Effect of periodic phase shift on acoustic cavitation induction with second-harmonic superimposition

Second-harmonic superimposition (SHS) is an ultrasound exposure method for efficiently inducing cavitation both in vitro and in vivo. Its efficiency is dependent on SH phase relative to the fundamental. In this study, we investigated the effect of periodical shifting of the relative phase on the cavitation induction by using SHS. In an aqueous solution, SH phase shifting with a shift pitch not larger than 0.5 π and a shift interval time not shorter than 30 ms was found to be as effective in inducing cavitation as the SH phase is fixed to the optimum phase. This result is important for the application of SHS to in vivo treatment in which it is difficult to maintain the relative phase in a target tissue constant because of movements and biophysical changes.