Ultrafast monitoring and control of subharmonic emissions of an unseeded bubble cloud during pulsed sonication

In the aim of limiting the destructive effects of collapsing bubbles, the regime of stable cavitation activity is currently targeted for sensitive therapeutic applications such as blood-brain barrier opening by ultrasound. This activity is quantified through the emergence of the subharmonic component of the fundamental frequency. Due to the intrinsically stochastic behavior of the cavitation phenomenon, a better control of the different (stable or inertial) cavitation regimes is a key requirement in the understanding of the mechanisms involving each bubble-induced mechanical effect. Current strategies applied to stable cavitation control rely on the use of either seeded microbubbles or a long-lasting pulse to reinitiate subharmonic emission. The present work aims at developing an ultrafast (inferior to 250 μs) monitoring and control of subharmonic emissions during long-pulsed (50 ms) sonication. The use of a FPGA-based feedback loop provides reproducible level of subharmonic emissions combined with temporal stability during the sonication duration. In addition, stable cavitation events are differentiated from the broadband noise characterizing inertial cavitation activity, with perspectives in the discrimination of the involved mechanisms underlying bubble-mediated therapeutic applications.