Multiplex molecular ultrasound imaging with perfluorocarbon nanodroplets

Abstract

Because of their small size and better stability, perfluorocarbon nanodroplets (PFNDs) are emerging as an appealing alternative to microbubbles as ultrasound imaging agents. We have developed a method to simultaneously distinguish between two populations of PFNDs in the same imaging volume. The method relies on synthesizing the PFNDs to contain cores with two different boiling points. One population of PFNDs contains a perfluoropentane core (boiling point = 28 °C), while the other contains a perfluorohexane core (boiling point = 56 °C). The low-boiling-point PFNDs undergo a single liquid-to-gas phase transition in response to an acoustic vaporizing trigger, while the high-boiling-point PFNDs recondense after each acoustic trigger, enabling repeated vaporization. We have optimized a custom imaging sequence including multiple acoustic triggers that can be used to accurately differentiate between the two populations of PFNDs. The estimated relative concentration between the two nanodroplets was highly linear (R2 and gt; 0.99) in phantom studies. The high-boiling-point PFNDs were molecularly targeted to the epidermal growth factor receptor. Cell-culture results demonstrate that the non-targeted PFNDs can act as a delivery control, enabling more precise molecular imaging. Overall, these results show that multiplex ultrasound imaging could be an effective method for a variety of molecular imaging applications.