Role of periodic shock waves in passive acoustic mapping of cavitation

The purpose of this work was to investigate the mechanistic, bubble-based source for passive acoustic mapping (PAM) reconstruction. Single cavitation clouds driven with f₀ = 254 kHz focused ultrasound, responding in the f₀/2 and f₀/3 subharmonic regimes, were monitored with an ultrasound scanning system and array transducer for PAM reconstruction. Simultaneously, high-speed imaging at 0.5×10⁶ frames per second, and a single element broadband passive cavitation detector (PCD) were used to identify key signals emitted by the cloud, and confirm that a single cloud had formed. The high-speed imaging and PCD data clearly demonstrate that the cloud generates periodic shock waves (PSWs) at frequencies subharmonic to f₀, along with acoustic emissions at f₀. The post-beamformed signal of PAM reveals features that are separated by a time interval approximately equal to the period of shock emission. We conclude that PSWs are a strong candidate for mediating PAM, which crucially, can be reconciled with reports in the literature of discrete, discontinuous cavitation activity.