Wilson, K. et al. Nat. Commun. 3, 618 (2012).

Photoacoustics—the use of light to generate acoustic signals inside a material—is garnering increased attention for in vivo tissue imaging. Of the available methods for creating a photoacoustic signal, thermal expansion has so far been the only one suitable for living organisms, and unfortunately the thermal expansion signal is very small. Exogenous contrast agents such as nanoparticles or liquid droplets of perfluorocarbons can increase the signal, but each has limitations. Wilson et al. now combine these two methods by creating photoacoustic nanodroplets composed of nanoscale perfluorocarbon droplets containing optically absorbing nanoparticles. The droplets are small enough to exit the bloodstream to reach target tissues, and light absorption by the nanoparticles heats and vaporizes the droplets. The resulting acoustic signals are an order of magnitude higher than the signals obtained from nanoparticles alone.