High spatial-resolution cavitation imaging of laser-triggered PFP droplets

Indocyanine green (ICG)-loaded droplets developed for photoacoustic imaging and drug delivery have been tested for cavitation imaging in vitro. Perfluoropentane mixed with ICG was vaporized with a short pulse laser and expanded to 3-5 times the initial diameter of 2-5 micron and generated a strong broadband photoacoustic signal. It was found these vaporized droplets could be cavitated and ruptured by a short ultrasound pulses. To test the feasibility of cavitation imaging of laser-triggered droplets, broadband cavitation imaging algorithms were implemented on a commercial ultrasound open platform with a linear array transducer, centered at 5 MHz. Less than 3 cycles ultrasound excitation pulse with a MI of 0.7 was used to insonify a polyethylene tube of 580 μm inner diameter containing droplets to induce inertial cavitation activity. The cavitation imaging was performed before and after vaporizing droplets by laser pulses. Broadband emissions of 5-10 MHz were observed only after droplets were vaporized. These preliminary results demonstrate the feasibility of high resolution cavitation imaging of vaporized droplets using a short ultrasound excitation pulse and could lead to further in vivo experiments.