In Vivo Visualization of Eye Vasculature Using Super-Resolution Ultrasound Microvessel Imaging

Impact of Elevated Intraocular Pressure on Lamina Cribrosa Oxygenation: A Combined Experimental-Computational Study on Monkeys

Purpose

To evaluate how lamina cribrosa (LC) oxygenation is affected by tissue distortions resulting from elevated intraocular pressure (IOP).

Design

Experimental study on 4 monkeys, histology, and computational analysis.

Subjects

Four healthy monkey eyes with OCT scans at IOPs of 10 to 60 mmHg.

Methods

Intraocular pressure-induced LC tissue deformations of a healthy monkey were measured in vivo using OCT images and digital volume correlation analysis techniques. Three-dimensional eye-specific models of the LC vasculature of 4 healthy monkey eyes were reconstructed using histology. The models were then used to compute LC oxygenation, first as reconstructed (baseline), and then with the LC vessels distorted according to the OCT-derived deformations. Two biomechanics-based mapping techniques were evaluated: cross-sectional and isotropic. The hemodynamics and oxygenations of the 4 LC vessel networks were evaluated at IOPs up to 60 mmHg to quantify the effects of IOP on LC oxygen supply, assorting the extent of LC tissue mild and severe hypoxia.

Main Outcome Measures

Intraocular pressure-induced deformation, vasculature structure, blood supply, and LC oxygenation.

Results

Intraocular pressure-induced deformations reduced LC oxygenation significantly and substantially. More than 20% of LC tissue suffered from mild hypoxia when IOP reached 30 mmHg. Extreme IOP (>50 mmHg) led to large severe hypoxia regions (>30%) in the isotropic mapping cases.

Conclusions

Our calculations predicted that moderately elevated IOP can lead to mild hypoxia in a substantial part of the LC, which, if sustained chronically, may contribute to neural tissue damage. For extreme IOP elevations, severe hypoxia was predicted, which would likely cause more immediate damage. Our findings suggest that despite the remarkable LC vascular robustness, IOP-induced distortions can potentially contribute to glaucomatous neuropathy.

Financial Disclosures

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Computationally Efficient SVD Filtering for Ultrasound Flow Imaging and Real-Time Application to Ultrafast Doppler

Over the past decade, ultrasound microvasculature imaging has seen the rise of highly sensitive techniques, such as ultrafast power Doppler (UPD) and ultrasound localization microscopy (ULM). The cornerstone of these techniques is the acquisition of a large number of frames based on unfocused wave transmission, enabling the use of singular value decomposition (SVD) as a powerful clutter filter to separate microvessels from surrounding tissue. Unfortunately, SVD is computationally expensive, hampering its use in real-time UPD imaging and weighing down the ULM processing chain, with evident impact in a clinical context. To solve this problem, we propose a new approach to implement SVD filtering, based on simplified and elementary operations that can be optimally parallelized on GPU (GPU sSVD), unlike standard SVD algorithms that are mainly serial. First, we show that GPU sSVD filters UPD and ULM data with high computational efficiency compared to standard SVD implementations, and without losing image quality. Second, we demonstrate that the proposed method is suitable for real-time operation. GPU sSVD was embedded in a research scanner, along with the spatial similarity matrix (SSM), a well-known efficient approach to automate the selection of SVD blood components. High real-time throughput of GPU sSVD is demonstrated when using large packets of frames, with and without SSM. For example, more than 15000 frames/s were filtered with 512 packet size on a 128 × 64 samples beamforming grid. Finally, GPU sSVD was used to perform, for the first time, UPD imaging with real-time and adaptive SVD filtering on healthy volunteers.

Multi-Parametric Retinal Microvascular Functional Perfusion Imaging Based on Dynamic Fundus Fluorescence Angiography

Retinal microvascular disease has caused serious visual impairment widely in the world, which can be hopefully prevented via early and precision microvascular hemodynamic diagnosis. Due to artifacts from choroidal microvessels and tiny movements, current fundus microvascular imaging techniques including fundus fluorescein angiography (FFA) precisely identify retinal microvascular microstructural damage and abnormal hemodynamic changes difficulty, especially in the early stage. Therefore, this study proposes an FFA-based multi-parametric retinal microvascular functional perfusion imaging (RM-FPI) scheme to assess the microstructural damage and quantify its hemodynamic distribution precisely. Herein, a spatiotemporal filter based on singular value decomposition combined with a lognormal fitting model was used to remove the above artifacts. Dynamic FFAs of patients (n = 7) were collected first. The retinal time fluorescence intensity curves were extracted and the corresponding perfusion parameters were estimated after decomposition filtering and model fitting. Compared with in vivo results without filtering and fitting, the signal-to-clutter ratio of retinal perfusion curves, average contrast, and resolution of RM-FPI were up to 7.32 ± 0.43 dB, 14.34 ± 0.24 dB, and 11.0 ± 2.0 µm, respectively. RM-FPI imaged retinal microvascular distribution and quantified its spatial hemodynamic changes, which further characterized the parabolic distribution of local blood flow within diameters ranging from 9 to 400 µm. Finally, RM-FPI was used to quantify, visualize, and diagnose the retinal hemodynamics of retinal vein occlusion from mild to severe. Therefore, this study provided a scheme for early and precision diagnosis of retinal microvascular disease, which might be beneficial in preventing its development.

Impact of anatomic variability and other vascular factors on lamina cribrosa hypoxia

Insufficient oxygenation in the lamina cribrosa (LC) may contribute to axonal damage and glaucomatous vision loss. To understand the range of susceptibilities to glaucoma, we aimed to identify key factors influencing LC oxygenation and examine if these factors vary with anatomical differences between eyes. We reconstructed 3D, eye-specific LC vessel networks from histological sections of four healthy monkey eyes. For each network, we generated 125 models varying vessel radius, oxygen consumption rate, and arteriole perfusion pressure. Using hemodynamic and oxygen supply modeling, we predicted blood flow distribution and tissue oxygenation in the LC. ANOVA assessed the significance of each parameter. Our results showed that vessel radius had the greatest influence on LC oxygenation, followed by anatomical variations. Arteriole perfusion pressure and oxygen consumption rate were the third and fourth most influential factors, respectively. The LC regions are well perfused under baseline conditions. These findings highlight the importance of vessel radius and anatomical variation in LC oxygenation, providing insights into LC physiology and pathology. Pathologies affecting vessel radius may increase the risk of LC hypoxia, and anatomical variations could influence susceptibility. Conversely, increased oxygen consumption rates had minimal effects, suggesting that higher metabolic demands, such as those needed to maintain intracellular transport despite elevated intraocular pressure, have limited impact on LC oxygenation.

Impact of elevated IOP on lamina cribrosa oxygenation; A combined experimental-computational study on monkeys

Purpose

Our goal is to evaluate how lamina cribrosa (LC) oxygenation is affected by the tissue distortions resulting from elevated IOP.

Design

Experimental study on monkeys.

Subjects

Four healthy monkey eyes with OCT scans with IOP of 10 to 50 mmHg, and then with histological sections of LC.

Methods

Since in-vivo LC oxygenation measurement is not yet possible, we used 3D eye-specific numerical models of the LC vasculature which we subjected to experimentally-derived tissue deformations. We reconstructed 3D models of the LC vessel networks of 4 healthy monkey eyes from histological sections. We also obtained in-vivo IOP-induced tissue deformations from a healthy monkey using OCT images and digital volume correlation analysis techniques. The extent that LC vessels distort under a given OCT-derived tissue strain remains unknown. We biomechanics-based mapping techniques: cross-sectional and isotropic. The hemodynamics and oxygenations of the four vessel networks were simulated for deformations at several IOPs up to 60mmHg. The results were used to determine the effects of IOP on LC oxygen supply, assorting the extent of tissue mild and severe hypoxia.

Main Outcome Measures

IOP-induced deformation, vasculature structure, blood supply, and oxygen supply for LC region.

Result

IOP-induced deformations reduced LC oxygenation significantly. More than 20% of LC tissue suffered from mild hypoxia when IOP reached 30 mmHg. Extreme IOP(>50mmHg) led to large severe hypoxia regions (>30%) in the isotropic mapping cases.

Conclusion

Our models predicted that moderately elevated IOP can lead to mild hypoxia in a substantial part of the LC, which, if sustained chronically, may contribute to neural tissue damage. For extreme IOP elevations, severe hypoxia was predicted, which would potentially cause more immediate damage. Our findings suggest that despite the remarkable LC vascular robustness, IOP-induced distortions can potentially contribute to glaucomatous neuropathy.

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.

Time Efficient Ultrasound Localization Microscopy Based on A Novel Radial Basis Function 2D Interpolation

Ultrasound localization microscopy (ULM) allows for the generation of super-resolved (SR) images of the vasculature by precisely localizing intravenously injected microbubbles. Although SR images may be useful for diagnosing and treating patients, their use in the clinical context is limited by the need for prolonged acquisition times and high frame rates. The primary goal of our study is to relax the requirement of high frame rates to obtain SR images. To this end, we propose a new time-efficient ULM (TEULM) pipeline built on a cutting-edge interpolation method. More specifically, we suggest employing Radial Basis Functions (RBFs) as interpolators to estimate the missing values in the 2-dimensional (2D) spatio-temporal structures. To evaluate this strategy, we first mimic the data acquisition at a reduced frame rate by applying a down-sampling (DS = 2, 4, 8, and 10) factor to high frame rate ULM data. Then, we up-sample the data to the original frame rate using the suggested interpolation to reconstruct the missing frames. Finally, using both the original high frame rate data and the interpolated one, we reconstruct SR images using the ULM framework steps. We evaluate the proposed TEULM using four in vivo datasets, a Rat brain (dataset A), a Rat kidney (dataset B), a Rat tumor (dataset C) and a Rat brain bolus (dataset D), interpolating at the in-phase and quadrature (IQ) level. Results demonstrate the effectiveness of TEULM in recovering vascular structures, even at a DS rate of 10 (corresponding to a frame rate of sub-100Hz). In conclusion, the proposed technique is successful in reconstructing accurate SR images while requiring frame rates of one order of magnitude lower than standard ULM.

The Robust Lamina Cribrosa Vasculature: Perfusion and Oxygenation Under Elevated Intraocular Pressure

Purpose

Elevated intraocular pressure (IOP) is thought to cause lamina cribrosa (LC) blood vessel distortions and potentially collapse, adversely affecting LC hemodynamics, reducing oxygenation, and triggering, or contributing to, glaucomatous neuropathy. We assessed the robustness of LC perfusion and oxygenation to vessel collapses.

Methods

From histology, we reconstructed three-dimensional eye-specific LC vessel networks of two healthy monkey eyes. We used numerical simulations to estimate LC perfusion and from this the oxygenation. We then evaluated the effects of collapsing a fraction of LC vessels (0%-36%). The collapsed vessels were selected through three scenarios: stochastic (collapse randomly), systematic (collapse strictly by the magnitude of local experimentally determined IOP-induced compression), and mixed (a combination of stochastic and systematic).

Results

LC blood flow decreased linearly as vessels collapsed-faster for stochastic and mixed scenarios and slower for the systematic one. LC regions suffering severe hypoxia (oxygen <8 mm Hg) increased proportionally to the collapsed vessels in the systematic scenario. For the stochastic and mixed scenarios, severe hypoxia did not occur until 15% of vessels collapsed. Some LC regions had higher perfusion and oxygenation as vessels collapsed elsewhere. Some severely hypoxic regions maintained normal blood flow. Results were equivalent for both networks and patterns of experimental IOP-induced compression.

Conclusions

LC blood flow was sensitive to distributed vessel collapses (stochastic and mixed) and moderately vulnerable to clustered collapses (systematic). Conversely, LC oxygenation was robust to distributed vessel collapses and sensitive to clustered collapses. Locally normal flow does not imply adequate oxygenation. The actual nature of IOP-induced vessel collapse remains unknown.

Quantitative Biomarkers Derived from a Novel, Contrast-Free Ultrasound, High-Definition Microvessel Imaging for Differentiating Choroidal Tumors

Angiogenesis has an essential role in the de novo evolution of choroidal melanoma as well as choroidal nevus transformation into melanoma. Differentiating early-stage melanoma from nevus is of high clinical importance; thus, imaging techniques that provide objective information regarding tumor microvasculature structures could aid accurate early detection. Herein, we investigated the feasibility of quantitative high-definition microvessel imaging (qHDMI) for differentiation of choroidal tumors in humans. This new ultrasound-based technique encompasses a series of morphological filtering and vessel enhancement techniques, enabling the visualization of tumor microvessels as small as 150 microns and extracting vessel morphological features as new tumor biomarkers. Distributional differences between the malignant melanomas and benign nevi were tested on 37 patients with choroidal tumors using a non-parametric Wilcoxon rank-sum test, and statistical significance was declared for biomarkers with p-values < 0.05. The ocular oncology diagnosis was choroidal melanoma (malignant) in 21 and choroidal nevus (benign) in 15 patients. The mean thickness of benign and malignant masses was 1.70 ± 0.40 mm and 3.81 ± 2.63 mm, respectively. Six HDMI biomarkers, including number of vessel segments (p = 0.003), number of branch points (p = 0.003), vessel density (p = 0.03), maximum tortuosity (p = 0.001), microvessel fractal dimension (p = 0.002), and maximum diameter (p = 0.003) exhibited significant distributional differences between the two groups. Contrast-free HDMI provided noninvasive imaging and quantification of microvessels of choroidal tumors. The results of this pilot study indicate the potential use of qHDMI as a complementary tool for characterization of small ocular tumors and early detection of choroidal melanoma.

Fast 3D Super-Resolution Ultrasound With Adaptive Weight-Based Beamforming

OBJECTIVE

Super-resolution ultrasound (SRUS) imaging through localising and tracking sparse microbubbles has been shown to reveal microvascular structure and flow beyond the wave diffraction limit. Most SRUS studies use standard delay and sum (DAS) beamforming, where high side lobes and broad main lobes make isolation and localisation of densely distributed bubbles challenging, particularly in 3D due to the typically small aperture of matrix array probes.

METHOD

This study aimed to improve 3D SRUS by implementing a new fast 3D coherence beamformer based on channel signal variance. Two additional fast coherence beamformers, that have been implemented in 2D were implemented in 3D for the first time as comparison: a nonlinear beamformer with p-th root compression and a coherence factor beamformer. The 3D coherence beamformers, together with DAS, were compared in computer simulation, on a microflow phantom and in vivo.

RESULTS

Simulation results demonstrated that all three adaptive weight-based beamformers can narrow the main lobe, suppress the side lobes, while maintaining the weaker scatter signals. Improved 3D SRUS images of microflow phantom and a rabbit kidney within a 3-second acquisition were obtained using the adaptive weight-based beamformers, when compared with DAS.

CONCLUSION

The adaptive weight-based 3D beamformers can improve the SRUS and the proposed variance-based beamformer performs best in simulations and experiments.

SIGNIFICANCE

Fast 3D SRUS would significantly enhance the potential utility of this emerging imaging modality in a broad range of biomedical applications.

In vivo ocular microvasculature imaging in rabbits with 3D ultrasound localization microscopy

Morphological and hemodynamic changes in the ocular vasculature are important signs of various ocular diseases. The evaluation of the ocular microvasculature with high resolution is valuable in comprehensive diagnoses. However, it is difficult for current optical imaging techniques to visualize the posterior segment and retrobulbar microvasculature due to the limited penetration depth of light, particularly when the refractive medium is opaque. Thus, we have developed a 3D ultrasound localization microscopy (ULM) imaging method to visualize the ocular microvasculature in rabbits with micron-scale resolution. We used a 32 × 32 matrix array transducer (center frequency: 8 MHz) with a compounding plane wave sequence and microbubbles. Block-wise singular value decomposition spatiotemporal clutter filtering and block-matching 3D denoising were implemented to extract the flowing microbubble signals at different imaging depths with high signal-to-noise ratios. The center points of microbubbles were localized and tracked in 3D space to achieve the micro-angiography. The in vivo results demonstrate the ability of 3D ULM to visualize the microvasculature of the eye in rabbits, where vessels down to 54 μm were successfully revealed. Moreover, the microvascular maps indicated the morphological abnormalities in the eye with retinal detachment. This efficient modality shows potential for use in the diagnosis of ocular diseases.

Detecting Early Ocular Choroidal Melanoma Using Ultrasound Localization Microscopy

Ocular choroidal melanoma (OCM) is the most common ocular primary malignant tumor in adults, and there is an increasing emphasis on its early detection and treatment worldwide. The main obstacle in early detection of OCM is its overlapping clinical features with benign choroidal nevus. Thus, we propose ultrasound localization microscopy (ULM) based on the image deconvolution algorithm to assist the diagnosis of small OCM in early stages. Furthermore, we develop ultrasound (US) plane wave imaging based on three-frame difference algorithm to guide the placement of the probe on the field of view. A high-frequency Verasonics Vantage system and an L22-14v linear array transducer were used to perform experiments on both custom-made modules in vitro and a SD rat with ocular choroidal melanoma in vivo. The results demonstrate that our proposed deconvolution method implement more robust microbubble (MB) localization, reconstruction of microvasculature network in a finer grid and more precise flow velocity estimation. The excellent performance of US plane wave imaging was successfully validated on the flow phantom and in an in vivo OCM model. In the future, the super-resolution ULM, a critical complementary imaging modality, can provide doctors with conclusive suggestions for early diagnosis of OCM, which is significant for the treatment and prognosis of patients.

Super-Resolution Ultrasound Through Sparsity-Based Deconvolution and Multi-Feature Tracking

Ultrasound super-resolution imaging through localisation and tracking of microbubbles can achieve sub-wave-diffraction resolution in mapping both micro-vascular structure and flow dynamics in deep tissue in vivo. Currently, it is still challenging to achieve high accuracy in localisation and tracking particularly with limited imaging frame rates and in the presence of high bubble concentrations. This study introduces microbubble image features into a Kalman tracking framework, and makes the framework compatible with sparsity-based deconvolution to address these key challenges. The performance of the method is evaluated on both simulations using individual bubble signals segmented from in vivo data and experiments on a mouse brain and a human lymph node. The simulation results show that the deconvolution not only significantly improves the accuracy of isolating overlapping bubbles, but also preserves some image features of the bubbles. The combination of such features with Kalman motion model can achieve a significant improvement in tracking precision at a low frame rate over that using the distance measure, while the improvement is not significant at the highest frame rate. The in vivo results show that the proposed framework generates SR images that are significantly different from the current methods with visual improvement, and is more robust to high bubble concentrations and low frame rates.

Super-Resolution Ultrasound Localization Microscopy for Visualization of the Ocular Blood Flow

OBJECTIVE

The ocular vascular system plays an important role in preserving the visual function. Alterations in either anatomy or hemodynamics of the eye may have adverse effects on vision. Thus, an imaging approach that can monitor alterations of ocular blood flow of the deep eye vasculature ranging from capillary-level vessels to large supporting vessels would be advantageous for detection of early stage retinal and optic nerve diseases.

METHODS

We propose a super-resolution ultrasound localization microscopy (ULM) technique that can assess both the microvessel and flow velocity of the deep eye with high resolution. Ultrafast plane wave imaging was acquired using an L22-14v linear array on a high frequency Verasonics Vantage system. A robust microbubble localization and tracking technique was applied to reconstruct ULM images. The experiment was first performed on pre-designed flow phantoms in vitro and then tested on a New Zealand white rabbit eye in vivo calibrated to various intraocular pressures (IOP) - 10 mmHg, 30 mmHg and 50 mmHg.

RESULTS

We demonstrated that retinal/choroidal vessels, central retinal artery, posterior ciliary artery, and vortex vein were all visible at high resolution. In addition, reduction of vascular density and flow velocity were observed with elevated IOPs.

CONCLUSION

These results indicate that super-resolution ULM is able to image the deep ocular tissue while maintaining high resolution that is comparable with optical coherence tomography angiography.

SIGNIFICANCE

Capability to detect subtle changes of blood flow may be clinically important in detecting and monitoring eye diseases such as glaucoma.

[Super-resolution ultrasound imaging : Methods and applications]

CLINICAL/METHODICAL ISSUE

The microvasculature plays an important role in many pathologic conditions but cannot be characterized in high resolution via conventional ultrasound methods.

STANDARD RADIOLOGICAL METHODS

Doppler-based techniques, contrast-enhanced sonography as well as dynamic contrast-enhanced computed tomography (CT) and magnetic resonance imaging (MRI) are commonly used to characterize tissue vascularization. However, these techniques cannot visualize the microvasculature adequately.

METHODICAL INNOVATION

Ultrasound localization microscopy (ULM) consists of contrast-enhanced ultrasound measurements in combination with a complex post-processing algorithm which detects microbubbles with high precision. The vasculature can then be visualized by accumulating the microbubble positions in a final image.

PERFORMANCE

Compared to conventional ultrasound techniques, ULM improves the image resolution by a factor of more than 10. This currently results in resolutions down to 10 µm and allows, therefore, the visualization of capillaries and the assessment of their perfusion. Also, this does not lead to a reduction of the penetration depth or the signal-to-noise ratio (SNR).

ACHIEVEMENT

The method enables the visualization of vascular structures in unsurpassed detail and has the potential to offer new possibilities for the diagnosis of various diseases and for gaining insights into physiological processes. However, ULM is not commercially available yet but is intensely being tested in clinical studies.

PRACTICAL RECOMMENDATIONS

ULM could potentially be applied to all fields in which the vasculature is relevant. Current fields of application include oncology, nephrology, and neurological research.

Magnetic Nano-Platform Enhanced iPSC-Derived Trabecular Meshwork Delivery and Tracking Efficiency

Purpose

Transplantation of stem cells to remodel the trabecular meshwork (TM) has become a new option for restoring aqueous humor dynamics and intraocular pressure homeostasis in glaucoma. In this study, we aimed to design a nanoparticle to label induced pluripotent stem cell (iPSC)-derived TM and improve the delivery accuracy and in vivo tracking efficiency.

Methods

PLGA-SPIO-Cypate (PSC) NPs were designed with polylactic acid-glycolic acid (PLGA) polymers as the backbone, superparamagnetic iron oxide (SPIO) nanoparticles, and near-infrared (NIR) dye cypate. In vitro assessment of cytotoxicity, iron content after NPs labeling, and the dual-model monitor was performed on mouse iPSC-derived TM (miPSC-TM) cells, as well as immortalized and primary human TM cells. Cell function after labeling, the delivery accuracy, in vivo tracking efficiency, and its effect on lowering IOP were evaluated following miPSC-TM transplantation in mice.

Results

Initial in vitro experiments showed that a single-time nanoparticles incubation was sufficient to label iPSC-derived TM and was not related to any change in both cell viability and fate. Subsequent in vivo evaluation revealed that the use of this nanoparticle not only improves the delivery accuracy of the transplanted cells in live animals but also benefits the dual-model tracking in the long term. More importantly, the use of the magnet triggers a temporary enhancement in the effectiveness of cell-based therapy in alleviating the pathologies associated with glaucoma.

Conclusion

This study provided a promising approach for enhancing both the delivery and in vivo tracking efficiency of the transplanted cells, which facilitates the clinical translation of stem cell-based therapy for glaucoma.

Characterization of the Ultrasound Localization Microscopy Resolution Limit in the Presence of Image Degradation

Ultrasound localization microscopy (ULM) has been able to overcome the diffraction limit of ultrasound imaging. The resolution limit of ULM has been previously modeled using the Cramér-Rao lower bound (CRLB). While this model has been validated in a homogeneous medium, it estimates a resolution limit, which has not yet been achieved in vivo. In this work, we investigated the effects of three sources of image degradation on the resolution limit of ULM. The Fullwave simulation tool was used to simulate acquisitions of transabdominal contrast-enhanced data at depth. The effects of reverberation clutter, trailing clutter, and phase aberration were studied. The resolution limit, in the presence of reverberation clutter alone, was empirically measured to be up to 39 times worse in the axial dimension and up to 2.1 times worse in the lateral dimension than the limit predicted by the CRLB. While reverberation clutter had an isotropic impact on the resolution, trailing clutter had a constant impact on both dimensions across all signal-to-trailing-clutter ratios (STCR). Phase aberration had a significant impact on the resolution limit over the studied analysis ranges. Phase aberration alone degraded the resolution limit up to 70 and 160 [Formula: see text] in the lateral and axial dimensions, respectively. These results illustrate the importance of phase aberration correction and clutter filtering in ULM postprocessing. The analysis results were demonstrated through the simulation of the ULM process applied to a cross-tube model that was degraded by each of the three aforementioned sources of degradation.

Super-Resolution Microscopy: Shedding New Light on In Vivo Imaging

Over the past two decades, super-resolution microscopy (SRM), which offered a significant improvement in resolution over conventional light microscopy, has become a powerful tool to visualize biological activities in both fixed and living cells. However, completely understanding biological processes requires studying cells in a physiological context at high spatiotemporal resolution. Recently, SRM has showcased its ability to observe the detailed structures and dynamics in living species. Here we summarized recent technical advancements in SRM that have been successfully applied to in vivo imaging. Then, improvements in the labeling strategies are discussed together with the spectroscopic and chemical demands of the fluorophores. Finally, we broadly reviewed the current applications for super-resolution techniques in living species and highlighted some inherent challenges faced in this emerging field. We hope that this review could serve as an ideal reference for researchers as well as beginners in the relevant field of in vivo super resolution imaging.

Automated Detection and Diagnosis of Diabetic Retinopathy: A Comprehensive Survey

Diabetic Retinopathy (DR) is a leading cause of vision loss in the world. In the past few years, artificial intelligence (AI) based approaches have been used to detect and grade DR. Early detection enables appropriate treatment and thus prevents vision loss. For this purpose, both fundus and optical coherence tomography (OCT) images are used to image the retina. Next, Deep-learning (DL)-/machine-learning (ML)-based approaches make it possible to extract features from the images and to detect the presence of DR, grade its severity and segment associated lesions. This review covers the literature dealing with AI approaches to DR such as ML and DL in classification and segmentation that have been published in the open literature within six years (2016-2021). In addition, a comprehensive list of available DR datasets is reported. This list was constructed using both the PICO (P-Patient, I-Intervention, C-Control, O-Outcome) and Preferred Reporting Items for Systematic Review and Meta-analysis (PRISMA) 2009 search strategies. We summarize a total of 114 published articles which conformed to the scope of the review. In addition, a list of 43 major datasets is presented.

Optimized Backing Layers Design for High Frequency Broad Bandwidth Ultrasonic Transducer

Ultrasonic transducers with broad bandwidth are considered to have high axial resolution and good ultrasound scanning flexibility for the clinical applications. The limitations of spatial resolution due to bandwidth are of great concern in ultrasound medical imaging. The method of acoustic impedance matching between the piezoelectric element and medium is commonly used to obtain broad bandwidth and high resolution. In this study, an optimized backing layer design was proposed to broaden the bandwidth by adding a tunable acoustic impedance matching layer of backing (AIMLB) between the backing layer and the piezoelectric ceramic element. The Mason equivalent circuit method was used to analyze the effect of the backing material composition and its structure on the bandwidth of the transducer. The optimized transducer was simulated using the finite-element method with the PZFlex software. Based on the PZFlex simulations, a 20-MHz ultrasonic transducer using the AIMLB with a bandwidth of approximately 92.29% was fabricated. The experimental results were in good agreement with the simulations. The ultrasonic imaging indicated that the designed ultrasonic transducer with an additional AIMLB had high performance with good imaging capability.

Structural and Functional Imaging of the Retina in Central Retinal Artery Occlusion - Current Approaches and Future Directions

Central retinal artery occlusion (CRAO) is a form of acute ischemic stroke which affects the retina. Intravenous thrombolysis is emerging as a compelling therapeutic approach. However, it is not known which patients may benefit from this therapy because there are no imaging modalities that adequately distinguish viable retina from irreversibly infarcted retina. The inner retina receives arterial supply from the central retinal artery and there is robust collateralization between this circulation and the outer retinal circulation, provided by the posterior ciliary circulation. Fundus photography can show canonical changes associated with CRAO including a cherry-red spot, arteriolar boxcarring and retinal pallor. Fluorescein angiography provides 2-dimensional imaging of the retinal circulation and can distinguish a complete from a partial CRAO as well as central versus peripheral retinal non-perfusion. Transorbital ultrasonography may assay flow through the central retinal artery and is useful in the exclusion of other orbital pathology that can mimic CRAO. Optical coherence tomography provides structural information on the different layers of the retina and exploratory work has described its utility in determining the time since onset of ischemia. Two experimental techniques are discussed. 1) Retinal functional imaging permits generation of capillary perfusion maps and can assay retinal oxygenation and blood flow velocity. 2) Photoacoustic imaging combines the principles of optical excitation and ultrasonic detection and - in animal studies - has been used to determine the retinal oxygen metabolic rate. Future techniques to determine retinal viability in clinical practice will require rapid, easily used, and reproducible methods that can be deployed in the emergency setting.

Retrobulbar blood flow in rat eyes during acute elevation of intraocular pressure

Most studies of the effect of acute elevation of intraocular pressure (IOP) on ocular blood-flow have utilized optical coherence tomography (OCT) to characterize retinal and choroidal flow and vascular density. This study investigates the effect of acute IOP elevation on blood flow velocity in the retrobulbar arteries and veins supplying and draining the eye, which, unlike the retinal and choroidal vasculature, are not directly compressed as IOP is increased. By cannulation of the anterior chamber of 20 Sprague-Dawley rats, we increased IOP in 10 mmHg steps from 10 to 60 mmHg and returned to 10 mmHg. After 1 min at each IOP (and 3 min after return to 10 mmHg), we acquired 18 MHz plane-wave ultrasound data at 3000 compound images/sec for 1.5 s. We produced color-flow Doppler images by digital signal processing of the ultrasound data, identified retrobulbar arteries and veins, generated spectrograms depicting flow velocity over the cardiac cycle and characterized changes of vascular density and perfusion in the orbit overall. Systolic, diastolic and mean velocities and resistive and pulsatile indices were determined from arterial spectrograms at each IOP level. Baseline mean arterial and mean venous velocities averaged 30.9 ± 10.8 and 8.5 ± 3.3 mm/s, respectively. Arterial velocity progressively decreased and resistance indices increased at and above an IOP of 30 mmHg. Mean arterial velocity at 60 mmHg dropped by 55% with respect to baseline, while venous velocity decreased by 20%. Arterial and venous velocities and resistance returned to near baseline after IOP was restored to 10 mmHg. Both vascular density and orbital perfusion decreased with IOP, but while perfusion returned to near normal when IOP returned to 10 mmHg, density remained reduced. Our findings are consistent with OCT-based studies showing reduced perfusion of the retina at levels comparable to retrobulbar arterial flow velocity change with increased IOP. The lesser effect on venous flow is possibly attributable to partial collapse of the venous lumen as volumetric venous outflow decreased at high IOP. The continued reduction in orbital vascular density 3 min after restoration of IOP to 10 mmHg might be attributable to persisting narrowing of capillaries, but this needs to be verified in future studies.

Current Development and Applications of Super-Resolution Ultrasound Imaging

Abnormal changes of the microvasculature are reported to be key evidence of the development of several critical diseases, including cancer, progressive kidney disease, and atherosclerotic plaque. Super-resolution ultrasound imaging is an emerging technology that can identify the microvasculature noninvasively, with unprecedented spatial resolution beyond the acoustic diffraction limit. Therefore, it is a promising approach for diagnosing and monitoring the development of diseases. In this review, we introduce current super-resolution ultrasound imaging approaches and their preclinical applications on different animals and disease models. Future directions and challenges to overcome for clinical translations are also discussed.

Piezoelectric Single Crystal Ultrasonic Transducer for Endoscopic Drug Release in Gastric Mucosa

Modern advanced minimally invasive surgery has been implemented for most of the significant gastrointestinal diseases. However, patients with coagulopathy or unresectable tumors cannot be cured by current treatment methods. Moreover, other existing medical devices for targeted drug release are too large to be applied in gastric endoscope because the diameter of the biopsy channel is smaller than 3 mm. To address it, in this work, we developed a piezoelectric single crystal ultrasonic transducer (the diameter was only 2.2 mm and the mass was 0.076 g) to produce acoustic waves, which could promote the drug release in the designed position of the digestive tract through an endoscope. It exhibited the electromechanical coupling coefficient of 0.36 and the center frequency of 6.9 MHz with the -6-dB bandwidth of 23%. In in vitro sonophoresis experiment, the gastric mucosa permeability to Bovine Serum Albumin increased about 5.6 times when the ultrasonic transducer was activated at 40 [Formula: see text] and 60% duty ratio, proving that employment of this transducer could facilitate drug penetration in the gastric mucosa. Meanwhile, the permeability could be adjusted by tuning the duty ratio of the ultrasonic transducer. The corresponding sonophoresis mechanism was related to the acoustic streaming and the thermal effect produced by the transducer. In addition, the measured maximum power density was 128 mW/cm² and the mechanical index of the ultrasonic transducer was 0.02. The results held a great implication for applications of the transducer for targeted drug release in the gastrointestinal tract.