The development of microbubble contrast agents has broadened the scope of medical ultrasound imaging. Along with dedicated imaging techniques, these agents provide enhanced echoes from the blood pool and have enabled diagnostic ultrasound to assess and quantify microvascular blood flow. Contrast-enhanced ultrasound is currently used worldwide with clinical indications in cardiology and radiology, and it continues to evolve and develop through innovative technological advancements. In this review article, we present an overview of the basic microbubble physics and bubble-specific imaging techniques that enable this modality, and follow this with a discussion on new and emerging applications.

• Finite-element modeling of a system of two encapsulated ultrasound contrast agents. • For equal-sized agent, resonance decreases and amplitude increases. • For unequal-sized agent, smaller bubble resonance increases and amplitude decreases. • A larger microbubble strongly influences the response of the smaller bubble. • Bubble-coupling leads to significant nanobubble vibrations at clinical frequencies.

With the widespread interest and uptake of super-resolution ultrasound (SRUS) through localization and tracking of microbubbles, also known as ultrasound localization microscopy (ULM), many localization and tracking algorithms have been developed. ULM can image many centimeters into tissue in-vivo and track microvascular flow non-invasively with sub-diffraction resolution. In a significant community effort, we organized a challenge, Ultrasound Localization and TRacking Algorithms for Super-Resolution (ULTRA-SR). The aims of this paper are threefold: to describe the challenge organization, data generation, and winning algorithms; to present the metrics and methods for evaluating challenge entrants; and to report results and findings of the evaluation. Realistic ultrasound datasets containing microvascular flow for different clinical ultrasound frequencies were simulated, using vascular flow physics, acoustic field simulation and nonlinear bubble dynamics simulation. Based on these datasets, 38 submissions from 24 research groups were evaluated against ground truth using an evaluation framework with six metrics, three for localization and three for tracking. In-vivo mouse brain and human lymph node data were also provided, and performance assessed by an expert panel. Winning algorithms are described and discussed. The publicly available data with ground truth and the defined metrics for both localization and tracking present a valuable resource for researchers to benchmark algorithms and software, identify optimized methods/software for their data, and provide insight into the current limits of the field. In conclusion, Ultra-SR challenge has provided benchmarking data and tools as well as direct comparison and insights for a number of the state-of-the art localization and tracking algorithms.

Localized and reversible plasma membrane disruption is a promising technique employed for the targeted deposition of exogenous therapeutic compounds for the treatment of disease. Indeed, the plasma membrane represents a significant barrier to successful delivery, and various physical methods using light, sound, and electrical energy have been developed to generate cell membrane perforations to circumvent this issue. To restore homeostasis and preserve viability, localized cellular repair mechanisms are subsequently triggered to initiate a rapid restoration of plasma membrane integrity. Here, we summarize the known emergency membrane repair responses, detailing the salient membrane sealing proteins as well as the underlying cytoskeletal remodeling that follows the physical induction of a localized plasma membrane pore, and we present an overview of potential modulation strategies that may improve targeted drug delivery approaches.

CRISPR-Cas9 ribonucleoproteins (RNPs) have been heavily considered for gene therapy due to their high on-target efficiency, rapid activity and lack of insertional mutagenesis relative to other CRISPR-Cas9 delivery formats. Genetic diseases such as hypertrophic cardiomyopathy currently lack effective treatment strategies and are prime targets for CRISPR-Cas9 gene editing technology. However, current in-vivo delivery strategies for Cas9 pose risks of unwanted immunogenic responses. This proof-of-concept study aimed to demonstrate that focused ultrasound (FUS) in combination with microbubbles can be used to deliver Cas9-sgRNA (single guide RNA) RNPs and functionally edit human induced pluripotent stem cells (hiPSCs) in-vitro, a model system that can be expanded to cardiovascular research via hiPSC-derived cardiomyocytes. Here, we first determine acoustic conditions suitable for the viable delivery of large proteins to hiPSC with clinical Definity® microbubble agents using our customized experimental platform. From here, we delivered Cas9-sgRNA RNP complexes targeting the EGFP (enhanced green fluorescent protein) gene to EGFP-expressing hiPSCs for EGFP knockout. Simultaneous acoustic cavitation detection during treatment confirmed a strong correlation between microbubble disruption and viable FUS-mediated protein delivery in hiPSCs. This study shows, for the first time, the potential for an FUS-mediated technique for targeted and precise CRISPR-Cas9 gene editing in human stem cells.