic field. Relatively early in the Output Display Standard development effort, the Food and Drug Administration indicated a need for a superior indicator for the potential for cavitation‐related bioeffects, initiating a search for such an index. The following paragraphs give an outline of the steps used to develop the Mechanical Index, its relevance as a potential bioeffects indicator, and some information on other exposure parameters involved in bioeffects research.