Temperature and size-dependence of the vaporization threshold of phase-shift emulsions

Abstract

Acoustic cavitation has proven to be important for several therapeutic applications of ultrasound. However, acoustic cavitation is difficult to initiate and sustain in the absence of cavitation nuclei, particularly in tissue. Phase shift emulsions are ideal candidates for cavitation nuclei for in vivo applications. These emulsions, which consist of superheated liquid perfluorocarbon droplets enclosed by albumin shells, may be vaporized with acoustic pulses, a process known as acoustic droplet vaporization (ADV). In this study, we determined the ADV threshold at 2 MHz as a function of temperature and droplet size. Studies were conducted with micro- and nano-sized emulsions in a closed-flow system immersed in a temperature controlled water bath. The emulsions were injected into a flow system and exposed to high intensity focused ultrasound (HIFU; acoustic parameters: 2 MHz, 10 cycles, 100 Hz pulse repetition frequency). A portable diagnostic ultrasound scanner was used to monitor for vaporization. Upon vaporization, the peak rarefactional pressure of the acoustic pulse and the water bath temperature were recorded. It was determined that the vaporization threshold was independent of droplet size and inversely proportional to temperature. The utility of these emulsions in cancer therapy, particularly for bubble-enhanced heating during HIFU exposure, will be discussed.