Super-Resolution Ultrasound Imaging for Analysis of Microbubbles Cluster by Acoustic Vortex Tweezers.

Using acoustic vortex tweezers (AVT) to spatially accumulate microbubbles (MBs) shows promise for enhancing drug delivery efficiency and reducing off-target effects. The strong echogenicity of accumulated MBs also improves diagnostics via conventional ultrasound (US) B-mode imaging. However, the annular high-pressure distribution of AVT inhibits MBs inflow at the inlet, reducing MBs collection. The spatial resolution of US B-mode imaging further limits theranostic applications of AVT-mediated MBs accumulation. To address these challenges, we integrated an AVT waveform with volumetric super-resolution imaging (VSRI) to monitor the dynamic growth of MBs cluster during accumulation. We used a 5-MHz 2D array transducer for VSRI, employing plane wave pulses interleaved with accumulating pulses to retain MBs at a flow rate of 0.023-0.047 mL/s in a 3-mm vessel phantom. An asymmetrical AVT waveform (AVT*) was produced by modulating the pressure at the MBs inlet compared to the outlet. The effectiveness was validated in rat cerebral vessels for real-time volumetric tracking of MBs clusters. Microscopy observations showed that AVT* could quickly gather flowing MBs into cluster without repelling them at a flow rate of 0.023 mL/s. Statistical results indicated that microscopic data correlated better with VSRI than with B-mode images, suggesting VSRI suffices to detect the dynamics of AVT*-actuated MBs accumulation in real-time. Additionally, VSRI detected a significant increase in MBs cluster size over time during AVT* in the superior sagittal sinus of the rat brain. These findings demonstrate that the proposed strategy can accumulate the flowing MBs at a desired location and simultaneously observe this phenomenon.