Fast and Selective Super-Resolution Ultrasound In Vivo With Acoustically Activated Nanodroplets

Abstract

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 microbubbles. 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 acoustically activatable contrast agents were used. We demonstrate their use with ultrasound mechanical indices well within recommended safety limits to enable fast on-demand sparse activation and destruction 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 a 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 (0.25 s) and diastole (0.25 s) with fast-AWSALM outdoing microbubble based ULM. In conclusion, we demonstrate the feasibility of fast and selective measurement of microvascular dynamics in vivo with subwavelength resolution using ultrasound and acoustically activatable nanodroplet contrast agents.

Nanodroplet-Based Super-Resolution Ultrasound Localization Microscopy

“…Dong et al introduced a type of blinking acoustic nanodroplets, which could achieve ULM imaging at quite high concentrations and have much higher efficiency of repetitive activation . Tang and his team developed nanodroplet-based AWSALM and fast-AWSALM and further verified the ability to visualize microvasculature in vivo and in vitro, respectively, which opens up the possibilities for super-resolution molecular imaging. ,, Nevertheless, there are a few challenges to be conquered to speed up the clinical translation of ULM. Out-of-plane motion, long acquisition time, and low frame rates are the major challenges faced by the clinical translation of ULM. ,− A previous study has attempted to overcome out-of-plane motion by implementing a 3D imaging technique .…”

“…Nanodroplets are the arguably the best solution to this trilemma, as they can be injected in high concentration and activated stochastically on demand, which therefore accelerates the image acquisition speed while covering the whole imaging area . Since the activations of nanodroplets are performed in a stochastic fashion and only a small subset of nanodroplets are activated at a given time, a high imaging resolution can be achieved through accurate localizations.…”

“…Due to the requirements for sparsity of flowing microbubbles, the microbubble-based ULM requires a long acquisition time, especially for small microvasculatures . Since not all nanodroplets are activated at the same time, activating nanodroplets detected by ultrasound can be achieved at very high agent concentrations, with or without flow . When it comes to cancer imaging, nanodroplet-based ULM has the potential to offer enhanced insights into tumor angiogenesis, characterized by the presence of vessels with very slow flows .…”

“…Until the next activation pulse, these activated nanodroplets will produce acoustic signals. First, the existing activated nanodroplets generated by the previous activation pulse will be destroyed; Second, a new subgroup of nanodroplets will be activated . The localizations of different subgroups of activated nanodroplets can be offered by this continuous activation and subsequent imaging pulses.…”

“…Due to the lower activation threshold of low-boiling-point nanodroplets, the acoustic pressure provided by the plane-wave imaging pulse is sufficient to activate it for imaging. , With plane waves, faster imaging and activation acquisition can be achieved without using focus-wave. Since merely the most easily activated droplets are activated by selected acoustic pressure, the subgroup of octafluoropropane nanodroplets will be sparsely activated by the first plane-wave imaging pulse, which is the principle of fast AWSALM . These activated droplets will generate contrast signals until the next plane-wave pulse.…”