Abstract
Acoustic droplet vaporization (ADV) is an important process that enables the theragnostic application of acoustically activated droplets, where the nucleation of inertial cavitation (IC) activity must be precisely controlled. This Letter describes threshold pressure measurements for ADV and acoustic emissions consistent with IC activity of lipid-shelled non-superheated perfluoropentane nanodroplets over a range of physiologically relevant concentrations at 1.1-MHz. Under the frequency investigated, results show that the thresholds were relatively independent of concentration for intermediate concentrations (105, 106, and 107 droplets/ml), thus indicating an optimal range of droplet concentrations for conducting threshold studies. For the highest concentration, the difference between the threshold for IC and the threshold for ADV was greatly reduced, suggesting that it might prove difficult to induce ADV without concomitant IC in applications that employ higher concentrations.
Highlights
IntroductionPhase-change nanodroplets exhibit promising potential for use as a next-generation ultrasound (US) contrast agent due to their chemical similarities with microbubbles while offering deeper penetration depth and longer circulation time. Applications of nanodroplets include B-mode US imaging, photoacoustic imaging, phase-contrast imaging, phase aberration correction, sonoporation therapy, blood-brain-barrier opening, embolotherapy and hyperthermia therapy, etc. Current formulations of droplets generally include a shell (composed of lipids, proteins, polymers, or surfactants) to provide Laplace pressure and a core [usually perfluorocarbons, including octafluoropropane (boiling point À39 C), decafluorobutane (boiling point À2 C), dodecafluoropentane (boiling point 29 C), decafluoropentane (boiling point 55 C), perfluorohexane (boiling point 58 C), etc.] to generate acoustic impedance mismatch after vaporization
Phase-change nanodroplets exhibit promising potential for use as a next-generation ultrasound (US) contrast agent due to their chemical similarities with microbubbles while offering deeper penetration depth and longer circulation time.1 Applications of nanodroplets include B-mode US imaging, photoacoustic imaging, phase-contrast imaging, phase aberration correction, sonoporation therapy, blood-brain-barrier opening, embolotherapy and hyperthermia therapy, etc.2 Current formulations of droplets generally include a shell to provide Laplace pressure and a core [usually perfluorocarbons, including octafluoropropane, decafluorobutane, dodecafluoropentane, decafluoropentane, perfluorohexane, etc.] to generate acoustic impedance mismatch after vaporization
For a quantitative description of phase change behavior, we considered the mean echo enhancement (MEE) of grayscale B-mode images acquired downstream of the excitation transducer focal volume as an indicator of Acoustic droplet vaporization (ADV)-induced phase change, and the walls of dialysis tube defined the region of interest (ROI)
Summary
Phase-change nanodroplets exhibit promising potential for use as a next-generation ultrasound (US) contrast agent due to their chemical similarities with microbubbles while offering deeper penetration depth and longer circulation time. Applications of nanodroplets include B-mode US imaging, photoacoustic imaging, phase-contrast imaging, phase aberration correction, sonoporation therapy, blood-brain-barrier opening, embolotherapy and hyperthermia therapy, etc. Current formulations of droplets generally include a shell (composed of lipids, proteins, polymers, or surfactants) to provide Laplace pressure and a core [usually perfluorocarbons, including octafluoropropane (boiling point À39 C), decafluorobutane (boiling point À2 C), dodecafluoropentane (boiling point 29 C), decafluoropentane (boiling point 55 C), perfluorohexane (boiling point 58 C), etc.] to generate acoustic impedance mismatch after vaporization. Phase-change nanodroplets exhibit promising potential for use as a next-generation ultrasound (US) contrast agent due to their chemical similarities with microbubbles while offering deeper penetration depth and longer circulation time.. In liquid emulsion form, the droplets have a relatively low scattering cross section due to their small size relative to the acoustic wavelength and similar characteristic impedance with the surrounding liquid. Upon liquid-gas phase transition, the resultant gas core (many times that of the original droplet) presents a much larger scattering cross section for ultrasonic waves. Such a process can be achieved acoustically, i.e., through acoustic droplet vaporization (ADV).. Such a process can be achieved acoustically, i.e., through acoustic droplet vaporization (ADV). The exact mechanism of ADV is still under investigation with several possibilities proposed, including cavitation, nucleation, droplet deformation, nonlinear propagation, and superharmonic focusing. After vaporization, under the influence of the US wave, the gaseous bubble would continue to oscillate, either stably or with sufficient energy to collapse inertially due to the momentum of surrounding liquid. Inertial cavitation (IC) is of interest due to its ability to induce mechanical and thermal bioeffects (i.e., jetting and enhanced heat deposition), which can cause potentially dangerous tissue disruption if induced in normal tissues
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