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.