Combined Multiwavelength Photoacoustic and Plane-Wave Ultrasound Imaging for Probing Dynamic Phase-Change Contrast Agents

Objective: The purpose of this study was to introduce combined multiwavelength photoacoustic (PA) and plane-wave ultrasound (US) imaging referred to as mwPA/pwUS imaging capable of probing the rapid dynamic behavior of optically activated phase-change contrast agents. Methods: A dedicated mwPA/pwUS imaging sequence was developed based on a programmable US system synchronized with a tunable laser to irradiate tissue with laser pulses at desired optical wavelengths and to acquire post laser pulse PA images followed by ultrafast plane-wave US images. To evaluate the mwPA/pwUS imaging, a capillary filled with optically responsive perfluorohexane nanodroplets (PFHnDs) containing a dye with the peak absorption at 760 nm was imaged with optical wavelengths ranging from 700 to 940 nm. The differences between post-laser ultrafast US images [i.e., differential US (ΔUS)] were taken to visualize the recondensation dynamics of PFHnDs at each wavelength. Results: The PA images of PFHnDs showed higher contrast near 760 nm wavelength, corresponding to the peak absorption of the dye encapsulated in the PFHnDs. Moreover, the ΔUS signals immediately after 760-nm pulsed laser irradiation were also high due to the increased US contrast associated with vaporized PFHnDs. Conclusion: The mwPA/pwUS imaging allowed for the US-based optical spectroscopic characterization of PFHnDs and their dynamics. Significance: The introduced mwPA/pwUS imaging sequence can be used in various clinical applications where both spectroscopic PA imaging of endogenous and/or exogenous chromophores and ultrafast US imaging of phase-change nanodroplets are desired.