Theoretical cavitation thresholds in vivo and their relationship to the mechanical index

Abstract

The mechanical index (MI) displayed on the screens of most modern diagnostic ultrasound machines is a relative indication of bioeffect risk. The current definition of the MI, the ratio of the rarefactional pressure in situ to the square root of the center frequency of the acoustic wave, is based on an analytic evaluation of the threshold for violent cavitation of pre-existing gas bubbles in biologically relevant fluids, i.e., water and blood. An alternative approach assuming spontaneous nucleation of bubbles when these liquids are under tension produced by an acoustic field indicated that the threshold was nearly independent of frequency in water but directly proportional to frequency in blood. More recently, it has been suggested that the lipid bilayers surrounding and within individual cells may rupture when subjected to a rarefactional pressure, thereby producing cavitation nuclei that may be further acted upon by the acoustic field. By combining theoretical thresholds for bilayer-nucleation and cavitation in tissue, it is shown that the maximum acoustic pressure permitted by the US FDA for the safe use of diagnostic ultrasound is lower than necessary in most circumstances. An evidence-based safety threshold for non-thermal effects of diagnostic ultrasound will be proposed. (NIH Grant R21EB013763-01)