Element Position Calibration for Matrix Array Transducers with Multiple Disjoint Piezoelectric Panels.

Two-dimensional ultrasound transducers enable the acquisition of fully volumetric data that have been demonstrated to provide greater diagnostic information in the clinical setting and are a critical tool for emerging ultrasound methods, such as super-resolution and functional imaging. This technology, however, is not without its limitations. Due to increased fabrication complexity, some matrix probes with disjoint piezoelectric panels may require initial calibration. In this manuscript, two methods for calibrating the element positions of the Vermon 1024-channel 8 MHz matrix transducer are detailed. This calibration is a necessary step for acquiring high resolution B-mode images while minimizing transducer-based image degradation. This calibration is also necessary for eliminating vessel-doubling artifacts in super-resolution images and increasing the overall signal-to-noise ratio (SNR) of the image. Here, we show that the shape of the point spread function (PSF) can be significantly improved and PSF-doubling artifacts can be reduced by up to 10 dB via this simple calibration procedure.

Ultrasound localization microscopy of the human kidney allograft on a clinical ultrasound scanner.

Chronic kidney disease is a major medical problem, causing more than a million deaths each year worldwide. Peripheral kidney microvascular damage characterizes most chronic kidney diseases, yet noninvasive and quantitative diagnostic tools to measure this are lacking. Ultrasound Localization Microscopy (ULM) can assess tissue microvasculature with unprecedented resolution. Here, we optimized methods on 35 kidney transplants and studied the feasibility of ULM in seven human kidney allografts with a standard low frame rate ultrasound scanner to access microvascular damage. Interlobar, arcuate, cortical radial vessels, and part of the medullary organization were visible on ULM density maps. The medullary vasa recta can be seen but are not as clear as the cortical vessels. Acquisition parameters were derived from Contrast-Enhanced Ultrasound examinations by increasing the duration of the recorded clip at the same plane. ULM images were compared with Color Doppler, Advanced Dynamic Flow, and Superb Microvascular Imaging with a contrast agent. Despite some additional limitations due to movement and saturation artifacts, ULM identified vessels two to four times thinner compared with Doppler modes. The mean ULM smallest analyzable vessel cross section was 0.3 ± 0.2 mm in the seven patients. Additionally, ULM was able to provide quantitative information on blood velocities in the cortical area. Thus, this proof-of-concept study has shown ULM to be a promising imaging technique for qualitative and quantitative microvascular assessment. Imaging native kidneys in patients with kidney diseases will be needed to identify their ULM biomarkers.

Ultrafast ultrasound localization microscopy for deep super-resolution vascular imaging.

Non-invasive imaging deep into organs at microscopic scales remains an open quest in biomedical imaging. Although optical microscopy is still limited to surface imaging owing to optical wave diffusion and fast decorrelation in tissue, revolutionary approaches such as fluorescence photo-activated localization microscopy led to a striking increase in resolution by more than an order of magnitude in the last decade. In contrast with optics, ultrasonic waves propagate deep into organs without losing their coherence and are much less affected by in vivo decorrelation processes. However, their resolution is impeded by the fundamental limits of diffraction, which impose a long-standing trade-off between resolution and penetration. This limits clinical and preclinical ultrasound imaging to a sub-millimetre scale. Here we demonstrate in vivo that ultrasound imaging at ultrafast frame rates (more than 500 frames per second) provides an analogue to optical localization microscopy by capturing the transient signal decorrelation of contrast agents--inert gas microbubbles. Ultrafast ultrasound localization microscopy allowed both non-invasive sub-wavelength structural imaging and haemodynamic quantification of rodent cerebral microvessels (less than ten micrometres in diameter) more than ten millimetres below the tissue surface, leading to transcranial whole-brain imaging within short acquisition times (tens of seconds). After intravenous injection, single echoes from individual microbubbles were detected through ultrafast imaging. Their localization, not limited by diffraction, was accumulated over 75,000 images, yielding 1,000,000 events per coronal plane and statistically independent pixels of ten micrometres in size. Precise temporal tracking of microbubble positions allowed us to extract accurately in-plane velocities of the blood flow with a large dynamic range (from one millimetre per second to several centimetres per second). These results pave the way for deep non-invasive microscopy in animals and humans using ultrasound. We anticipate that ultrafast ultrasound localization microscopy may become an invaluable tool for the fundamental understanding and diagnostics of various disease processes that modify the microvascular blood flow, such as cancer, stroke and arteriosclerosis.

Novel Perfluorinated Triblock Amphiphilic Copolymers for Lipid-Shelled Microbubble Stabilization.

Amphiphilic triblock (Atri) copolymers made of perfluorinated alkyl chain linked to hydrocarbon chain and methoxy-poly(ethylene glycol) of three different molecular weights were synthesized. In vitro evaluation demonstrated that these new compounds were noncytotoxic. Characterization and interaction of each triblock copolymer with a branched polyamine myristoyl lipid (2-{3[bis-(3-amino-propyl)-amino]-propylamino}- N-ditetradecyl carbamoyl methyl-acetamide, DMAPAP) were studied by the Langmuir film method and thermal analysis. The triblock copolymer/cationic lipids (1:10, w/w) were mixed with perfluorobutane gas to form microbubbles (MBs). The latter were characterized by optical microscopy to get the microbubble size and concentration by densimetry to determine the amount of encapsulated gas and by ultrasound to assess oscillation properties. Atri with the lowest and intermediate weights were shown to interact with the cationic lipid DMAPAP and stabilize the Langmuir film. In that case, monodisperse microbubbles ranging from 2.3 ± 0.1 to 2.8 ± 0.1 µm were obtained. The proportion of encapsulated gas within the MB shell increased up to 3 times after the incorporation of the copolymer with the lowest and intermediate weights. Moreover, the acoustic response of the microbubbles was maintained in the presence of the copolymers.

Comb-Like Fluorophilic-Lipophilic-Hydrophilic Polymers for Nanocapsules as Ultrasound Contrast Agents.

Imaging the enhanced permeation and retention effect by ultrasound is hindered by the large size of commercial ultrasound contrast agents (UCAs). To obtain nanosized UCAs, triblock copolymers of poly(ethylene glycol)-polylactide-poly(1 H,1 H,2 H,2 H-heptadecafluorodecyl methacrylate) (PEG-PLA-PFMA) with distinct numbers of perfluorinated pendant chains (5, 10, or 20) are synthesized by a combination of ring-opening polymerization and atom transfer radical polymerization. Nanocapsules (NCs) containing perfluorooctyl bromide (PFOB) intended as UCAs are obtained with a 2-fold increase in PFOB encapsulation efficiency in fluorinated NCs as compared with plain PEG-PLA NCs thanks to fluorous interactions. NC morphology is strongly influenced by the number of perfluorinated chains and the amount of polymer used for formulation, leading to peculiar capsules with several PFOB cores at high PEG-PLA-PFMA20 amount and single-cored NCs with a thinner shell at low fluorinated polymer amount, as confirmed by small-angle neutron scattering. Finally, fluorinated NCs yield higher in vitro ultrasound signal compared with PEG-PLA NCs, and no in vitro cytotoxicity is induced by fluorinated polymers and their degradation products. Our results highlight the benefit of adding comb-like fluorinated blocks in PEG-PLA polymers to modify the nanostructure and enhance the echogenicity of nanocapsules intended as UCAs.

Transcranial functional ultrasound imaging of the brain using microbubble-enhanced ultrasensitive Doppler.

Functional ultrasound (fUS) is a novel neuroimaging technique, based on high-sensitivity ultrafast Doppler imaging of cerebral blood volume, capable of measuring brain activation and connectivity in rodents with high spatiotemporal resolution (100µm, 1ms). However, the skull attenuates acoustic waves, so fUS in rats currently requires craniotomy or a thinned-skull window. Here we propose a non-invasive approach by enhancing the fUS signal with a contrast agent, inert gas microbubbles. Plane-wave illumination of the brain at high frame rate (500Hz compounded sequence with three tilted plane waves, PRF=1500Hz with a 128 element 15MHz linear transducer), yields highly-resolved neurovascular maps. We compared fUS imaging performance through the intact skull bone (transcranial fUS) versus a thinned-skull window in the same animal. First, we show that the vascular network of the adult rat brain can be imaged transcranially only after a bolus intravenous injection of microbubbles, which leads to a 9dB gain in the contrast-to-tissue ratio. Next, we demonstrate that functional increase in the blood volume of the primary sensory cortex after targeted electrical-evoked stimulations of the sciatic nerve is observable transcranially in presence of contrast agents, with high reproducibility (Pearson's coefficient ρ=0.7±0.1, p=0.85). Our work demonstrates that the combination of ultrafast Doppler imaging and injection of contrast agent allows non-invasive functional brain imaging through the intact skull bone in rats. These results should ease non-invasive longitudinal studies in rodents and open a promising perspective for the adoption of highly resolved fUS approaches for the adult human brain.

High spatiotemporal control of spontaneous reactions using ultrasound-triggered composite droplets.

Achieving high spatial and temporal control over a spontaneous reaction is a particularly challenging task with potential breakthroughs in various fields of research including surface patterning and drug delivery. We report here an exceptionally effective method that allows attaining such control. This method relies on a remotely triggered ultrasound-induced release of a reactant encapsulated in a composite microdroplet of liquid perfluorohexane. More specifically, the demonstration was achieved by locally applying a focused 2.25 MHz transducer onto a microfluidic channel in which were injected composite microdroplets containing a solution of an azidocoumarin and an external flow containing a reactive alkyne.

Volumetric ultrasound localization microscopy with diverging cylindrical waves.

Transcranial ultrasound plays a limited role in neuroradiology due to its lack of resolution, planar imaging, and user-dependency. By breaching the diffraction limit using injected microbubbles, volumetric ultrasound localization microscopy (ULM) could help alleviate those issues. However, performing 3D ultrasound imaging at a high frame rate with sufficient signal-to-noise ratio to track individual microbubbles through the skull remains a challenge, especially with a portable scanner. In this study, we describe a ULM sequence suitable for volumetric transcranial imaging exploiting cylindrical emissions on multiplexed matrix probes, through simulations, hydrophone measurements, and flow phantoms. This geometry leads to a doubling of the peak acoustic pressure, up to 400 kPa, with respect to spherical emission and improved volume rate, up to 180 Hz. Cylindrical emissions also improve ULM saturation rate by 60% through a skull phantom. The assessment of microbubble velocity was also improved from 33% error in the average flow measured with spherical waves to a 5% error with cylindrical waves. Conversely, we demonstrate the detrimental impacts of cylindrical waves toward the field of view and isotropic sensitivity. Nevertheless, due to its enhanced signal-to-noise ratio and 3D nature, such a cylindrical volumetric sequence could be beneficial for ULM as a diagnostic tool in humans, especially when portability is a necessity.

Proof of Concept of an Affordable, Compact and Transcranial Submillimeter Accurate Ultrasound-Based Tracking System.

In neurosurgery, a current challenge is to provide localized therapy in deep and difficult-to-access brain areas with millimeter accuracy. In this prospect, new surgical devices such as microrobots are being developed, which require controlled inbrain navigation to ensure the safety and efficiency of the intervention. In this context, the device tracking technology has to answer a three-sided challenge: invasiveness, performance, and facility of use. Although ultrasound seems appropriate for transcranial tracking, the skull remains an obstacle because of its significant acoustic perturbations. A compact and affordable ultrasound-based tracking system that minimizes skull-related disturbances is proposed here. This system consists of three emitters fixed on the patient's head and a one-millimeter receiver embedded in the surgical device. The 3D position of the receiver is obtained by trilateration based on time of flight measurements. The system demonstrates a submillimeter tracking accuracy through an 8.9 mm thick skull plate phantom. This result opens multiple perspectives in terms of millimeter accurate navigation for a large number of neurobiomedical devices.

RF-ULM: Ultrasound Localization Microscopy Learned from Radio-Frequency Wavefronts.

In Ultrasound Localization Microscopy (ULM), achieving high-resolution images relies on the precise localization of contrast agent particles across a series of beamformed frames. However, our study uncovers an enormous potential: The process of delay-and-sum beamforming leads to an irreversible reduction of Radio-Frequency (RF) channel data, while its implications for localization remain largely unexplored. The rich contextual information embedded within RF wavefronts, including their hyperbolic shape and phase, offers great promise for guiding Deep Neural Networks (DNNs) in challenging localization scenarios. To fully exploit this data, we propose to directly localize scatterers in RF channel data. Our approach involves a custom super-resolution DNN using learned feature channel shuffling, non-maximum suppression, and a semi-global convolutional block for reliable and accurate wavefront localization. Additionally, we introduce a geometric point transformation that facilitates seamless mapping to the B-mode coordinate space. To understand the impact of beamforming on ULM, we validate the effectiveness of our method by conducting an extensive comparison with State-Of-The-Art (SOTA) techniques. We present the inaugural in vivo results from a wavefront-localizing DNN, highlighting its real-world practicality. Our findings show that RF-ULM bridges the domain shift between synthetic and real datasets, offering a considerable advantage in terms of precision and complexity. To enable the broader research community to benefit from our findings, our code and the associated SOTA methods are made available at https://github.com/hahnec/rf-ulm.

Molecular Simulation of Covalent Adaptable Networks and Vitrimers: A Review.

Covalent adaptable networks and vitrimers are novel polymers with dynamic reversible bond exchange reactions for crosslinks, enabling them to modulate their properties between those of thermoplastics and thermosets. They have been gathering interest as materials for their recycling and self-healing properties. In this review, we discuss different molecular simulation efforts that have been used over the last decade to investigate and understand the nanoscale and molecular behaviors of covalent adaptable networks and vitrimers. In particular, molecular dynamics, Monte Carlo, and a hybrid of molecular dynamics and Monte Carlo approaches have been used to model the dynamic bond exchange reaction, which is the main mechanism of interest since it controls both the mechanical and rheological behaviors. The molecular simulation techniques presented yield sufficient results to investigate the structure and dynamics as well as the mechanical and rheological responses of such dynamic networks. The benefits of each method have been highlighted. The use of other tools such as theoretical models and machine learning has been included. We noticed, amongst the most prominent results, that stress relaxes as the bond exchange reaction happens, and that at temperatures higher than the glass transition temperature, the self-healing properties are better since more bond BERs are observed. The lifetime of dynamic covalent crosslinks follows, at moderate to high temperatures, an Arrhenius-like temperature dependence. We note the modeling of certain properties like the melt viscosity with glass transition temperature and the topology freezing transition temperature according to a behavior ruled by either the Williams-Landel-Ferry equation or the Arrhenius equation. Discrepancies between the behavior in dissociative and associative covalent adaptable networks are discussed. We conclude by stating which material parameters and atomistic factors, at the nanoscale, have not yet been taken into account and are lacking in the current literature.

Whole organ volumetric sensing Ultrasound Localization Microscopy for characterization of kidney structure.

Glomeruli are the filtration units of the kidney and their function relies heavily on their microcirculation. Despite its obvious diagnostic importance, an accurate estimation of blood flow in the capillary bundle within glomeruli defies the resolution of conventional imaging modalities. Ultrasound Localization Microscopy (ULM) has demonstrated its ability to image in-vivo deep organs in the body. Recently, the concept of sensing ULM or sULM was introduced to classify individual microbubble behavior based on the expected physiological conditions at the micrometric scale. In the kidney of both rats and humans, it revealed glomerular structures in 2D but was severely limited by planar projection. In this work, we aim to extend sULM in 3D to image the whole organ and in order to perform an accurate characterization of the entire kidney structure. The extension of sULM into the 3D domain allows better localization and more robust tracking. The 3D metrics of velocity and pathway angular shift made glomerular mask possible. This approach facilitated the quantification of glomerular physiological parameter such as an interior traveled distance of approximately 7.5 ± 0.6 microns within the glomerulus. This study introduces a technique that characterize the kidney physiology which can serve as a method to facilite pathology assessment. Furthermore, its potential for clinical relevance could serve as a bridge between research and practical application, leading to innovative diagnostics and improved patient care..

Visualization of Renal Glomeruli in Human Native Kidneys With Sensing Ultrasound Localization Microscopy.

OBJECTIVES: Kidney diseases significantly impact individuals' quality of life and strongly reduce life expectancy. Glomeruli play a crucial role in kidney function. Current imaging techniques cannot visualize them due to their small size. Sensing ultrasound localization microscopy (sULM) has shown promising results for visualizing in vivo the glomeruli of human kidney grafts. This study aimed to evaluate the ability of sULM to visualize glomeruli in vivo in native human kidneys despite their depth and a shorter duration of ultrasound acquisition limited by the period of the patient's apnea. Sensing ultrasound localization microscopy parameters in native kidneys and kidney grafts and their consequence regarding glomeruli detection were also compared. MATERIALS AND METHODS: Exploration by sULM was conducted in 15 patients with native kidneys and 5 with kidney allografts. Glomeruli were counted using a normalized distance metric projected onto sULM density maps. The difference in the acquisition time, the kidney depth, and the frame rate between native kidneys and kidney grafts and their consequence regarding glomeruli detection were assessed. RESULTS: Glomerular visualization was achieved in 12 of 15 patients with native kidneys. It failed due to impossible breath-holding for 2 patients and a too-deep kidney for 1 patient. Sensing ultrasound localization microscopy found 16 glomeruli per square centimeter in the native kidneys (6-31) and 33 glomeruli per square centimeter in kidney transplant patients (18-55). CONCLUSIONS: This study demonstrated that sULM can visualize glomeruli in native human kidneys in vivo. The proposed method may have many hypothetical applications, including biomarker development, assisting biopsy, or potentially avoiding it. It establishes a framework for improving the detection of local microstructural pathology, influencing the evaluation of allografts, and facilitating disease monitoring in the native kidney.

Size and phase preservation of amorphous calcium carbonate nanoparticles in aqueous media using different types of lignin for contrast-enhanced ultrasound imaging.

HYPOTHESIS: Calcium carbonate (CaCO3) nanoparticles could have great potential for contrast-enhanced ultrasound imaging (CEUS) due to their gas-generating properties and sensitivity to physiological conditions. However, the use of nano CaCO3 for biomedical applications requires the assistance of stabilizers to control the size and avoid the fast dissolution/recrystallization of the particles when exposed to aqueous conditions. EXPERIMENTS: Herein, we report the stabilization of nano CaCO3 using lignin, and synthesized core-shell amorphous CaCO3-lignin nanoparticles (LigCC NPs) with a diameter below 100 nm. We have then investigated the echogenicity of the LigCC NPs by monitoring the consequent generation of contrast in vitro for 90 min in linear and non-linear B-mode imaging. FINDINGS: This research explores how lignin type and structure affect stabilization efficiency, lignin structuration around CaCO3 cores, and particle echogenicity. Interestingly, by employing lignin as the stabilizer, it becomes possible to maintain the echogenic properties of CaCO3, whereas the use of lipid coatings prevents the production of signal generation in ultrasound imaging. This work opens new avenue for CEUS imaging of the vascular and extravascular space using CaCO3, as it highlights the potential to generate contrast for extended durations at physiological pH by utilizing the amorphous phase of CaCO3.

ULTRA-SR Challenge: Assessment of Ultrasound Localization and TRacking Algorithms for Super-Resolution Imaging.

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.

Sensing ultrasound localization microscopy for the visualization of glomeruli in living rats and humans.

BACKGROUND: Estimation of glomerular function is necessary to diagnose kidney diseases. However, the study of glomeruli in the clinic is currently done indirectly through urine and blood tests. A recent imaging technique called Ultrasound Localization Microscopy (ULM) has appeared. It is based on the ability to record continuous movements of individual microbubbles in the bloodstream. Although ULM improved the resolution of vascular imaging up to tenfold, the imaging of the smallest vessels had yet to be reported. METHODS: We acquired ultrasound sequences from living humans and rats and then applied filters to divide the data set into slow-moving and fast-moving microbubbles. We performed a double tracking to highlight and characterize populations of microbubbles with singular behaviors. We decided to call this technique "sensing ULM" (sULM). We used post-mortem micro-CT for side-by-side confirmation in rats. FINDINGS: In this study, we report the observation of microbubbles flowing in the glomeruli in living humans and rats. We present a set of analysis tools to extract quantitative information from individual microbubbles, such as remanence time or normalized distance. INTERPRETATION: As glomeruli play a key role in kidney function, it would be possible that their observation yields a deeper understanding of the kidney. It could also be a tool to diagnose kidney diseases in patients. More generally, it will bring imaging capabilities closer to the functional units of organs, which is a key to understand most diseases, such as cancer, diabetes, or kidney failures. FUNDING: This study was funded by the European Research Council under the European Union Horizon H2020 program (ERC Consolidator grant agreement No 772786-ResolveStroke).

rtPA-loaded fucoidan polymer microbubbles for the targeted treatment of stroke.

Systemic injection of thrombolytic drugs is the gold standard treatment for non-invasive blood clot resolution. The most serious risks associated with the intravenous injection of tissue plasminogen activator-like proteins are the bleeding complication and the dose related neurotoxicity. Indeed, the drug has to be injected in high concentrations due to its short half-life, the presence of its natural blood inhibitor (PAI-1) and the fast hepatic clearance (0.9 mg/kg in humans, 10 mg/kg in mouse models). Overall, there is a serious need for a dose-reduced targeted treatment to overcome these issues. We present in this article a new acoustic cavitation-based method for polymer MBs synthesis, three times faster than current hydrodynamic-cavitation method. The generated MBs are ultrasound responsive, stable and biocompatible. Their functionalization enabled the efficient and targeted treatment of stroke, without side effects. The stabilizing shell of the MBs is composed of Poly-Isobutyl Cyanoacrylate (PIBCA), copolymerized with fucoidan. Widely studied for its targeting properties, fucoidan exhibit a nanomolar affinity for activated endothelium and activated platelets (P-selectins). Secondly, the thrombolytic agent (rtPA) was loaded onto microbubbles (MBs) with a simple adsorption protocol. Hence, the present study validated the in vivo efficiency of rtPA-loaded Fuco MBs to be over 50 % more efficient than regular free rtPA injection for stroke resolution. In addition, the relative injected rtPA grafted onto targeting MBs was 1/10th of the standard effective dose (1 mg/kg in mouse). As a result, no hemorrhagic event, BBB leakage nor unexpected tissue distribution were observed.

In vivo targeted delivery of large payloads with an ultrasound clinical scanner.

PURPOSE: Performing drug-delivery with an ultrasonic imaging scanner in situ could drastically simplify treatment and improve its specificity. Our objective is to deliver large amounts of an encapsulated agent in vivo using a clinical ultrasound scanner with a millimetric resolution. This study describes the encapsulation of fluorescein within ultrasound-inducible composite droplets and its targeted release in predefined zones in the liver of rats. METHODS: An aqueous solution of fluorescein was encapsulated within perfluorocarbon liquid in 4 µm monodisperse droplets using a microfluidic system. The agent was then injected within the femoral vein of 12 rats. After exploratory ultrasound imaging, the sonographer defined five zones in the liver and a release sequence was initiated on the same apparatus. The surface of the liver was observed under fluorescence macroscopy and intraoperative fluorescence camera in vivo, before liver samples were sliced for pathology. RESULTS: Following the conversion of the droplets, a 25 dB increase in contrast was observed in the zones selected by the sonographer. These hyperechoic regions were colocalized with the bright fluorescent spots observed on the surface of the liver. A minimum peak-negative pressure of 2.6 MPa, which is within regulations for imaging pulses, was required for the delivery of the content of the droplets. The tissue and cellular structures were not affected by the exposure to the release sequence. CONCLUSIONS: Since composite droplets can carry various therapeutic and imaging agents, they could deliver such agents specifically in any organ accessible to ultrasound.

3D transcranial ultrasound localization microscopy for discrimination between ischemic and hemorrhagic stroke in early phase.

Early diagnosis is a critical part of the emergency care of cerebral hemorrhages and ischemia. A rapid and accurate diagnosis of strokes reduces the delays to appropriate treatments and a better functional recovery. Currently, CTscan and MRI are the gold standards with constraints of accessibility, availability, and possibly some contraindications. The development of Ultrasound Localization Microscopy (ULM) has enabled new perspectives to conventional transcranial ultrasound imaging with increased sensitivity, penetration depth, and resolution. The possibility of volumetric imaging has increased the field-of-view and provided a more precise description of the microvascularisation. In this study, rats (n = 9) were subjected to thromboembolic ischemic stroke or intracerebral hemorrhages prior to volumetric ULM at the early phases after onsets. Although the volumetric ULM performed in the early phase of ischemic stroke revealed a large hypoperfused area in the cortical area of the occluded artery, it showed a more diffused hypoperfusion in the hemorrhagic model. Respective computations of a Microvascular Diffusion Index highlighted different patterns of perfusion loss during the first 24 h of these two strokes' subtypes. Our study provides the first proof that this methodology should allow early discrimination between ischemic and hemorrhagic stroke with a potential toward diagnosis and monitoring in clinic.

Performance benchmarking of microbubble-localization algorithms for ultrasound localization microscopy.

Ultrafast ultrasound localization microscopy can be used to detect the subwavelength acoustic scattering of intravenously injected microbubbles to obtain haemodynamic maps of the vasculature of animals and humans. The quality of the haemodynamic maps depends on signal-to-noise ratios and on the algorithms used for the localization of the microbubbles and the rendering of their trajectories. Here we report the results of benchmarking of the performance of seven microbubble-localization algorithms. We used metrics for localization errors, localization success rates, processing times and a measure of the reprojection of the localization of the microbubbles on the original beamformed grid. We combined eleven metrics into an overall score and tested the algorithms in three simulated microcirculation datasets, and in angiography datasets of the brain of a live rat after craniotomy, an excised rat kidney and a mammary tumour in a live mouse. The algorithms, metrics and datasets, which we have made openly available at https://github.com/AChavignon/PALA and https://doi.org/10.5281/zenodo.4343435 , will facilitate the identification or generation of optimal microbubble-localization algorithms for specific applications.