In vivo whole brain microvascular imaging in mice using transcranial 3D Ultrasound Localization Microscopy

Role of astrocytes and microglia in hepatic encephalopathy associated with advanced chronic liver disease: lessons from animal studies

Hepatic encephalopathy, defined as neuropsychiatric dysfunction secondary to liver disease, is a frequent decompensating event in cirrhosis. Its clinical impact is highlighted by a notable increase in patient mortality rates and a concomitant reduction in overall quality of life. Systemically, liver disease, liver function failure, portosystemic shunting, and associated multi-organ dysfunction result in the increase of disease-causing neurotoxins in the circulation, which impairs cerebral homeostasis. Key circulating neurotoxins are ammonia and inflammatory mediators. In the brain, pathophysiology is less well understood, but is thought to be driven by glial cell dysfunction. Astrocytes are the only brain resident cells that have ammonia-metabolizing machinery and are therefore putatively most susceptible to ammonia elevation. Based on a large body of mostly in vitro evidence, ammonia-induced cellular and molecular disturbances include astrocyte swelling and oxidative stress. Microglia, the brain resident macrophages, have been linked to the translation of systemic inflammation to the brain microenvironment. Recent evidence from animal studies has provided novel insights into old and new downstream effects of astrocyte and microglial dysfunction such as toxin clearance disruption and myeloid cell attraction to the central nervous system parenchyma. Furthermore, state of the art research increasingly implicates neuronal dysfunction and possibly even irreversible neuronal cell death. Cell-type specific investigation in animal models highlights the need for critical revision of the contribution of astrocytes and microglia to well-established and novel cellular and molecular alterations in hepatic encephalopathy. In this review, we therefore give a current and comprehensive overview of causes, features, and consequences of astrocyte and microglial dysfunction in hepatic encephalopathy, including areas of interest for future investigation.

Advancements in Three-Dimensional Super-Resolution Ultrasound Imaging: A Narrative Review

The lack of sensibility of traditional ultrasound (US) imaging to the slow blood flow in small vessels resulted in the development of microbubble (MB) contrast agents. These MBs are given intravenously, and US imaging can detect them quite effectively. This noninvasive imaging method, known as contrast-enhanced US (CEUS), now makes it possible to accurately assess tissue perfusion and blood flow. Though CEUS offers several benefits, diffraction restricts the spatial resolution of all US imaging systems to length scales equal to roughly half the wavelength of the transmitted US beam. Based on individual MB detection and localization, the recently developed super-resolution US (SRUS) imaging method has shown unprecedentedly high spatial resolution exceeding the physical diffraction limit. It is now possible to visualize the microvasculature beyond the diffraction-limited resolution by localizing spatially isolated MBs across several frames. The highest resolution possible at clinical US frequencies can be on the order of several micrometers when tissue and probe motion are not present. Enhancing the functional study of tissue microvascular networks with structural data could lead to improved disease management. Through the localization and tracking of MBs, SRUS may reconstruct images of the microvasculature with resolution exceeding the diffraction limit in both 2-dimensional (2D) and 3-dimensional (3D) space. In contrast to the 2D approach, 3D SRUS imaging does not suffer from out-of-plane motion and can offer volumetric coverage with super-resolution in all three dimensions. Research has used two primary methods for 3D SRUS imaging including arrays that can electronically gather volumetric information or mechanically scanning the volume with a linear probe to produce a stack of 2D SRUS images. This manuscript aims to offer a comprehensive review of 3D SRUS imaging, clarifying methodologies, clinical applications, and notable challenges that could motivate future research and help facilitate clinical translation.

Microvascular blood flow ultrasound imaging with microbubble-based H-Scan technology

Ultrasound blood flow imaging plays a crucial role in the diagnosis of cardiovascular and cerebrovascular diseases. Conventional ultrafast ultrasound plane-wave imaging techniques have limited capabilities in microvascular imaging. To enhance the quality of blood flow imaging, this study proposes a microbubble-based H-Scan ultrasound imaging technique. This technique utilizes high-order H-Scan to detect the Rayleigh scattering contributed by blood flow and microbubbles at certain concentrations. The detected results are then processed in the B channel using methods such as clutter filtering based on Casorati matrix singular value decomposition (Casorati-SVD). Compared with the control group without H-Scan, the signal-to-noise ratio (SNR) of the experimental group using the microbubble-based H-Scan ultrasound imaging technique was increased by 38.61% on average and the contrast signal-to-noise ratio (CNR) was increased by 39.5% on average. The improved image quality of microvascular flow imaging was visibly enhanced. This method demonstrates significant advantages in enhancing the sensitivity and accuracy of ultrasound blood flow imaging, indicating considerable potential for clinical applications.

The Potential for Blood-Brain Barrier Disruption During Transcranial Ultrasound Super-Resolution Imaging

Super-resolution (SR) ultrasound imaging dramatically improves the resolution of microvascular images beyond the diffraction limit. It is well-established that ultrasound with contrast agents, such as those used in SR, can open the blood-brain barrier (BBB) under certain conditions. This study aimed to quantify potential BBB disruption (BBBD) during 3-D transcranial ultrasound SR as a function of mechanical index (MI) and pressure distribution. Mice were imaged using conventional 3-D SR sequences. BBBD was quantified through fluorescence imaging of dye extravasation for mechanical indices in the range of 0-0.78 (measured in water). The results suggest that when 3-D SR images are acquired at 0.78 MI, BBBD occurs; however, imaging was achievable without significant dye extravasation below 0.78. In addition, hydrophone measurements and 3-D simulations were performed to estimate local pressure distributions in the brain. For a transducer surface MI of 0.64, estimates of the local MI within the brain averaged 0.18 ± 0.09, ranging from 0.03 to 0.44, with no significant BBBD observed. However, for a transducer surface MI of 0.78, significant BBBD was observed (p < 0.05), where the values in the brain range from 0.04 to 0.53, averaging 0.22 ± 0.11. This suggests that the local MI that generates BBB bio-effects is within the range of 0.44-0.53 MI. For the 0.78 MI case, 4.8% of the total brain volume had a pressure above 0.44 MI according to hydrophone measurements (4.8 mm2 of the 100 mm2 2-D plane scanned) and 11.10% according to simulations (74.6 mm3 of the 777.5 mm3 volume).

Angle-Independent Blood Flow Velocity Measurement With Ultrasound Speckle Decorrelation Analysis

Precise measurement of the blood flow velocity in major arteries is important for the assessment of circulation dysfunction but challenging when using a one-dimensional (1D) ultrasound transducer array. Current available ultrasound velocimetry methods are susceptible to the probe-to-vessel angle and require the vessels to be well-aligned within the imaging plane of the 1D transducer array. In this study, a novel angle-independent velocimetry (VT-vUS) based on the ultrasound speckle decorrelation analysis of the ultrasound field signal is proposed to measure the blood flow velocity using a conventional 1D ultrasound transducer array. We first introduced the principle and evaluated this technique with numerical simulation and phantom experiments, which demonstrated that VT-vUS can accurately reconstruct the velocity magnitude of blood flow at arbitrary probe-to-vessel angles for different preset flow speeds (up to ~2.5 m/s). Further, we applied VT-vUS to measure the pulsatile flow of the radial artery and carotid artery in a healthy volunteer. Results show that the absolute velocity profiles obtained with VT-vUS at different probe-to-vessel angles have high consistency and agree well with the absolute speed obtained with the color Doppler-corrected velocimetry throughout the cardiac cycle. With the ability to alleviate the dependency on probe-to-vessel angle, VT-vUS has the potential for circulation-related disease screening in clinical practices.

Microfluidic nanobubbles produced using a micromixer for ultrasound imaging and gene delivery

Ultrasound (US)-mediated delivery is considered relatively safe and achieves tissue-specific targeting by simply adjusting the application site of the physical energy. Moreover, combining US with micro- or nanobubbles (MBs or NBs), which serve as US contrast agents, enhances the delivery of drugs, genes, and nucleic acids which also functioning as a tool for US. The performance of US-responsive MBs and NBs, including their therapeutic outcomes, is influenced by the bubble manufacturing methods. Furthermore, productivity and scalability must also be considered for clinical applications. Among various NBs fabrication techniques, microfluidic technology has emerged as a promising approach. However, the potential of NBs generated by microfluidics for drug delivery remains unexplored. In this study, US-responsive NBs were prepared using a microfluidic device, providing a single step gas-filling operation and rapid production method not only for US imaging but also for gene delivery. The effectiveness of these NBs was subsequently evaluated. The preparation conditions for the microfluidic NBs (MF-NBs) were optimized based on their physical properties, including particle size, number concentration, and their performance as US agents. Gene delivery capability was assessed in various tissues, including muscles, heart, kidney, and brain. The results demonstrated that MF-NBs exhibit high monodispersity, enhance US imaging, achieve widespread distribution following administration (including in brain tissue), and enable gene delivery to irradiated areas. These findings suggest that MF-NBs, with their high productivity and uniformity, are promising candidates for practical applications in US imaging, gene delivery, and nucleic acid delivery systems.

Pruning Sparse Tensor Neural Networks Enables Deep Learning for 3D Ultrasound Localization Microscopy

Ultrasound Localization Microscopy (ULM) is a non-invasive technique that allows for the imaging of micro-vessels in vivo, at depth and with a resolution on the order of ten microns. ULM is based on the sub-resolution localization of individual microbubbles injected in the bloodstream. Mapping the whole angioarchitecture requires the accumulation of microbubbles trajectories from thousands of frames, typically acquired over a few minutes. ULM acquisition times can be reduced by increasing the microbubble concentration, but requires more advanced algorithms to detect them individually. Several deep learning approaches have been proposed for this task, but they remain limited to 2D imaging, in part due to the associated large memory requirements. Herein, we propose the use of sparse tensor neural networks to enable deep learning-based 3D ULM by improving memory scalability with increased dimensionality. We study several approaches to efficiently convert ultrasound data into a sparse format and study the impact of the associated loss of information. When applied in 2D, the sparse formulation reduces the memory requirements by a factor 2 at the cost of a small reduction of performance when compared against dense networks. In 3D, the proposed approach reduces memory requirements by two order of magnitude while largely outperforming conventional ULM in high concentration settings. We show that Sparse Tensor Neural Networks in 3D ULM allow for the same benefits as dense deep learning based method in 2D ULM i.e. the use of higher concentration in silico and reduced acquisition time.

Transcranial brain-wide functional ultrasound and ultrasound localization microscopy in mice using multi-array probes

Functional ultrasound imaging (fUS) and ultrasound localization microscopy (ULM) are advanced ultrasound imaging modalities for assessing both functional and anatomical characteristics of the brain. However, the application of these techniques at a whole-brain scale has been limited by technological challenges. While conventional linear acoustic probes provide a narrow 2D field of view and matrix probes lack sufficient sensitivity for 3D transcranial fUS, multi-array probes have been developed to combine high sensitivity to blood flow with fast 3D acquisitions. In this study, we present a novel approach for the combined implementation of transcranial whole-brain fUS and ULM in mice using a motorized multi-array probe. This technique provides high-resolution, non-invasive imaging of neurovascular dynamics across the entire brain. Our findings reveal a significant correlation between absolute cerebral blood volume (ΔCBV) increases and microbubble speed, indicating vessel-level dependency of the evoked response. However, the lack of correlation with relative CBV (rCBV) suggests that fUS cannot distinguish functional responses alterations across different arterial vascular compartments. This methodology holds promise for advancing our understanding of neurovascular coupling and could be applied in brain disease diagnostics and therapeutic monitoring.

Optimizingin vivodata acquisition for robust clinical microvascular imaging using ultrasound localization microscopy

Objective. Ultrasound localization microscopy (ULM) enables microvascular imaging at spatial resolutions beyond the acoustic diffraction limit, offering significant clinical potentials. However, ULM performance relies heavily on microbubble (MB) signal sparsity, the number of detected MBs, and signal-to-noise ratio (SNR), all of which vary in clinical scenarios involving bolus MB injections. These sources of variations underscore the need to optimize MB dosage, data acquisition timing, and imaging settings in order to standardize and optimize ULM of microvasculature. This pilot study aims to investigate the temporal changes in MB signals during bolus injections in both pig and human models to optimize data acquisition for clinical ULM.Approach.Quantitative indices, mainly including individual MB SNR, normalized cross-correlation (NCC) of the MB signal with the point-spread function, and the number of localizable MBs, were developed to evaluate MB signal quality and guide the selection of acquisition timing. The effects of transmitted voltage and dosage on signal quality for MB localization were also explored.Main results. In both pig and human studies, MB localization quality (primarily indicated by NCC) reached a minimum at peak MB concentration, then improved as MB counts decreased during the wash-out phase. An optimal acquisition window was identified by balancing localization quality (empirically, NCC > 0.57) and MB concentration. In the pig model, a relatively short time window (approximately 10 s) for optimal acquisition was identified during the rapid wash-out phase, highlighting the need for real-time MB signal monitoring during data acquisition. The slower wash-out phase in humans allowed for a more flexible imaging window of 1-2 min, while trade-offs were observed between localization quality and MB density (or acquisition length) at different wash-out phase timings. Guided by these findings, robust ULM imaging was achieved in both pig and human kidneys using a short period of data acquisition (3.6 s and 9.6 s of data), demonstrating its feasibility in clinical practice.Significance.This study provides insights into optimizing data acquisition for consistent and reproducible ULM, paving the way for its standardization and broader clinical applications.

Noninvasive optical monitoring of cerebral hemodynamics in a preclinical model of neonatal intraventricular hemorrhage

Intraventricular hemorrhage (IVH) is a common complication in premature infants and is associated with white matter injury and long-term neurodevelopmental disabilities. Standard diagnostic tools such as cranial ultrasound and MRI are widely used in both preclinical drug development and clinical practice to detect IVH. However, these methods are limited to endpoint assessments of blood accumulation and do not capture real-time changes in germinal matrix blood flow leading to IVH. This limitation could potentially result in missed opportunities to advance drug candidates that may have protective effects against IVH. In this pilot study, we aimed to develop a noninvasive optical approach using diffuse correlation spectroscopy (DCS) to monitor real-time hemodynamic changes associated with hemorrhagic events and pre-hemorrhagic blood flow in a preclinical rabbit model of IVH. DCS measurements were conducted during the experimental induction of IVH, and results were compared with ultrasound and histological analysis to validate findings. Significant changes in hemodynamics were detected in all animals subjected to IVH-inducing procedures, including those that did not show clear positive results on ultrasound 18 h later. The study revealed progressively elevated coefficients of variation in blood flow, largely driven by temporal fluctuations in the <0.25 Hz range. Our findings suggest that real-time optical monitoring with DCS can provide critical insights heralding pathological blood flow changes, offering a more sensitive and informative tool for evaluating potential therapeutics that may help avert the progression to IVH.

Microvascular heterogeneity exploration in core and invasive zones of orthotopic rat glioblastoma via ultrasound localization microscopy

BACKGROUND

We studied the microvascular structure and function of in situ glioblastoma using ultrasound localization microscopy (ULM).

METHODS

The in vivo study was conducted via craniotomy in six Sprague-Dawley rats. Capillary pattern, capillary hemodynamics, and functional quantitative parameters were compared among tumor core, invasive zone, and normal brain tissue with ex vivo micro-computed tomography (micro-CT) and scanning electron microscopy. Correlations between quantitative parameters and histopathological vascular density (VD-H), proliferation index, and histopathological vascular maturity index (VMI-H) were evaluated. Kruskal-Wallis H, ANOVA, Mann-Whitney U, Pearson, and Spearman correlation statistics were used.

RESULTS

Compared to the tumor core, the invasive zone exhibited higher microvascularity structural disorder and complexity, increased hemodynamic heterogeneity, higher local blood flow perfusion (p ≤ 0.033), and slightly lower average flow velocity (p = 0.873). Significant differences were observed between the invasive zone and normal brain tissue across all parameters (p ≤ 0.001). ULM demonstrated higher microstructural resolution compared to micro-CT and a nonsignificant difference compared to scanning electron microscopy. The invasive zone vascular density correlated with VD-H (r = 0.781, p < 0.001). Vessel diameter (r = 0.960, p < 0.001), curvature (r = 0.438, p = 0.047), blood flow velocity (r = 0.487, p = 0.025), and blood flow volume (r = 0.858, p < 0.001) correlated with proliferation index. Vascular density (r = -0.444, p = 0.044) and fractal dimension (r = -0.933, p < 0.001) correlated with VMI-H.

CONCLUSION

ULM provided high-resolution, noninvasive imaging of glioblastoma microvascularity, offering insights into structural/functional abnormalities.

RELEVANCE STATEMENT

ULM technology based on ultrafast ultrasound can accurately quantify the microvessels of glioblastoma, providing a new method for evaluating the effectiveness of antiangiogenic therapy and visualizing disease progression. This method may facilitate early therapeutic assessment.

KEY POINTS

ULM reliably captures the vascular structures and hemodynamic features of glioblastoma in rats. Micro-CT and scanning electron microscopy validated its effectiveness in microvascular non-invasion characterization. ULM is expected to effectively evaluate glioblastoma anti-vascular therapy response.

Ultrasound Super-Resolution Imaging of Neonatal Cerebral Vascular Reorganization

During the first days of neonatal growth, the central nervous system (CNS) develops self-regulatory mechanisms to ensure constant cerebral perfusion. However, this vascular neogenesis takes place at a microscopic scale that cannot be observed with current clinical imaging techniques. Ultrasound localization microscopy (ULM) allows us to observe micro-vessels of the order of a few microns at depths of several centimeters. This can be done using conventional clinical ultrasound scanners and contrast sequences (CEUS). In this study, ULM is used to observe the human microvasculature in neonatal patients undergoing treatment for life-threatening malformations forming direct connections between the cerebral arterial and venous systems. It is observed that neuroendovascular treatment of neonatal arteriovenous malformations causes remodeling and reorganization of the cerebral vasculature by also activating corticomedullary vascular connections. ULM enables us to follow microvascular changes in human neonates with high spatio-temporal resolution. ULM may provide a novel clinical translatable tool, particularly including cerebral imaging in very young patients.

Non-invasive real-time pulsed Doppler assessment of blood flow in mouse ophthalmic artery

Non-invasive and high-temporal resolution methods for characterizing blood flow in mouse cranial arteries, such as the ophthalmic artery (OphA), are lacking. We present an application of pulsed Doppler ultrasound to provide real-time, non-invasive measurement of blood flow velocity in the OphA through an identified soft tissue window in the mouse head. We confirmed the identity of the artery and mapped its origin from the internal carotid artery by a combination of microcomputed tomography (microCT) vascular imaging and transient occlusion of the internal carotid artery. Application of our approach demonstrated sex differences in the OphA vasodilative response to agonists. We also evaluated real-time flow characteristics in the OphA in response to transient carotid artery ligation. The method will provide a simple and low-cost approach for screening drugs targeting ophthalmic blood flow and can be used as a more accessible surrogate of cerebral blood flow in both acute and longitudinal imaging studies.

Non-Invasive Deep-Brain Imaging With 3D Integrated Photoacoustic Tomography and Ultrasound Localization Microscopy (3D-PAULM)

Photoacoustic computed tomography (PACT) is a proven technology for imaging hemodynamics in deep brain of small animal models. PACT is inherently compatible with ultrasound (US) imaging, providing complementary contrast mechanisms. While PACT can quantify the brain's oxygen saturation of hemoglobin (sO , US imaging can probe the blood flow based on the Doppler effect. Further, by tracking gas-filled microbubbles, ultrasound localization microscopy (ULM) can map the blood flow velocity with sub-diffraction spatial resolution. In this work, we present a 3D deep-brain imaging system that seamlessly integrates PACT and ULM into a single device, 3D-PAULM. Using a low ultrasound frequency of 4 MHz, 3D-PAULM is capable of imaging the brain hemodynamic functions with intact scalp and skull in a totally non-invasive manner. Using 3D-PAULM, we studied the mouse brain functions with ischemic stroke. Multi-spectral PACT, US B-mode imaging, microbubble-enhanced power Doppler (PD), and ULM were performed on the same mouse brain with intrinsic image co-registration. From the multi-modality measurements, we further quantified blood perfusion, sO2, vessel density, and flow velocity of the mouse brain, showing stroke-induced ischemia, hypoxia, and reduced blood flow. We expect that 3D-PAULM can find broad applications in studying deep brain functions on small animal models.

A Tracking Prior to Localization Workflow for Ultrasound Localization Microscopy

Ultrasound Localization Microscopy (ULM) has proven effective in resolving microvascular structures and local mean velocities at sub-diffraction-limited scales, offering high-resolution imaging capabilities. Dynamic ULM (DULM) enables the creation of angiography or velocity movies throughout cardiac cycles. Currently, these techniques rely on a Localization-and-Tracking (LAT) workflow consisting in detecting microbubbles (MB) in the frames before pairing them to generate tracks. While conventional LAT methods perform well at low concentrations, they suffer from longer acquisition times and degraded localization and tracking accuracy at higher concentrations, leading to biased angiogram reconstruction and velocity estimation. In this study, we propose a novel approach to address these challenges by reversing the current workflow. The proposed method, Tracking-and-Localization (TAL), relies on first tracking the MB and then performing localization. Through comprehensive benchmarking using both in silico and in vivo experiments and employing various metrics to quantify ULM angiography and velocity maps, we demonstrate that the TAL method consistently outperforms the reference LAT workflow. Moreover, when applied to DULM, TAL successfully extracts velocity variations along the cardiac cycle with improved repeatability. The findings of this work highlight the effectiveness of the TAL approach in overcoming the limitations of conventional LAT methods, providing enhanced ULM angiography and velocity imaging.

3D ultrasound localization microscopy of the nonhuman primate brain

BACKGROUND

Haemodynamic changes occur in stroke and neurodegenerative diseases. Developing imaging techniques allowing the in vivo visualisation and quantification of cerebral blood flow would help better understand the underlying mechanism of these cerebrovascular diseases.

METHODS

3D ultrasound localization microscopy (ULM) is a recently developed technology that can map the microvasculature of the brain at large depth and has been mainly used until now in rodents. In this study, we tested the feasibility of 3D ULM of the nonhuman primate (NHP) brain with a single 256-channel programmable ultrasound scanner.

FINDINGS

We achieved a highly resolved vascular map of the macaque brain at large depth (down to 3 cm) in presence of craniotomy and durectomy using an 8-MHz multiplexed matrix probe. We were able to distinguish vessels as small as 26.9 μm. We also demonstrated that transcranial imaging of the macaque brain at similar depth was feasible using a 3-MHz probe and achieved a resolution of 60 μm.

INTERPRETATION

This work paves the way to clinical applications of 3D ULM. In particular, transcranial 3D ULM in humans could become a tool for the non-invasive study and monitoring of the brain cerebrovascular changes occurring in neurological diseases.

FUNDING

This work was supported by the New Frontier in Research Fund (NFRFE-2022-00590), by the Canada Foundation for Innovation under grant 38095, by the Natural Sciences and Engineering Research Council of Canada (NSERC) under discovery grant RGPIN-2020-06786, by Brain Canada under grant PSG2019, and by the Canadian Institutes of Health Research (CIHR) under grant PJT-156047 and MPI-452530. Computing support was provided by the Digital Research Alliance of Canada.

Non-invasive volumetric ultrasound localization microscopy detects vascular changes in mice with Alzheimer's disease

Alzheimer's Disease (AD) is the most common form of dementia and one of the leading causes of death. AD is known to be correlated to tortuosity in the microvasculature as well as decreases in blood flow throughout the brain. However, the mechanisms behind these changes and their causal relation to AD are poorly understood. Methods: Here, we use volumetric ultrasound localization microscopy (ULM) to non-invasively and quantitatively compare the microvascular morphology and flow dynamics of five wildtype (WT) and five APPNL-G-F Knock-in mice, a mouse model of AD, across a 1cmx1cmx1cm brain volume and in four specific brain regions: the hippocampal formation, thalamus, hypothalamus, and cerebral cortex. Results: Comparisons between groups showed a significant increase in tortuosity, as measured by the Sum of Angles Metric (SOAM), throughout the brain (p < 0.01) and the hypothalamus (p = 0.01), in mice with AD. While differences in mean velocity (p < 0.01) and blood flow (p=0.04) were detected across the whole brain, their effect size was small and no differences were detected in the four selected regions. There was a significant decrease in the linear log relationship between vessel diameter and blood flow, with AD mice experiencing a lower slope than WT mice across the whole brain volume (p = 0.02) and in the hippocampal formation (p = 0.05), a region affected by Amyloid Beta plaques in this mouse model. The AD mice had higher blood flows in smaller vessels and smaller blood flows in larger vessels than the WT mice. Conclusions: This preliminary demonstrates that the imaging technique can be used for non-invasive, longitudinal, volumetric assessment of AD, which may allow for investigation into the poorly understood microvascular degeneration associated with AD through time as well as the development of early diagnostic techniques.

Optimizing In Vivo Data Acquisition for Robust Clinical Microvascular Imaging Using Ultrasound Localization Microscopy

Ultrasound localization microscopy (ULM) enables microvascular imaging at spatial resolutions beyond the acoustic diffraction limit, offering significant clinical potentials. However, ULM performance relies heavily on microbubble (MB) signal sparsity, the number of detected MBs, and signal-to-noise ratio (SNR), all of which vary in clinical scenarios involving bolus MB injections. These sources of variations underscore the need to optimize MB dosage, data acquisition timing, and imaging settings in order to standardize and optimize ULM of microvasculature. This pilot study investigated temporal changes in MB signals during bolus injections in both pig and human models to optimize data acquisition for clinical ULM. Quantitative indices were developed to evaluate MB signal quality, guiding selection of acquisition timing that balances the MB localization quality and adequate MB counts. The effects of transmitted voltage and dosage were also explored. In the pig model, a relatively short window (approximately 10 seconds) for optimal acquisition was identified during the rapid wash-out phase, highlighting the need for real-time MB signal monitoring during data acquisition. The slower wash-out phase in humans allowed for a more flexible imaging window of 1-2 minutes, while trade-offs were observed between localization quality and MB density (or acquisition length) at different wash-out phase timings. Guided by these findings, robust ULM imaging was achieved in both pig and human kidneys using a short period of data acquisition, demonstrating its feasibility in clinical practice. This study provides insights into optimizing data acquisition for consistent and reproducible ULM, paving the way for its standardization and broader clinical applications.

Non-invasive 4D transcranial functional ultrasound and ultrasound localization microscopy for multimodal imaging of neurovascular response

A long-standing goal of neuroimaging is the non-invasive volumetric assessment of whole brain function and structure at high spatial and temporal resolutions. Functional ultrasound (fUS) and ultrasound localization microscopy (ULM) are rapidly emerging techniques that promise to bring advanced brain imaging and therapy to the clinic with the safety and low-cost advantages associated with ultrasound. fUS has been used to study cerebral hemodynamics at high temporal resolutions while ULM has been used to study cerebral microvascular structure at high spatial resolutions. These two methods have complementary spatio-temporal characteristics, making them ideally suited for multimodal imaging, but both suffer from limitations associated with transcranial ultrasound imaging. Here, these two methods are combined on the same data acquisition, completely non-invasively, using contrast-enhancements, which solves the dual challenges of sensitivity during transcranial imaging and the ability to implement super-resolution. From this combined approach, the cerebral blood flow, activated brain region, brain connectivity, vessel diameter, and vessel velocity were all calculated from the same data acquisition. During stimulation periods, there was a statistically significant (p<0.0001) increase in cerebral blood flow, diameter, and global velocity, but a decrease in velocity in the activated region. Additionally, the global flow increased (p=0.11) and connectivity decreased (24.7%) when compared to baseline. This multimodal approach allows for the study of the relationship between cerebral hemodynamics (30 ms resolution) and the microvasculature (14.6 μm resolution) using one ultrasound scan.

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

Using acoustic vortex tweezers (AVTs) 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 2-D 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 (SSS) 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.

Deep Learning in Ultrasound Localization Microscopy: Applications and Perspectives

Ultrasound localization microscopy (ULM) is a novel super-resolution imaging technique that can image the vasculature in vivo at depth with resolution far beyond the conventional limit of diffraction. By relying on the localization and tracking of clinically approved microbubbles injected in the blood stream, ULM can provide not only anatomical visualization but also hemodynamic quantification of the microvasculature. Several deep learning approaches have been proposed to address challenges in ULM including denoising, improving microbubble localization, estimating blood flow velocity, or performing aberration correction. Proposed deep learning methods often outperform their conventional counterparts by improving image quality and reducing processing time. In addition, their robustness to high concentrations of microbubbles can lead to reduced acquisition times in ULM, addressing a major hindrance to ULM clinical application. Herein, we propose a comprehensive review of the diversity of deep learning applications in ULM focusing on approaches assuming a sparse microbubble distribution. We first provide an overview of how existing studies vary in the constitution of their datasets or in the tasks targeted by the deep learning model. We also take a deeper look into the numerous approaches that have been proposed to improve the localization of microbubbles since they differ highly in their formulation of the optimization problem, their evaluation, or their network architectures. We finally discuss the current limitations and challenges of these methods, as well as the promises and potential of deep learning for ULM in the future.

Transcutaneous Imaging of Rabbit Kidney Using 3-D Acoustic Wave Sparsely Activated Localization Microscopy With a Row-Column-Addressed Array

OBJECTIVE

Super-resolution ultrasound (SRUS) imaging through localizing and tracking microbubbles, also known as ultrasound localization microscopy (ULM), can produce sub-diffraction resolution images of micro-vessels. We have recently demonstrated 3-D selective SRUS with a matrix array and phase change contrast agents (PCCAs). However, this method is limited to a small field of view (FOV) and by the complex hardware required.

METHOD

This study proposed 3-D acoustic wave sparsely activated localization microscopy (AWSALM) using PCCAs and a 128+128 row-column-addressed (RCA) array, which offers ultrafast acquisition with over 6 times larger FOV and 4 times reduction in hardware complexity than a 1024-element matrix array. We first validated this method on an in-vitro microflow phantom and subsequently demonstrated non-invasively on a rabbit kidney in-vivo.

RESULTS

Our results show that 3-D AWSALM images of the phantom covering a mm volume can be generated under 5 seconds with an 8 times resolution improvement over the system point spread function. The full volume of the rabbit kidney can be covered to generate 3-D microvascular structure, flow speed and direction super-resolution maps under 15 seconds, combining the large FOV of RCA with the high resolution of SRUS. Additionally, 3-D AWSALM is selective and can visualize the microvasculature within the activation volume and downstream vessels in isolation. Sub-sets of the kidney microvasculature can be imaged through selective activation of PCCAs.

CONCLUSION

Our study demonstrates large FOV 3-D AWSALM using an RCA probe.

SIGNIFICANCE

3-D AWSALM offers an unique in-vivo imaging tool for fast, selective and large FOV vascular flow mapping.

Optically Validated Microvascular Phantom for Super-Resolution Ultrasound Imaging

Super-resolution ultrasound (SRUS) visualizes microvasculature beyond the ultrasound (US) diffraction limit (wavelength( )/2) by localizing and tracking spatially isolated microbubble (MB) contrast agents. SRUS phantoms typically consist of simple tube structures, where diameter channels below m are not available. Furthermore, these phantoms are generally fragile and unstable, have limited ground truth validation, and their simple structure limits the evaluation of SRUS algorithms. To aid SRUS development, robust and durable phantoms with known and physiologically relevant microvasculature are needed for repeatable SRUS testing. This work proposes a method to fabricate durable microvascular phantoms that allow optical gauging for SRUS validation. The methodology used a microvasculature negative print embedded in a Polydimethylsiloxane (PDMS) to fabricate a microvascular phantom. Branching microvascular phantoms with variable microvascular density were demonstrated with optically validated vessel diameters down to m ( ; m). SRUS imaging was performed and validated with optical measurements. The average SRUS error was m ( ) with a standard deviation error of m. The average error decreased to m ( ) once the number of localized MBs surpassed 1000 per estimated diameter. In addition, less than 10% variance of acoustic and optical properties and the mechanical toughness of the phantoms measured a year after fabrication demonstrated their long-term durability. This work presents a method to fabricate durable and optically validated complex microvascular phantoms which can be used to quantify SRUS performance and facilitate its further development.

Volumetric Ultrasound Localization Microscopy

Super-resolution ultrasound (SRUS) has evolved significantly with the advent of ultrasound localization microscopy (ULM). This technique enables subwavelength resolution imaging using microbubble contrast agents. Initially confined to 2-D imaging, ULM has progressed toward volumetric approaches, allowing for comprehensive 3-D visualization of microvascular networks. This review explores the technological advancements and challenges associated with volumetric ULM, focusing on key aspects such as transducer design, acquisition speed, data processing algorithms, or integration into clinical practice. We discuss the limitations of traditional 2-D ULM, including dependence on precise imaging plane selection and compromised resolution in microvasculature quantification. In contrast, volumetric ULM offers enhanced spatial resolution and allows motion correction in all directions, promising transformative insights into microvascular pathophysiology. By examining current research and future directions, this review highlights the potential of volumetric ULM to contribute significantly to diagnostic across various medical conditions, including cancers, arteriosclerosis, strokes, diabetes, and neurodegenerative diseases.

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 3-D ultrasound imaging at a high frame rate with sufficient signal-to-noise ratio (SNR) 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 the ULM saturation rate by 60% through a skull phantom. The assessment of microbubble velocity was also improved from a 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 SNR and 3-D nature, such a cylindrical volumetric sequence could be beneficial for ULM as a diagnostic tool in humans, especially when portability is a necessity.

Novel intravital approaches to quantify deep vascular structure and perfusion in the aging mouse brain using ultrasound localization microscopy (ULM)

Intra-vital visualization of deep cerebrovascular structures and blood flow in the aging brain has been a difficult challenge in the field of neurovascular research, especially when considering the key role played by the cerebrovasculature in the pathogenesis of both vascular cognitive impairment and dementia (VCID) and Alzheimer's disease (AD). Traditional imaging methods face difficulties with the thicker skull of older brains, making high-resolution imaging and cerebral blood flow (CBF) assessment challenging. However, functional ultrasound (fUS) imaging, an emerging non-invasive technique, provides real-time CBF insights with notable spatial-temporal resolution. This study introduces an enhanced longitudinal fUS method for aging brains. Using elderly (24-month C57BL/6) mice, we detail replacing the skull with a polymethylpentene window for consistent fUS imaging over extended periods. Ultrasound localization mapping (ULM), involving the injection of a microbubble (<<10 μm) suspension allows for recording of high-resolution microvascular vessels and flows. ULM relies on the localization and tracking of single circulating microbubbles in the blood flow. A FIJI-based analysis interprets these high-quality ULM visuals. Testing on older mouse brains, our method successfully unveils intricate vascular specifics even in-depth, showcasing its utility for longitudinal studies that require ongoing evaluations of CBF and vascular aspects in aging-focused research.

3D light-sheet fluorescence microscopy in preclinical and clinical drug discovery

Light-sheet fluorescence microscopy (LSFM) combined with tissue clearing has emerged as a powerful technology in drug discovery. LSFM is applicable to a variety of samples, from rodent organs to clinical tissue biopsies, and has been used for characterizing drug targets in tissues, demonstrating the biodistribution of pharmaceuticals and determining their efficacy and mode of action. LSFM is scalable to high-throughput analysis and provides resolution down to the single cell level. In this review, we describe the advantages of implementing LSFM into the drug discovery pipeline and highlight recent advances in this field.

Super-resolution ultrasound imaging reveals temporal cerebrovascular changes with disease progression in female 5×FAD mouse model of Alzheimer's disease: correlation with pathological impairments

BACKGROUND

Vascular dysfunction is closely associated with the progression of Alzheimer's disease (AD). A critical research gap exists that no studies have explored the in vivo temporal changes of cerebrovascular alterations with AD progression in mouse models, encompassing both structure and flow dynamics at micron-scale resolution across the early, middle, and late stages of the disease.

METHODS

In this study, ultrasound localisation microscopy (ULM) was applied to image the cerebrovascular alterations of the transgenic female 5×FAD mouse model across different stages of disease progression: early (4 months), moderate (7 months), and late (12 months). Age-matched non-transgenic (non-Tg) littermates were used as controls. Immunohistology examinations were performed to evaluate the influence of disease progression on the β-amyloid (Aβ) load and microvascular alterations, including morphological changes and the blood-brain barrier (BBB) leakage.

FINDINGS

Our findings revealed a significant decline in both vascular density and flow velocity in the retrosplenial cortex of 5×FAD mice at an early stage, which subsequently became more pronounced in the visual cortex and hippocampus as the disease progressed. Additionally, we observed a reduction in vascular length preceding diminished flow velocities in cortical penetrating arterioles during the early stages. The quantification of vascular metrics derived from ULM imaging showed significant correlations with those obtained from vascular histological images. Immunofluorescence staining identified early vascular abnormalities in the retrosplenial cortex. As the disease advanced, there was an exacerbation of Aβ accumulation and BBB disruption in a regionally variable manner. The vascular changes observed through ULM imaging exhibited a negative correlation with amyloid load and were associated with the compromise of the BBB integrity.

INTERPRETATION

Through high-resolution, in vivo imaging of cerebrovasculature, this study reveals significant spatiotemporal dysfunction in cerebrovascular dynamics accompanying disease progression in a mouse model of AD, enhancing our understanding of its pathophysiology.

FUNDING

This study is supported by grants from National Key Research and Development Program of China (2020YFA0908800), National Natural Science Foundation of China (12074269, 82272014, 82327804, 62071310), Shenzhen Basic Science Research (20220808185138001, JCYJ20220818095612027, JCYJ20210324093006017), STI 2030-Major Projects (2021ZD0200500) and Guangdong Natural Science Foundation (2024A1515012591, 2024A1515011342).

Dynamic Ultrasound Localization Microscopy Without ECG-Gating

OBJECTIVE

Dynamic Ultrasound Localization Microscopy (DULM) has first been developed for non-invasive Pulsatility measurements in the rodent brain. DULM relies on the localization and tracking of microbubbles (MBs) injected into the bloodstream, to obtain highly resolved velocity and density cine-loops. Previous DULM techniques required ECG-gating, limiting its application to specific datasets, and increasing acquisition time. The objective of this study is to eliminate the need for ECG-gating in DULM experiments by introducing a motion-matching method for time registration.

METHODS

We developed a motion-matching algorithm based on tissue Doppler that leverages the cyclic tissue motion within the brain. Tissue Doppler was estimated for each group of frames in the acquisitions, at multiple locations identified as local maxima in the skin above the skull. Subsequently, each group of frames was time-registered to a reference group by delaying it based on the maximum correlation value between their respective tissue Doppler signals. This synchronization ensured that each group of frames aligned with the brain tissue motion of the reference group, and consequently, with its cardiac cycle. As a result, velocities of MBs could be averaged to retrieve flow velocity variations over time.

RESULTS

Initially validated in ECG-gated acquisitions in a rat model (n = 1), the proposed method was successfully applied in a mice model in 2D (n = 3) and in a feline model in 3D (n = 1). Performing time-registration with the proposed motion-matching method or by using ECG-gating leads to similar results. For the first time, dynamic velocity and density cine-loops were extracted without the need for any information on the animal ECG, and complex dynamic markers such as the Pulsatility index were estimated.

CONCLUSION

Results suggest that DULM can be performed without external gating, enabling the use of DULM on any ULM dataset where enough MBs are detectable. Time registration by motion-matching represents a significant advancement in DULM techniques, making DULM more accessible by simplifying its experimental complexity.

Evaluation of tumor microvasculature with 3D ultrasound localization microscopy based on 2D matrix array

OBJECTIVE

Evaluation of tumor microvascular morphology is of great significance in tumor diagnosis, therapeutic effect prediction, and surgical planning. Recently, two-dimensional ultrasound localization microscopy (2DULM) has demonstrated its superiority in the field of microvascular imaging. However, it suffers from planar dependence and is unintuitive. We propose a novel three-dimensional ultrasound localization microscopy (3DULM) to avoid these limitations.

METHODS

We investigated 3DULM based on a 2D array for tumor microvascular imaging. After intravenous injection of contrast agents, all elements of the 2D array transmit and receive signals to ensure a high and stable frame rate. Microbubble signal extraction, filtering, positioning, tracking, and other processing were used to obtain a 3D vascular map, flow velocity, and flow direction. To verify the effectiveness of 3DULM, it was validated on double helix tubes and rabbit VX2 tumors. Cisplatin was used to verify the ability of 3DULM to detect microvascular changes during tumor treatment.

RESULTS

In vitro, the sizes measured by 3DULM at 3 mm and 13 mm were 178 μ m and 182 μ m , respectively. In the rabbit tumors, we acquired 9000 volumes to reveal vessels about 30 μ m in diameter, which surpasses the diffraction limit of ultrasound in traditional ultrasound imaging, and the results matched with micro-angiography. In addition, there were significant changes in vascular density and curvature between the treatment and control groups.

CONCLUSIONS

The effectiveness of 3DULM was verified in vitro and in vivo. Hence, 3DULM may have potential applications in tumor diagnosis, tumor treatment evaluation, surgical protocol guidance, and cardiovascular disease.

CLINICAL RELEVANCE STATEMENT

3D ultrasound localization microscopy is highly sensitive to microvascular changes; thus, it has clinical potential for tumor diagnosis and treatment evaluation.

KEY POINTS

• 3D ultrasound localization microscopy is demonstrated on double helix tubes and rabbit VX2 tumors. • 3D ultrasound localization microscopy can reveal vessels about 30 μ m in diameter-far smaller than traditional ultrasound. • This form of imaging has potential applications in tumor diagnosis, tumor treatment evaluation, surgical protocol guidance, and cardiovascular disease.

Multiparametric Brain Hemodynamics Imaging Using a Combined Ultrafast Ultrasound and Photoacoustic System

Studying brain-wide hemodynamic responses to different stimuli at high spatiotemporal resolutions can help gain new insights into the mechanisms of neuro- diseases and -disorders. Nonetheless, this task is challenging, primarily due to the complexity of neurovascular coupling, which encompasses interdependent hemodynamic parameters including cerebral blood volume (CBV), cerebral blood flow (CBF), and cerebral oxygen saturation (SO2). The current brain imaging technologies exhibit inherent limitations in resolution, sensitivity, and imaging depth, restricting their capacity to comprehensively capture the intricacies of cerebral functions. To address this, a multimodal functional ultrasound and photoacoustic (fUSPA) imaging platform is reported, which integrates ultrafast ultrasound and multispectral photoacoustic imaging methods in a compact head-mountable device, to quantitatively map individual dynamics of CBV, CBF, and SO2 as well as contrast agent enhanced brain imaging at high spatiotemporal resolutions. Following systematic characterization, the fUSPA system is applied to study brain-wide cerebrovascular reactivity (CVR) at single-vessel resolution via relative changes in CBV, CBF, and SO2 in response to hypercapnia stimulation. These results show that cortical veins and arteries exhibit differences in CVR in the stimulated state and consistent anti-correlation in CBV oscillations during the resting state, demonstrating the multiparametric fUSPA system's unique capabilities in investigating complex mechanisms of brain functions.

Ultrasound Speckle Decorrelation Analysis-Based Velocimetry for 3D-Velocity-Components Measurement Using a 1D Transducer Array

Ultrasound velocimetry has been widely used for blood flow imaging. However, the flow measurements are constrained to resolve the in-plane 2D flow components when using a 1D transducer array. In this work, an ultrasound speckle decorrelation analysis-based velocimetry (3C-vUS) is proposed for 3D velocity components measurement using a 1D transducer array. The 3C-vUS theory is first derived and validated with numerical simulations and phantom experiments. The in vivo testing results show that 3C-vUS can accurately measure the blood flow 3D-velocity-components of the human carotid artery at arbitrary probe-to-vessel angles throughout the cardiac cycle. With such capability, the 3C-vUS will alleviate the requirement of operators and promote disease screening for blood flow-related disorders.

Microbubble-enhanced transcranial MR-guided focused ultrasound brain hyperthermia: heating mechanism investigation using finite element method

Recently, our group developed a synergistic brain drug delivery method to achieve simultaneous transcranial hyperthermia and localized blood-brain barrier opening via MR-guided focused ultrasound (MRgFUS). In a rodent model, we demonstrated that the ultrasound power required for transcranial MRgFUS hyperthermia was significantly reduced by injecting microbubbles (MBs). However, the specific mechanisms underlying the power reduction caused by MBs remain unclear. The present study aims to elucidate the mechanisms of MB-enhanced transcranial MRgFUS hyperthermia through numerical studies using the finite element method. The microbubble acoustic emission (MAE) and the viscous dissipation (VD) were hypothesized to be the specific mechanisms. Acoustic wave propagation was used to model the FUS propagation in the brain tissue, and a bubble dynamics equation for describing the dynamics of MBs with small shell thickness was used to model the MB oscillation under FUS exposures. A modified bioheat transfer equation was used to model the temperature in the rodent brain with different heat sources. A theoretical model was used to estimate the bubble shell's surface tension, elasticity, and viscosity losses. The simulation reveals that MAE and VD caused a 40.5% and 52.3% additional temperature rise, respectively. Compared with FUS only, MBs caused a 64.0% temperature increase, which is consistent with our previous animal experiments. Our investigation showed that MAE and VD are the main mechanisms of MB-enhanced transcranial MRgFUS hyperthermia.

3D Acoustic Wave Sparsely Activated Localization Microscopy With Phase Change Contrast Agents

OBJECTIVE

The aim of this study is to demonstrate 3-dimensional (3D) acoustic wave sparsely activated localization microscopy (AWSALM) of microvascular flow in vivo using phase change contrast agents (PCCAs).

MATERIALS AND METHODS

Three-dimensional AWSALM using acoustically activable PCCAs was evaluated on a crossed tube microflow phantom, the kidney of New Zealand White rabbits, and the brain of C57BL/6J mice through intact skull. A mixture of C 3 F 8 and C 4 F 10 low-boiling-point fluorocarbon gas was used to generate PCCAs with an appropriate activation pressure. A multiplexed 8-MHz matrix array connected to a 256-channel ultrasound research platform was used for transmitting activation and imaging ultrasound pulses and recording echoes. The in vitro and in vivo echo data were subsequently beamformed and processed using a set of customized algorithms for generating 3D super-resolution ultrasound images through localizing and tracking activated contrast agents.

RESULTS

With 3D AWSALM, the acoustic activation of PCCAs can be controlled both spatially and temporally, enabling contrast on demand and capable of revealing 3D microvascular connectivity. The spatial resolution of the 3D AWSALM images measured using Fourier shell correlation is 64 μm, presenting a 9-time improvement compared with the point spread function and 1.5 times compared with half the wavelength. Compared with the microbubble-based approach, more signals were localized in the microvasculature at similar concentrations while retaining sparsity and longer tracks in larger vessels. Transcranial imaging was demonstrated as a proof of principle of PCCA activation in the mouse brain with 3D AWSALM.

CONCLUSIONS

Three-dimensional AWSALM generates volumetric ultrasound super-resolution microvascular images in vivo with spatiotemporal selectivity and enhanced microvascular penetration.

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.

Biopathologic Characterization and Grade Assessment of Breast Cancer With 3-D Multiparametric Ultrasound Combining Shear Wave Elastography and Backscatter Tensor Imaging

OBJECTIVE

Despite recent improvements in medical imaging, the final diagnosis and biopathologic characterization of breast cancers currently still requires biopsies. Ultrasound is commonly used for clinical examination of breast masses. B-Mode and shear wave elastography (SWE) are already widely used to detect suspicious masses and differentiate benign lesions from cancers. But additional ultrasound modalities such as backscatter tensor imaging (BTI) could provide relevant biomarkers related to tissue organization. Here we describe a 3-D multiparametric ultrasound approach applied to breast carcinomas in the aims of (i) validating the ability of BTI to reveal the underlying organization of collagen fibers and (ii) assessing the complementarity of SWE and BTI to reveal biopathologic features of diagnostic interest.

METHODS

Three-dimensional SWE and BTI were performed ex vivo on 64 human breast carcinoma samples using a linear ultrasound probe moved by a set of motors. Here we describe a 3-D multiparametric representation of the breast masses and quantitative measurements combining B-mode, SWE and BTI.

RESULTS

Our results reveal for the first time that BTI can capture the orientation of the collagen fibers around tumors. BTI was found to be a relevant marker for assessing cancer stages, revealing a more tangent tissue orientation for in situ carcinomas than for invasive cancers. In invasive cases, the combination of BTI and SWE parameters allowed for classification of invasive tumors with respect to their grade with an accuracy of 95.7%.

CONCLUSION

Our results highlight the potential of 3-D multiparametric ultrasound imaging for biopathologic characterization of breast tumors.

Specific and Nonuniform Brain States during Cold Perception in Mice

The quest to decode the complex supraspinal mechanisms that integrate cutaneous thermal information in the central system is still ongoing. The dorsal horn of the spinal cord is the first hub that encodes thermal input which is then transmitted to brain regions via the spinothalamic and thalamocortical pathways. So far, our knowledge about the strength of the interplay between the brain regions during thermal processing is limited. To address this question, we imaged the brains of adult awake male mice in resting state using functional ultrasound imaging during plantar exposure to constant and varying temperatures. Our study reveals for the first time the following: (1) a dichotomy in the response of the somatomotor-cingulate cortices and the hypothalamus, which was never described before, due to the lack of appropriate tools to study such regions with both good spatial and temporal resolutions. (2) We infer that cingulate areas may be involved in the affective responses to temperature changes. (3) Colder temperatures (ramped down) reinforce the disconnection between the somatomotor-cingulate and hypothalamus networks. (4) Finally, we also confirm the existence in the mouse brain of a brain mode characterized by low cognitive strength present more frequently at resting neutral temperature. The present study points toward the existence of a common hub between somatomotor and cingulate regions, whereas hypothalamus functions are related to a secondary network.

A Multiplexed 32 × 32 2D Matrix Array Transducer for Flexible Sub-Aperture Volumetric Ultrasound Imaging

A fully-sampled two-dimensional (2D) matrix array ultrasonic transducer is essential for fast and accurate three-dimensional (3D) volumetric ultrasound imaging. However, these arrays, usually consisting of thousands of elements, not only face challenges of poor performance and complex wiring due to high-density elements and small element sizes but also put high requirements for electronic systems. Current commercially available fully-sampled matrix arrays, dividing the aperture into four fixed sub-apertures to reduce system channels through multiplexing are widely used. However, the fixed sub-aperture configuration limits imaging flexibility and the gaps between sub-apertures lead to reduced imaging quality. In this study, we propose a high-performance multiplexed matrix array by the design of 1-3 piezocomposite and gapless sub-aperture configuration, as well as optimized matching layer materials. Furthermore, we introduce a sub-aperture volumetric imaging method based on the designed matrix array, enabling high-quality and flexible 3D ultrasound imaging with a low-cost 256-channel system. The influence of imaging parameters, including the number of sub-apertures and steering angle on imaging quality was investigated by simulation, in vitro and in vivo imaging experiments. The fabricated matrix array has a center frequency of 3.4 MHz and a -6 dB bandwidth of above 70%. The proposed sub-aperture volumetric imaging method demonstrated a 10% improvement in spatial resolution, a 19% increase in signal-to-noise ratio, and a 57.7% increase in contrast-to-noise ratio compared with the fixed sub-aperture array imaging method. This study provides a new strategy for high-quality volumetric ultrasound imaging with a low-cost system.

Reconstructing microvascular network skeletons from 3D images: What is the ground truth?

Structural changes to microvascular networks are increasingly highlighted as markers of pathogenesis in a wide range of disease, e.g. Alzheimer's disease, vascular dementia and tumour growth. This has motivated the development of dedicated 3D imaging techniques, alongside the creation of computational modelling frameworks capable of using 3D reconstructed networks to simulate functional behaviours such as blood flow or transport processes. Extraction of 3D networks from imaging data broadly consists of two image processing steps: segmentation followed by skeletonisation. Much research effort has been devoted to segmentation field, and there are standard and widely-applied methodologies for creating and assessing gold standards or ground truths produced by manual annotation or automated algorithms. The Skeletonisation field, however, lacks widely applied, simple to compute metrics for the validation or optimisation of the numerous algorithms that exist to extract skeletons from binary images. This is particularly problematic as 3D imaging datasets increase in size and visual inspection becomes an insufficient validation approach. In this work, we first demonstrate the extent of the problem by applying 4 widely-used skeletonisation algorithms to 3 different imaging datasets. In doing so we show significant variability between reconstructed skeletons of the same segmented imaging dataset. Moreover, we show that such a structural variability propagates to simulated metrics such as blood flow. To mitigate this variability we introduce a new, fast and easy to compute super metric that compares the volume, connectivity, medialness, bifurcation point identification and homology of the reconstructed skeletons to the original segmented data. We then show that such a metric can be used to select the best performing skeletonisation algorithm for a given dataset, as well as to optimise its parameters. Finally, we demonstrate that the super metric can also be used to quickly identify how a particular skeletonisation algorithm could be improved, becoming a powerful tool in understanding the complex implication of small structural changes in a network.

Photoacoustic imaging of squirrel monkey cortical responses induced by peripheral mechanical stimulation

Non-human primates (NHPs) are crucial models for studies of neuronal activity. Emerging photoacoustic imaging modalities offer excellent tools for studying NHP brains with high sensitivity and high spatial resolution. In this research, a photoacoustic microscopy (PAM) device was used to provide a label-free quantitative characterization of cerebral hemodynamic changes due to peripheral mechanical stimulation. A 5 × 5 mm area within the somatosensory cortex region of an adult squirrel monkey was imaged. A deep, fully connected neural network was characterized and applied to the PAM images of the cortex to enhance the vessel structures after mechanical stimulation on the forelimb digits. The quality of the PAM images was improved significantly with a neural network while preserving the hemodynamic responses. The functional responses to the mechanical stimulation were characterized based on the improved PAM images. This study demonstrates capability of PAM combined with machine learning for functional imaging of the NHP brain.

Multi-Scale Volumetric Dynamic Optoacoustic and Laser Ultrasound (OPLUS) Imaging Enabled by Semi-Transparent Optical Guidance

Major biological discoveries are made by interrogating living organisms with light. However, the limited penetration of un-scattered photons within biological tissues limits the depth range covered by optical methods. Deep-tissue imaging is achieved by combining light and ultrasound. Optoacoustic imaging exploits the optical generation of ultrasound to render high-resolution images at depths unattainable with optical microscopy. Recently, laser ultrasound has been suggested as a means of generating broadband acoustic waves for high-resolution pulse-echo ultrasound imaging. Herein, an approach is proposed to simultaneously interrogate biological tissues with light and ultrasound based on layer-by-layer coating of silica optical fibers with a controlled degree of transparency. The time separation between optoacoustic and ultrasound signals collected with a custom-made spherical array transducer is exploited for simultaneous 3D optoacoustic and laser ultrasound (OPLUS) imaging with a single laser pulse. OPLUS is shown to enable large-scale anatomical characterization of tissues along with functional multi-spectral imaging of chromophores and assessment of cardiac dynamics at ultrafast rates only limited by the pulse repetition frequency of the laser. The suggested approach provides a flexible and scalable means for developing a new generation of systems synergistically combining the powerful capabilities of optoacoustics and ultrasound imaging in biology and medicine.

Super-Resolution Ultrasound Reveals Cerebrovascular Impairment in a Mouse Model of Alzheimer's Disease

Increasing evidence has suggested a link between cerebrovascular disease and the cognitive impairment associated with Alzheimer's disease. However, detailed descriptions of microvascular changes across brain regions and how they relate to other more traditional pathology have been lacking. Additionally, the efforts to elucidate the interplay between cerebral microvascular function and Alzheimer's disease progression are complicated by the necessity of probing deep-brain structures since early-stage Alzheimer's disease typically involves hippocampal pathology. The purpose of this study was to examine changes in microvascular dynamics in a mouse model of Alzheimer's disease using cohorts that were age-matched to wild-type controls. Data from both sexes were included in this study. Super-resolution ultrasound localization microscopy revealed microvascular functional and structural features throughout the whole brain depth to visualize and quantify. We found that functional decreases in hippocampal and entorhinal flow velocity preceded structural derangements in regional vascular density. Co-registered histological sectioning confirmed the regionalized perfusion deficits seen on ultrasound imaging, which were co-localized with amyloid beta plaque deposition. In addition to providing global vascular quantifications of deep brain structures with a high local resolution, this technology also permitted velocity-profile analysis of individual vessels and, in some cases, allowed for decoupling of arterial and venous flow contributions. These data suggest that microvascular pathology is an early and pervasive feature of Alzheimer's disease and may represent a novel therapeutic target for this disease.

Ultrasound Flow Imaging Study on Rat Brain with Ultrasound and Light Stimulations

Functional ultrasound (fUS) flow imaging provides a non-invasive method for the in vivo study of cerebral blood flow and neural activity. This study used functional flow imaging to investigate rat brain's response to ultrasound and colored-light stimuli. Male Long-Evan rats were exposed to direct full-field strobe flashes light and ultrasound stimulation to their retinas, while brain activity was measured using high-frequency ultrasound imaging. Our study found that light stimuli, particularly blue light, elicited strong responses in the visual cortex and lateral geniculate nucleus (LGN), as evidenced by changes in cerebral blood volume (CBV). In contrast, ultrasound stimulation elicited responses undetectable with fUS flow imaging, although these were observable when directly measuring the brain's electrical signals. These findings suggest that fUS flow imaging can effectively differentiate neural responses to visual stimuli, with potential applications in understanding visual processing and developing new diagnostic tools.

Quantitative pulsatility measurements using 3D dynamic ultrasound localization microscopy

A rise in blood flow velocity variations (i.e. pulsatility) in the brain, caused by the stiffening of upstream arteries, is associated with cognitive impairment and neurodegenerative diseases. The study of this phenomenon requires brain-wide pulsatility measurements, with large penetration depth and high spatiotemporal resolution. The development of dynamic ultrasound localization microscopy (DULM), based on ULM, has enabled pulsatility measurements in the rodent brain in 2D. However, 2D imaging accesses only one slice of the brain and measures only 2D-projected and hence biased velocities . Herein, we present 3D DULM: using a single ultrasound scanner at high frame rate (1000-2000 Hz), this method can produce dynamic maps of microbubbles flowing in the bloodstream and extract quantitative pulsatility measurements in the cat brain with craniotomy and in the mouse brain through the skull, showing a wide range of flow hemodynamics in both large and small vessels. We highlighted a decrease in pulsatility along the vascular tree in the cat brain, which could be mapped with ultrasound down to a few tens of micrometers for the first time. We also performed an intra-animal validation of the method by showing consistent measurements between the two sides of the Willis circle in the mouse brain. Our study provides the first step towards a new biomarker that would allow the detection of dynamic abnormalities in microvessels in the brain, which could be linked to early signs of neurodegenerative diseases.

Phase Aberration Correction for In Vivo Ultrasound Localization Microscopy Using a Spatiotemporal Complex-Valued Neural Network

Ultrasound Localization Microscopy (ULM) can map microvessels at a resolution of a few micrometers ( [Formula: see text]). Transcranial ULM remains challenging in presence of aberrations caused by the skull, which lead to localization errors. Herein, we propose a deep learning approach based on recently introduced complex-valued convolutional neural networks (CV-CNNs) to retrieve the aberration function, which can then be used to form enhanced images using standard delay-and-sum beamforming. CV-CNNs were selected as they can apply time delays through multiplication with in-phase quadrature input data. Predicting the aberration function rather than corrected images also confers enhanced explainability to the network. In addition, 3D spatiotemporal convolutions were used for the network to leverage entire microbubble tracks. For training and validation, we used an anatomically and hemodynamically realistic mouse brain microvascular network model to simulate the flow of microbubbles in presence of aberration. The proposed CV-CNN performance was compared to the coherence-based method by using microbubble tracks. We then confirmed the capability of the proposed network to generalize to transcranial in vivo data in the mouse brain (n=3). Vascular reconstructions using a locally predicted aberration function included additional and sharper vessels. The CV-CNN was more robust than the coherence-based method and could perform aberration correction in a 6-month-old mouse. After correction, we measured a resolution of [Formula: see text] for younger mice, representing an improvement of 25.8%, while the resolution was improved by 13.9% for the 6-month-old mouse. This work leads to different applications for complex-valued convolutions in biomedical imaging and strategies to perform transcranial ULM.

Broad Elevation Projection Super-Resolution Ultrasound (BEP-SRUS) Imaging With a 1-D Unfocused Linear Array

Super-resolution ultrasound (SRUS) through localizing spatially isolated microbubbles (MBs) has been demonstrated to overcome the wave diffraction limit and reveal the microvascular structure and flow information at the microscopic scale. However, 3-D SRUS imaging remains a challenge due to the fabrication and computational complexity of 2-D matrix array probes. Inspired by X-ray radiography which can present information within a volume in a single projection image with much simpler hardware than X-ray computerized tomography (CT), this study investigates the feasibility of broad elevation projection super-resolution (BEP-SR) ultrasound using a 1-D unfocused linear array. Both simulation and in vitro experiments were conducted on 3-D microvessel phantoms. In vivo demonstration was done on the Rabbit kidney. Data from a 1-D linear array with and without an elevational focus were synthesized by summing up row signals acquired from a 2-D matrix array with and without delays. A full 3-D reconstruction was also generated as the reference, using the same data of the 2-D matrix array but without summing row signals. Results show that using an unfocused 1-D array probe, BEP-SR can capture significantly more information within a volume in both vascular structure and flow velocity than the conventional 1-D elevational-focused probe. Compared with the 2-D projection image of the full 3-D SRUS results using the 2-D array probe with the same aperture size, the 2-D projection SRUS image of BEP-SR has similar volume coverage, using 32 folds fewer independent elements. This study demonstrates BEP-SR's ability of high-resolution imaging of microvascular structures and flow velocity within a 3-D volume at significantly reduced costs. The proposed BEP method could significantly benefit the clinical translation of the SRUS imaging technique by making it more affordable and repeatable.

Superficial Bifurcated Microflow Phantom for High-Frequency Ultrasound Applications

OBJECTIVE

To evaluate and optimize high-frequency ultrasound (HFUS) imaging techniques that visualize the morphology of microscale vasculatures, many studies have used flow phantoms with straight channels. However, the previous phantoms lack the complexity of microvessels to simulate a realistic vascular environment in a shallow depth. This study was aimed at devising a new protocol for fabrication of a microflow phantom with bifurcated geometry at a superficial region.

METHODS

The proposed protocol involved the following features: (i) a bifurcated flow tract model 300 µm in diameter was debossed on the surface of a tissue slab made of polyvinyl alcohol cryogel, and (ii) a wall-less lumen was created via bonding tissue slabs to put a lid on the debossed flow tract. The structure of the created microflow phantom was evaluated using 2-D and 3-D power Doppler imaging with a 30 MHz HFUS modality.

RESULTS

Ultrasound imaging revealed that the desired flow tract with bifurcation was successfully created in the phantom at a depth of 2-5 mm from the ultrasound probe. The diameters of the flow tract measured in the axial direction were 307 ± 3.7 µm in the parent branch and 232 ± 18.2 and 256 ± 23.3 µm in the two daughter branches, respectively.

CONCLUSION

The experiments revealed that the proposed protocol for creating a microscale intricate flow tract with desired dimensions and depth is valid. This new phantom will facilitate further improvement in the ultrasound technologies for the precise visualization of superficial complex vasculatures such as those in skin layers.

High-Spatiotemporal-Resolution Ultrasound Flow Imaging to Determine Cerebrovascular Hemodynamics in Alzheimer's Disease Mice Model

Although the relationships of cerebrovascular hemodynamic dysfunction with neurodegenerative diseases remain unclear, many studies have indicated that poor cerebral perfusion accelerates the progression of neurodegenerative diseases, such as Alzheimer's disease (AD). Small animal models are widely used in AD research. However, providing an imaging modality with a high spatiotemporal resolution and sufficiently large field of view to assess cerebrovascular hemodynamics in vivo remains a challenge. The present study proposes a novel technique for high-spatiotemporal-resolution vector micro-Doppler imaging (HVμDI) based on contrast-free ultrafast high frequency ultrasound imaging to visualize the cerebrovascular hemodynamics of the mouse, with a data acquisition time of 0.4 s, a minimal detectable vessel size of 38 µm, and a temporal resolution of 500 Hz. In vivo experiments are conducted on wild-type and AD mice. Cerebrovascular hemodynamics are quantified using the cerebral vascular density, diameter, velocity, tortuosity, cortical flow pulsatility, and instant flow direction variations. Results reveal that AD significantly change the cerebrovascular hemodynamics. HVμDI offers new opportunities for in vivo analysis of cerebrovascular hemodynamics in neurodegenerative pathologies in preclinical animal research.

Three-dimensional super resolution ultrasound imaging with a multi-frequency hemispherical phased array

BACKGROUND

High resolution imaging of the microvasculature plays an important role in both diagnostic and therapeutic applications in the brain. However, ultrasound pulse-echo sonography imaging the brain vasculatures has been limited to narrow acoustic windows and low frequencies due to the distortion of the skull bone, which sacrifices axial resolution since it is pulse length dependent.

PURPOSE

To overcome the detect limit, a large aperture 256-module sparse hemispherical transmit/receive array was used to visualize the acoustic emissions of ultrasound-vaporized lipid-coated decafluorobutane nanodroplets flowing through tube phantoms and within rabbit cerebral vasculature in vivo via passive acoustic mapping and super resolution techniques.

METHODS

Nanodroplets were vaporized with 55 kHz burst-mode ultrasound (burst length = 145 μs, burst repetition frequency = 9-45 Hz, peak negative acoustic pressure = 0.10-0.22 MPa), which propagates through overlying tissues well without suffering from severe distortions. The resulting emissions were received at a higher frequency (612 or 1224 kHz subarray) to improve the resulting spatial resolution during passive beamforming. Normal resolution three-dimensional images were formed using a delay, sum, and integrate beamforming algorithm, and super-resolved images were extracted via Gaussian fitting of the estimated point-spread-function to the normal resolution data.

RESULTS

With super resolution techniques, the mean lateral (axial) full-width-at-half-maximum image intensity was 16 ± 3 (32 ± 6) μm, and 7 ± 1 (15 ± 2) μm corresponding to ∼1/67 of the normal resolution at 612 and 1224 kHz, respectively. The mean positional uncertainties were ∼1/350 (lateral) and ∼1/180 (axial) of the receive wavelength in water. In addition, a temporal correlation between nanodroplet vaporization and the transmit waveform shape was observed, which may provide the opportunity to enhance the signal-to-noise ratio in future studies.

CONCLUSIONS

Here, we demonstrate the feasibility of vaporizing nanodroplets via low frequency ultrasound and simultaneously performing spatial mapping via passive beamforming at higher frequencies to improve the resulting spatial resolution of super resolution imaging techniques. This method may enable complete four-dimensional vascular mapping in organs where a hemispherical array could be positioned to surround the target, such as the brain, breast, or testicles.

Dissecting Multiparametric Cerebral Hemodynamics using Integrated Ultrafast Ultrasound and Multispectral Photoacoustic Imaging

Understanding brain-wide hemodynamic responses to different stimuli at high spatiotemporal resolutions can help study neuro-disorders and brain functions. However, the existing brain imaging technologies have limited resolution, sensitivity, imaging depth and provide information about only one or two hemodynamic parameters. To address this, we propose a multimodal functional ultrasound and photoacoustic (fUSPA) imaging platform, which integrates ultrafast ultrasound and multispectral photoacoustic imaging methods in a compact head-mountable device, to quantitatively map cerebral blood volume (CBV), cerebral blood flow (CBF), oxygen saturation (SO2) dynamics as well as contrast agent enhanced brain imaging with high spatiotemporal resolutions. After systematic characterization, the fUSPA system was applied to quantitatively study the changes in brain hemodynamics and vascular reactivity at single vessel resolution in response to hypercapnia stimulation. Our results show an overall increase in brain-wide CBV, CBF, and SO2, but regional differences in singular cortical veins and arteries and a reproducible anti-correlation pattern between venous and cortical hemodynamics, demonstrating the capabilities of the fUSPA system for providing multiparametric cerebrovascular information at high-resolution and sensitivity, that can bring insights into the complex mechanisms of neurodiseases.