Near-infrared Photoacoustic imaging (NIR-PA) can enable deep-tissue imaging, yet clinical translation has been hindered by a lack of suitable NIR-PA contrast agents. The FDA-approved Indocyanine green (ICG) dye is a promising candidate, but it offers limited targeting ability and poor stability. To address this unmet need, we examined three novel ICG-based platforms in the form of DNA scaffolds, J-aggregates, and nanobubbles. We demonstrate that all three platforms yield a PA signal stronger than whole blood at concentrations as low as 45 µM and are amenable to molecular targeting.
Nanobubbles (NBs) have demonstrable potential for ultrasound imaging and therapeutic applications. Recent studies have even shown their capacity for cellular internalization, which has important implications for their in-vivo stability and bioaccumulation. Traditional methods for observing NBs often involve fluorescence labelling, which can influence NB behaviour. Moreover, these methods are unsuitable for detecting intact (acoustically active) NBs within a cellular environment. This study introduces a label-free approach employing optical coherence tomography (OCT) to investigate the temporal variations in speckle intensity of the OCT backscatter signal of cells interacting with NBs. The temporal variations in the signal intensity of cell aggregates result from the motion of subcellular scatterers within the cellular environment. In this work, we investigate whether internalized NBs modify the temporal variations in the signal intensity. For our experimental imaging set-up, we used a Thorlabs MEMS-VCSEL Swept Source OCT system with a central wavelength of 1300 nm to acquire M-Mode and B-Mode acquisitions. PC3 prostate cancer cells and in-house lipid-shelled NBs were used. The sensitivity of the speckle decorrelation technique was tested on our system using an intensity autocorrelation function on polystyrene microspheres and diluted NBs. Our study demonstrates that speckle decorrelation OCT can effectively detect NBs within a compact cell pellet under specific conditions and was verified using contrast-enhanced ultrasound. This approach provides an additional optical method for NB detection within cellular environments and holds the potential for broader applications in detecting NBs in in-vivo applications.
Significance: An effective contrast agent for concurrent multimodal photoacoustic (PA) and ultrasound (US) imaging must have both high optical absorption and high echogenicity. Integrating a highly absorbing dye into the lipid shell of gas core nanobubbles (NBs) adds PA contrast to existing US contrast agents but may impact agent ultrasonic response.
Aim: We report on the development and ultrasonic characterization of lipid-shell stabilized C3F8 NBs with integrated Sudan Black (SB) B dye in the shell as dual-modal PA-US contrast agents.
Approach: Perfluoropropane NBs stabilized with a lipid shell including increasing concentrations of SB B dye were formulated by amalgamation (SBNBs). Physical properties of SBNBs were characterized using resonant mass measurement, transmission electron microscopy and pendant drop tensiometry. Concentrated bubble solutions were imaged for 8 min to assess signal decay. Diluted bubble solutions were stimulated by a focused transducer to determine the response of individual NBs to long cycle (30 cycle) US. For assessment of simultaneous multimodal contrast, bulk populations of SBNBs were imaged using a PA and US imaging platform.
Results: We produced high agent yield (∼1011) with a mean diameter of ∼200 to 300 nm depending on SB loading. A 40% decrease in bubble yield was measured for solutions with 0.3 and 0.4 mg / ml SB. The addition of SB to the shell did not substantially affect NB size despite an increase in surface tension by up to 8 mN / m. The bubble decay rate increased after prolonged exposure (8 min) by dyed bubbles in comparison to their undyed counterparts (2.5-fold). SB in bubble shells increased gas exchange across the shell for long cycle US. PA imaging of these agents showed an increase in power (up to 10 dB) with increasing dye.
Conclusions: We added PA contrast function to NBs. The addition of SB increased gas exchange across the NB shell. This has important implications in their use as multimodal agents.
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