Nucleation threshold of carbon black ultrasound contrast agent

Most ultrasound contrast agents used in ultrasonic imaging comprise shell-encapsulated microbubbles, whose ingredients have been associated with adverse bioeffects. In this study, we investigated the nucleation behaviour of carbon black dispersion, whose hydrophobic nanoparticles are used intradermally. For a hypothetical, perfectly spherical carbon black particle surrounded by a perfectly spherical gaseous void, we derived a theoretical nucleation threshold of only 1.3× the resting radius. Carbon black particles and aggregates thereof were investigated using high-speed photography during 1.0 MHz sonication. The nucleation threshold found experimentally is lower than the Blake cavitation threshold of 2.0× the resting radius of free, unencapsulated microbubbles. Therefore, carbon black dispersion may be a promising ultrasound contrast agent.