Optical Droplet Vaporization of Micron-sized Perfluorocarbon Droplets and their Photoacoustic Detection

摘要

An acoustic and photoacoustic characterization of micron-sized perfluorocarbon (PFC) droplets is presented. PFC droplets are currently being investigated as acoustic and photoacoustic contrast agents, as well as cancer therapy agents. Pulse echo measurements at 375 MHz were used to determine the PFC diameter of nine droplets, ranging from 3.2 to 6.7 μm, and the PFC sound velocity, ranging from 311 to 406 m/s. An average sound velocity of 379 ±18 m/s was calculated if droplets smaller than the ultrasonic beam width of 4.0 μm were excluded. Optical droplet vaporization, where vaporization of a single droplet occurred upon laser irradiation of sufficient intensity (above 1.5 J/cm~2), was verified using acoustic methods. The ultrasonic backscatter amplitude, acoustic impedance and attenuation increased after vaporization, consistent with a phase change from a liquid to gas core. Using laser irradiation values below the vaporization threshold, photoacoustic measurements were used to compare the photoacoustic ultrasound emission spectra of three droplets of varying diameters to a theoretical model. Good agreement of the experimental results with theoretical predictions of the ultrasound spectral features were observed over the bandwidth of the 375 MHz transducer.