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Mapping microbubble viscosity using fluorescence lifetime imaging of molecular rotors

Encapsulated microbubbles are well established as highly effective contrast agents for ultrasound imaging. Central to their use is accurate characterization of their viscoelastic surface properties, but methods for mapping these are lacking. This paper presents a new strategy for performing these measurements using a small fluorophore or “molecular rotor” embedded in the microbubble surface, whose fluorescence lifetime is related to the viscosity of its surroundings. This allows quantitative shell viscosity measurement, and has revealed that individual bubbles can have heterogeneous viscosity distributions. Eckersley devised the concept together with Stride and Kuimova and developed the analytical methods essential for this study.