Remotely triggered contrast nanoagent for ultrasound and photoacoustic imaging

Abstract

Ultrasound (US) is a real time, noninvasive, and cost effective imaging modality, capable of visualizing tissue morphology and even some physiological properties of tissue. Photoacoustic (PA) imaging is capable of depth-resolved assessment of the optical properties inside tissue, providing a functional visualization of tissue. In both techniques, contrast agents (microbubbles in US and plasmonic nanoparticles in PA) are used for cellular and molecular imaging. We propose a single, nano-sized, remotely triggered contrast agent that simultaneously provides US and PA contrast enhancement. The US/PA contrast agent, photoacoustic droplet (PAD), was synthesized through an emulsion technique to incorporate plasmonic nanoparticles into a liquid perfluorocarbon droplet covered with bovine serum albumin (BSA). This facile synthesis method produced multifaceted agents as small as 50 nm. The agents were then suspended in a polyacrylamide phantom for imaging experiments. US images were captured using a 7 MHz US transducer before and after laser irradiation. The PADs remained inactive and showed no significant contrast until remotely activated by pulsed laser irradiation. The PAD enhances the contrast in both US and PA imaging simultaneously via three forms of signal production: vaporization, thermoelastic expansion, and acoustic impedance mismatch. The resulting contrast was increased up to 17 dB in PA images. The developed platform, while showing excellent contrast enhancement capabilities, could also be used for many different applications including photothermal therapy, drug delivery and release, molecular targeting, and sonoporation.