Super-Resolution Ultrasound Based Cell Tracking With Polymeric Nanobubbles.

Tracking the fate of transplanted cells in vivo remains a major challenge for the development and monitoring of cell-based therapies. Here, we report a nanobubble (NB) based acoustic labeling strategy that enables ultrasound localization microscopy (ULM) to track transplanted cells in tumors with single-cell-scale sensitivity and super-resolution spatial precision. We developed stable, biocompatible poly(butyl cyanoacrylate) NB that are efficiently internalized by cells and generate strong intracellular ultrasound signals. Furthermore, ULM allows the real-time visualization of individual cells, their localization, and trajectory reconstruction in flow phantoms and murine breast cancers. In the latter, NB-labeled bone marrow-derived mononuclear cells were tracked in vivo following intra-arterial injection, whereas unlabeled cells remained undetectable. By integrating cell trajectories with microbubble-based super-resolution vascular maps, our approach allows identification of the blood vessels that delivered the cells. This clinically compatible imaging strategy expands the application of ULM beyond vascular mapping and provides a platform for high-resolution, dynamic monitoring of cell-based therapies.