Laser-induced vaporization and acoustic-induced cavitation of droplets

In this study, micro-droplets developed for photoacoustic imaging and drug delivery have been evaluated in vitro. Induced by a short laser pulse, perfluoropentane mixed with optical dye was vaporized and expanded up to about 20 times of initial diameter of 3–10 micron, generating strong broadband photoacoustic signals. It was found these vaporized droplets became less stable and prone to rupture to ultrasound pulses. The broadband inertial cavitation signals by a very short ultrasound pulse can provide high spatial resolution in passive cavitation imaging. For feasibility test, passive cavitation imaging algorithms were implemented in a commercial ultrasound open platform with a linear array transducer, centered at 5 MHz. The system was synchronized with a 1 MHz unfocused single element transducer. 700kPa single-cycle excitation ultrasound pulse was induced to a 580 μm inner diameter polyethylene tube containing micro-droplets. The cavitation imaging was performed before and after vaporizing droplets by laser. Broadband emissions of 3–7MHz were observed only with vaporized droplets. These preliminary results show the feasibility of cavitation imaging of vaporized droplets with a short ultrasound excitation pulse for improved spatial resolution and could lead to further in-vivo experiments.