  Perfusion by the microcirculation is key to the development, maintenance and
pathology of tissue. Its measurement with high spatiotemporal resolution is
consequently valuable but remains a challenge in deep tissue. Ultrasound
Localization Microscopy (ULM) provides very high spatiotemporal resolution but
the use of microbubbles requires low contrast agent concentrations, a long
acquisition time, and gives little control over the spatial and temporal
distribution of the bubbles. The present study is the first to demonstrate
Acoustic Wave Sparsely-Activated Localization Microscopy (AWSALM) and
fast-AWSALM for in vivo super-resolution ultrasound imaging, offering contrast
on demand and vascular selectivity. Three different formulations of
sono-switchable contrast agents were tested. We demonstrate their use with
ultrasound mechanical indices well within recommended safety limits to enable
fast on-demand sparse switching at very high agent concentrations. We produce
super-localization maps of the rabbit renal vasculature with acquisition times
between 5.5 s and 0.25 s, and an 4-fold improvement in spatial resolution. We
present the unique selectivity of AWSALM in visualizing specific vascular
branches and downstream microvasculature, and we show super-localized kidney
structures in systole and diastole with fast-AWSALM. In conclusion we
demonstrate the feasibility of fast and selective measurement of microvascular
dynamics in vivo with subwavelength resolution using ultrasound and
sono-switchable nanodroplets.
