Carbocyanine dye-patterned microbubbles as ultrasound-modulated fluorescent contrast agents

Abstract

Combining fluorescence and acoustic techniques enables the characterization of biochemical environments with optical sensors while utilizing the penetration depth and spatial resolution of focused ultrasound. Localized light generation within a small tissue volume can yield spatially resolved chemical information useful for delineating tissue pathology such as cancerous lesions. Our work develops fluorescent microbubble contrast agents that generate light in the focal zone of an ultrasound beam through modulation of their fluorescence intensity. Microbubbles were designed with a self-quenching lipophilic dye patterned within the lipid layer, enabling the dye molecules to quench and dequench with the ultrasound-driven particle size oscillations. Distinct dye patterns were generated on the microbubble surfaces and investigated for their ultrasound-driven fluorescence response. By amplifying the microbubble fluorescence intensity modulations using a lock-in amplifier, the optical signal from these locally activated particles was detected within an optically scattering environment and at significant depth. We additionally observed that these microbubbles also displayed harmonic oscillations of fluorescence intensity beyond the ultrasound driving frequency which could be used to further improve the signal-to-noise ratio for detection. This technique could enable sensitive optical imaging with ultrasound-scale millimeter-level spatial resolution, providing a valuable tool to address the challenge of optical imaging in deep tissue.