Spatio-temporal mapping and characterization of acoustic cavitation seeded by microbubbles and solid microparticles during focused ultrasound exposure.

Cavitation nuclei are often used to seed and promote acoustic cavitation in therapeutic applications. However, the effects of fluid velocity and ultrasound exposure parameters peak rarefactional pressure (Pr), pulse duration (PD), and pulse repetition frequency (PRF) on the spatial distribution, type, magnitude, and number of acoustic cavitation events for each nuclei remains poorly understood. In this study, a 1.6-mm diameter tunnel phantom (3% agar) was perfused (fluid velocity: 10-40 mm/s) with either microbubbles (SonoVue) or hydrophobic solid microparticles (TALC) and exposed to 74 different acoustic parameter combinations (frequency: 0.5 MHz, Pr: 150-1500 kPa, PD: 1-100,000 cycles, PRF: 1-50 Hz, number of pulses: 10-250). Spatial mapping of passively acquired acoustic cavitation emissions was performed with a 64-element array coaxial to the focused ultrasound transducer. At pressures above the cavitation threshold, cavitation activity generated from microbubbles was significantly reduced and spatially biased upstream after the first pulse at high PRFs relative to the fluid velocity. On the other hand, solid microparticles had no spatial bias and no significant reduction in the energy of acoustic emissions after the first pulse. Whereas microbubbles may be destroyed, and therefore, cease to act as cavitation nuclei, solid microparticles do not suffer from depletion of energy with high PRFs