Charles C. Church
Molecular Biosystems, Inc., 10030 Barnes Canyon Rd., San Diego, CA 92121
The mechanical index (MI) is a means of providing the users of diagnostic ultrasound equipment with a real-time, on screen display of the potential for a given exposure to produce tissue damage by mechanical means. As originally conceived, MI was based on a calculation of the lowest rarefactional pressure P[sub r], needed to produce the growth and subsequent collapse of an optimally sized, pre-existing gas bubble such that the peak temperature attained upon collapse was at least 5000 K. While the assumption that stabilized microbubbles exist within the body may be necessary when assessing the safety of medical exposures to ultrasound, that assumption is unproven. If it is assumed that there are no pre-existing, stabilized gas bubbles in tissue, it is possible to calculate a ``maximum threshold'' rarefactional pressure amplitude P[sub rmt], above which cavitation is highly probable. By combining the theory for spontaneous homogeneous nucleation and the rate of cavitation observed during clinical lithotripsy procedures, an estimate of this (frequency-independent) threshold may be obtained. Assuming that an ``acceptable'' cavitation rate in tissue is 10[sup -20] events/m[sup 3]/s and that an additional clinical safety factor of 2 is desirable, it is possible to show that P[sub rmt]=3 MPa. For comparison, at a frequency of 5 MHz, if MI=0.4, then P[sub r]=0.9 MPa; if MI=1.0, P[sub r]=2.2 MPa. [Work supported by NIH.]