Label free measurement of retinal blood cell flux, velocity, hematocrit and capillary width in the living mouse eye

High-speed, phase contrast retinal and blood flow imaging using an adaptive optics partially confocal multi-line ophthalmoscope

High-speed, phase contrast retinal and blood flow imaging using an adaptive optics partially confocal multi-line ophthalmosocope (AO-pcMLO) is described. It allows for simultaneous confocal and phase contrast imaging with various directional multi-line illumination by using a single 2D camera and a digital micromirror device (DMD). Both vertical and horizontal line illumination directions were tested, for photoreceptor and vascular imaging. The phase contrast imaging provided improved visualization of retinal structures such as cone inner segments, vessel walls and red blood cells with images being acquired at frame rates up to 500 Hz. Blood flow velocities of small vessels (<40 µm in diameter) were measured using kymographs for capillaries and cross-correlation between subsequent images for arterioles or venules. Cardiac-related pulsatile patterns were observed with normal resting heart-beat rate, and instantaneous blood flow velocities from 0.7 to 20 mm/s were measured.

Changes to the shape, orientation and packing of red cells as a function of retinal capillary size

The free diameter of a red blood cell exceeds the lumen diameter of capillaries in the central nervous system, requiring significant deformation of cells. However the deformations undertaken in vivo are not well established due to the difficulty in observing cellular capillary flow in living human tissue. Here, we used high resolution adaptive optics imaging to non-invasively track 17,842 red blood cells in transit through 121 unique capillary segments of diameter 8 µm or less in the retina of 3 healthy human subjects. Within each vessel, a 2D en face profile was generated for the "average cell", whose shape was then inferred in 3D based on the key assumption of a circular capillary cross-section. From this we estimated the average volume, surface area, orientation, and separation between red cells within each capillary tube. Our results showed a network filtration effect, whereby narrower vessels were more likely to contain smaller cells (defined by surface area, which is thought not to vary during a cell's passage through the vascular system). A bivariate linear model showed that for larger cells in narrower vessels: cells re-orient themselves to align with the flow axis, their shape becomes more elongated, there are longer gaps between successive cells, and remarkably, that cell volume is less which implies the ejection of water from cells to facilitate capillary transit. Taken together, these findings suggest that red cells pass through retinal capillaries with some reluctance. A biphasic distribution for cell orientation and separation was evident, indicating a "tipping point" for vessels narrower than approx. 5 µm. This corresponds closely to the typical capillary lumen diameter, and may maximize sensitivity of cellular flow to small changes in diameter. We suggest that the minimization of unnecessary oxygen exchange, and hence of damage via reactive oxygen pathways, may have provided evolutionary pressure to ensure that capillary lumens are generally narrower than red blood cells.

Validation of an automated method for studying retinal capillary blood flow

Two major approaches for tracking cellular motion across a range of biological tissues are the manual labelling of cells, and automated analysis of spatiotemporal information represented in a kymograph. Here we compare these two approaches for the measurement of retinal capillary flow, a particularly noisy application due to the low intrinsic contrast of single red blood cells (erythrocytes). Image data were obtained using a flood-illuminated adaptive optics ophthalmoscope at 750 nm, allowing the acquisition of flow information over several cardiac cycles which provided key information in evaluating tracking accuracy. Our results show that in addition to being much faster, the automated method is more accurate in the face of rapid flow and reduced image contrast. This study represents the first validation of commonly used kymograph approaches to capillary flow analysis.

Mapping the human parafoveal vascular network to understand flow variability in capillaries

Capillary flow is known to be non-homogenous between vessels and variable over time, for reasons that are poorly understood. The local properties of individual vessels have been shown to have limited explanatory power in this regard. This exploratory study investigates the association of network-level properties such as vessel depth, branch order, and distance from the feeding arteriole with capillary flow. Detailed network connectivity analysis was undertaken in 3 healthy young subjects using flood-illuminated adaptive optics retinal imaging, with axial depth of vessels determined via optical coherence tomography angiography. Forty-one out of 70 vessels studied were of terminal capillary type, i.e. fed from an arterial junction and drained by a venous junction. Approximately half of vessel junctions were amenable to fitting with a model of relative branch diameters, with only a few adhering to Murray's Law. A key parameter of the model (the junction exponent) was found to be inversely related to the average velocity (r = -0.59, p = 0.015) and trough velocity (r = -0.67, p = 0.004) in downstream vessels. Aspects of cellular flow, such as the minimum velocity, were also moderately correlated (r = 0.46, p = 0.009) with distance to the upstream feeding arteriole. Overall, this study shows that capillary network topology contributes significantly to the flow variability in retinal capillaries in human eyes. Understanding the heterogeneity in capillary flow is an important first step before pathological flow states can be properly understood. These results show that flow within capillary vessels is not affected by vessel depths but significantly influenced by the upstream feeder distance as well as the downstream vessel junction exponents, but there remains much to be uncovered regarding healthy capillary flow.

Flow Heterogeneity and Factors Contributing to the Variability in Retinal Capillary Blood Flow

Purpose

Capillary flow plays an important role in the nourishment and maintenance of healthy neural tissue and can be observed directly and non-invasively in the living human retina. Despite their importance, patterns of normal capillary flow are not well understood due to limitations in spatial and temporal resolution of imaging data.

Methods

Capillary flow characteristics were studied in the retina of three healthy young individuals using a high-resolution adaptive optics ophthalmoscope. Imaging with frame rates of 200 to 300 frames per second was sufficient to capture details of the single-file flow of red blood cells in capillaries over the course of about 3 seconds.

Results

Erythrocyte velocities were measured from 72 neighboring vessels of the parafoveal capillary network for each subject. We observed strong variability among vessels within a given subject, and even within a given imaged field, across a range of capillary flow parameters including maximum and minimum velocities, pulsatility, abruptness of the systolic peak, and phase of the cardiac cycle. The observed variability was not well explained by "local" factors such as the vessel diameter, tortuosity, length, linear cell density, or hematocrit of the vessel. Within a vessel, a moderate relation between the velocities and hematocrit was noted, suggesting a redistribution of plasma between cells with changes in flow.

Conclusions

These observations advance our fundamental understanding of normal capillary physiology and raise questions regarding the potential role of network-level effects in explaining the observed flow heterogeneity.

Differential reductions in the capillary red-blood-cell flux between retina and brain under chronic global hypoperfusion

Significance

It has been hypothesized that abnormal microcirculation in the retina might predict the risk of ischemic damages in the brain. Direct comparison between the retinal and the cerebral microcirculation using similar animal preparation and under similar experimental conditions would help test this hypothesis.

Aim

We investigated capillary red-blood-cell (RBC) flux changes under controlled conditions and bilateral-carotid-artery-stenosis (BCAS)-induced hypoperfusion, and then compared them with our previous measurements performed in the brain.

Approach

We measured capillary RBC flux in mouse retina with two-photon microscopy using a fluorescence-labeled RBC-passage approach. Key physiological parameters were monitored during experiments to ensure stable physiology.

Results

We found that under the controlled conditions, capillary RBC flux in the retina was much higher than in the brain (i.e., cerebral cortical gray matter and subcortical white matter), and that BCAS induced a much larger decrease in capillary RBC flux in the retina than in the brain.

Conclusions

We demonstrated a two-photon microscopy-based technique to efficiently measure capillary RBC flux in the retina. Since cerebral subcortical white matter often exhibits early pathological developments due to global hypoperfusion, our results suggest that retinal microcirculation may be utilized as an early marker of brain diseases involving global hypoperfusion.

High-resolution structural and functional retinal imaging in the awake behaving mouse

The laboratory mouse has provided tremendous insight to the underpinnings of mammalian central nervous system physiology. In recent years, it has become possible to image single neurons, glia and vascular cells in vivo by using head-fixed preparations combined with cranial windows to study local networks of activity in the living brain. Such approaches have also succeeded without the use of general anesthesia providing insights to the natural behaviors of the central nervous system. However, the same has not yet been developed for the eye, which is constantly in motion. Here we characterize a novel head-fixed preparation that enables high-resolution adaptive optics retinal imaging at the single-cell level in awake-behaving mice. We reveal three new functional attributes of the normal eye that are overlooked by anesthesia: 1) High-frequency, low-amplitude eye motion of the mouse that is only present in the awake state 2) Single-cell blood flow in the mouse retina is reduced under anesthesia and 3) Mouse retinae thicken in response to ketamine/xylazine anesthesia. Here we show key benefits of the awake-behaving preparation that enables study of retinal physiology without anesthesia to study the normal retinal physiology in the mouse.

High-speed measurement of retinal arterial blood flow in the living human eye with adaptive optics ophthalmoscopy

We present a technique to measure the rapid blood velocity in large retinal vessels with high spatiotemporal resolution. Red blood cell motion traces in the vessels were non-invasively imaged using an adaptive optics near-confocal scanning ophthalmoscope at a frame rate of 200 fps. We developed software to measure blood velocity automatically. We demonstrated the ability to measure the spatiotemporal profiles of the pulsatile blood flow with a maximum velocity of 95-156 mm/s in retinal arterioles with a diameter >100 µm. High-speed and high-resolution imaging increased the dynamic range, enhanced sensitivity, and improved the accuracy when studying retinal hemodynamics.

Introduction to the Feature Issue on Adaptive Optics for Biomedical Applications

The guest editors introduce a feature issue commemorating the 25th anniversary of adaptive optics in biomedical research.

Measuring red blood cell shape in the human retina

The free diameter of a red blood cell generally exceeds the lumen diameter of capillaries in the central nervous system, requiring significant cellular deformation. However, the deformations undertaken are not well established under natural conditions due to the difficulty in observing corpuscular flow in vivo. Here we describe a novel, to the best of our knowledge, method to noninvasively study the shape of red blood cells as they traverse the narrow capillary networks of the living human retina, using high-speed adaptive optics. One hundred and twenty-three capillary vessels were analyzed in three healthy subjects. For each capillary, image data were motion-compensated and then averaged over time to reveal the appearance of the blood column. Data from hundreds of red blood cells were used to profile the average cell in each vessel. Diverse cellular geometries were observed across lumens ranging from 3.2 to 8.4 µm in diameter. As capillaries narrowed, cells transitioned from rounder to more elongated shapes and from being counter-aligned to aligned with the axis of flow. Remarkably, in many vessels the red blood cells maintained an oblique orientation relative to the axis of flow.

Evolution of adaptive optics retinal imaging [Invited]

This review describes the progress that has been achieved since adaptive optics (AO) was incorporated into the ophthalmoscope a quarter of a century ago, transforming our ability to image the retina at a cellular spatial scale inside the living eye. The review starts with a comprehensive tabulation of AO papers in the field and then describes the technological advances that have occurred, notably through combining AO with other imaging modalities including confocal, fluorescence, phase contrast, and optical coherence tomography. These advances have made possible many scientific discoveries from the first maps of the topography of the trichromatic cone mosaic to exquisitely sensitive measures of optical and structural changes in photoreceptors in response to light. The future evolution of this technology is poised to offer an increasing array of tools to measure and monitor in vivo retinal structure and function with improved resolution and control.

Visualization of erythrocyte stasis in the living human eye in health and disease

Blood cells trapped in stasis have been reported within the microcirculation, but their relevance to health and disease has not been established. In this study, we introduce an in vivo imaging approach that reveals the presence of a previously-unknown pool of erythrocytes in stasis, located within capillary segments of the CNS, and present in 100% of subjects imaged. These results provide a key insight that blood cells pause as they travel through the choroidal microvasculature, a vascular structure that boasts the highest blood flow of any tissue in the body. Demonstration of clinical utility using deep learning reveals that erythrocyte stasis is altered in glaucoma, indicating the possibility of more widespread changes in choroidal microvascular than previously realized. The ability to monitor the choroidal microvasculature at the single cell level may lead to novel strategies for tracking microvascular health in glaucoma, age-related macular degeneration, and other neurodegenerative diseases.

Methods to measure blood flow and vascular reactivity in the retina

Disturbances of retinal perfusion are involved in the onset and maintenance of several ocular diseases, including diabetic retinopathy, glaucoma, and retinal vascular occlusion. Hence, knowledge on ocular vascular anatomy and function is highly relevant for basic research studies and for clinical judgment and treatment. The retinal vasculature is composed of the superficial, intermediate, and deep vascular layer. Detection of changes in blood flow and vascular diameter especially in smaller vessels is essential to understand and to analyze vascular diseases. Several methods to evaluate blood flow regulation in the retina have been described so far, but no gold standard has been established. For highly reliable assessment of retinal blood flow, exact determination of vessel diameter is necessary. Several measurement methods have already been reported in humans. But for further analysis of retinal vascular diseases, studies in laboratory animals, including genetically modified mice, are important. As for mice, the small vessel size is challenging requiring devices with high optic resolution. In this review, we recapitulate different methods for retinal blood flow and vessel diameter measurement. Moreover, studies in humans and in experimental animals are described.

Advanced Optical Wavefront Technologies to Improve Patient Quality of Vision and Meet Clinical Requests

Adaptive optics (AO) is employed for the continuous measurement and correction of ocular aberrations. Human eye refractive errors (lower-order aberrations such as myopia and astigmatism) are corrected with contact lenses and excimer laser surgery. Under twilight vision conditions, when the pupil of the human eye dilates to 5-7 mm in diameter, higher-order aberrations affect the visual acuity. The combined use of wavefront (WF) technology and AO systems allows the pre-operative evaluation of refractive surgical procedures to compensate for the higher-order optical aberrations of the human eye, guiding the surgeon in choosing the procedure parameters. Here, we report a brief history of AO, starting from the description of the Shack-Hartmann method, which allowed the first in vivo measurement of the eye's wave aberration, the wavefront sensing technologies (WSTs), and their principles. Then, the limitations of the ocular wavefront ascribed to the IOL polymeric materials and design, as well as future perspectives on improving patient vision quality and meeting clinical requests, are described.

Mechanical regulation of the early stages of angiogenesis

Favouring or thwarting the development of a vascular network is essential in fields as diverse as oncology, cardiovascular disease or tissue engineering. As a result, understanding and controlling angiogenesis has become a major scientific challenge. Mechanical factors play a fundamental role in angiogenesis and can potentially be exploited for optimizing the architecture of the resulting vascular network. Largely focusing on in vitro systems but also supported by some in vivo evidence, the aim of this Highlight Review is dual. First, we describe the current knowledge with particular focus on the effects of fluid and solid mechanical stimuli on the early stages of the angiogenic process, most notably the destabilization of existing vessels and the initiation and elongation of new vessels. Second, we explore inherent difficulties in the field and propose future perspectives on the use of in vitro and physics-based modelling to overcome these difficulties.

In Vivo Capillary Structure and Blood Cell Flux in the Normal and Diabetic Mouse Eye

Purpose

To characterize the early structural and functional changes in the retinal microvasculature in response to hyperglycemia in the Ins2^Akita mouse.

Methods

A custom phase-contrast adaptive optics scanning light ophthalmoscope was used to image retinal capillaries of 9 Ins2^Akita positive (hyperglycemic) and 9 Ins2^Akita negative (euglycemic) mice from postnatal weeks 5 to 18. A 15 kHz point scan was used to image capillaries and measure red blood cell flux at biweekly intervals; measurements were performed manually. Retinal thickness and fundus photos were captured monthly using a commercial scanning laser ophthalmoscope/optical coherence tomography. Retinal thickness was calculated using a custom algorithm. Blood glucose and weight were tracked throughout the duration of the study.

Results

Elevated blood glucose (>250 mg/dL) was observed at 4 to 5 weeks of age in Ins2^Akita mice and remained elevated throughout the study, whereas euglycemic littermates maintained normal glucose levels. There was no significant difference in red blood cell flux, capillary anatomy, lumen diameter, or occurrence of stalled capillaries between hyperglycemic and euglycemic mice between postnatal weeks 5 and 18. Hyperglycemic mice had a thinner retina than euglycemic littermates (p < 0.001), but retinal thickness did not change with duration of hyperglycemia despite glucose levels that were more than twice times normal.

Conclusions

In early stages of hyperglycemia, retinal microvasculature structure (lumen diameter, capillary anatomy) and function (red blood cell flux, capillary perfusion) were not impaired despite 3 months of chronically elevated blood glucose. These findings suggest that hyperglycemia alone for 3 months does not alter capillary structure or function in profoundly hyperglycemic mice.

Computational investigation of blood cell transport in retinal microaneurysms

Microaneurysms (MAs) are one of the earliest clinically visible signs of diabetic retinopathy (DR). MA leakage or rupture may precipitate local pathology in the surrounding neural retina that impacts visual function. Thrombosis in MAs may affect their turnover time, an indicator associated with visual and anatomic outcomes in the diabetic eyes. In this work, we perform computational modeling of blood flow in microchannels containing various MAs to investigate the pathologies of MAs in DR. The particle-based model employed in this study can explicitly represent red blood cells (RBCs) and platelets as well as their interaction in the blood flow, a process that is very difficult to observe in vivo. Our simulations illustrate that while the main blood flow from the parent vessels can perfuse the entire lumen of MAs with small body-to-neck ratio (BNR), it can only perfuse part of the lumen in MAs with large BNR, particularly at a low hematocrit level, leading to possible hypoxic conditions inside MAs. We also quantify the impacts of the size of MAs, blood flow velocity, hematocrit and RBC stiffness and adhesion on the likelihood of platelets entering MAs as well as their residence time inside, two factors that are thought to be associated with thrombus formation in MAs. Our results show that enlarged MA size, increased blood velocity and hematocrit in the parent vessel of MAs as well as the RBC-RBC adhesion promote the migration of platelets into MAs and also prolong their residence time, thereby increasing the propensity of thrombosis within MAs. Overall, our work suggests that computational simulations using particle-based models can help to understand the microvascular pathology pertaining to MAs in DR and provide insights to stimulate and steer new experimental and computational studies in this area.

Imaging the dynamics of individual processes of microglia in the living retina in vivo

Microglia are an essential population of resident immune cells in the central nervous system (CNS) and retina. These microscopic cells possess sub-cellular processes that make them challenging to image due to limited resolution and contrast. The baseline behavior of microglial processes in the living retina has been poorly characterized, and yet are essential to understanding how these cells respond under conditions of health, development, stress and disease. Here we use in vivo adaptive optics scanning light ophthalmoscopy combined with time-lapse imaging and quantification of process motility, to reveal the detailed behavior of microglial cells in a population of healthy mice. We find microglial processes to be dynamic at all branch-levels, from primary to end-protrusions. Cell-processes remodel at average speeds of 0.6 ± 0.4 µm/min with growth and deletion bursts of 0-7.6 µm/min. Longitudinal imaging in the same mice showed cell-somas to remain stable over seconds to minutes, but show migration over days to months. In addition to characterizing in vivo process motility and Sholl analysis using a microglial reporter mouse, we also demonstrate that microglia can be imaged without fluorescent labels at all. Phase-contrast imaging using safe levels of near-infrared light successfully imaged microglia soma and process remodeling with micron-level detail noninvasively, confirmed by simultaneous imaging of fluorescent microglial cells in transgenic mice. This label-free approach provides a new opportunity to investigate CNS immune system noninvasively without requiring transgenic or antibody labeling which could have off-target effects of changing normal microglial behavior. Additionally, CNS microglia study can now be conducted without the need for cranial window surgery which have the potential to change their behavior due to local or systemic inflammation.

Development of a Preclinical Laser Speckle Contrast Imaging Instrument for Assessing Systemic and Retinal Vascular Function in Small Rodents

Purpose

To develop and test a non-contact, contrast-free, retinal laser speckle contrast imaging (LSCI) instrument for use in small rodents to assess vascular anatomy, quantify hemodynamics, and measure physiological changes in response to retinal vascular dysfunction over a wide field of view (FOV).

Methods

A custom LSCI instrument capable of wide-field and non-contact imaging in small rodents was constructed. The effect of camera gain, laser power, and exposure duration on speckle contrast variance was standardized before the repeatability of LSCI measurements was determined in vivo. Finally, the ability of LSCI to detect alterations in local and systemic vascular function was evaluated using a laser-induced branch retinal vein occlusion and isoflurane anesthesia model, respectively.

Results

The LSCI system generates contrast-free maps of retinal blood flow with a 50° FOV at >376 frames per second (fps) and under a short exposure duration (>50 µs) with high reliability (intraclass correlation R = 0.946). LSCI was utilized to characterize retinal vascular anatomy affected by laser injury and longitudinally measure alterations in perfusion and blood flow profile. Under varied doses of isoflurane, LSCI could assess cardiac and systemic vascular function, including heart rate, peripheral resistance, contractility, and pulse propagation.

Conclusions

We present a LSCI system for detecting anatomical and physiological changes in retinal and systemic vascular health and function in small rodents.

Translational Relevance

Detecting and quantifying early anatomical and physiological changes in vascular function in animal models of retinal, systemic, and neurodegenerative diseases could strengthen our understanding of disease progression and enable the identification of new prognostic and diagnostic biomarkers for disease management and for assessing treatment efficacies.

Optical Coherence Tomography Angiography in the Thirteen-Lined Ground Squirrel

Purpose

To assess the performance of two spectral-domain optical coherence tomography-angiography systems in a natural model of hypoperfusion: the hibernating thirteen-lined ground squirrel (13-LGS).

Methods

Using a high-speed (130 kHz) OCT-A system (HS-OCT-A) and a commercial OCT (36 kHz; Bioptigen Envisu; BE-OCT-A), we imaged the 13-LGS retina throughout its hibernation cycle. Custom software was used to extract the superior, middle, and deep capillary plexus (SCP, MCP, and DCP, respectively). The retinal vasculature was also imaged with adaptive optics scanning light ophthalmoscopy (AOSLO) during torpor to visualize individual blood cells. Finally, correlative histology with immunolabeled or DiI-stained vasculature was performed.

Results

During euthermia, vessel density was similar between devices for the SCP and MCP (P = 0.88, 0.72, respectively), with a small difference in the DCP (−1.63 ± 1.54%, P = 0.036). Apparent capillary dropout was observed during torpor, but recovered after forced arousal, and this effect was exaggerated in high-speed OCT-A imaging. Based on cell flux measurements with AOSLO, increasing OCT-A scan duration by ∼1000× would avoid the apparent capillary dropout artifact. High correspondence between OCT-A (during euthermia) and histology enabled lateral scale calibration.

Conclusions

While the HS-OCT-A system provides a more efficient workflow, the shorter interscan interval may render it more susceptible to the apparent capillary dropout artifact. Disambiguation between capillary dropout and transient ischemia can have important implications in the management of retinal disease and warrants additional diagnostics.

Translational Relevance

The 13-LGS provides a natural model of hypoperfusion that may prove valuable in modeling the utility of OCT-A in human pathologies associated with altered blood flow.

Intravital Imaging of Circulating Red Blood Cells in the Retinal Vasculature of Growing Mice

Purpose

To establish a custom-built, high-speed 90 frame-per-second laser-scanning confocal microscope for real-time in vivo retinal imaging of individual flowing red blood cells (RBCs) in retinal vasculature of live mouse model.

Methods

Fluorescently labeled RBCs were injected into mice of different ages (3 to 62 weeks old). Anti-CD31 antibody conjugated with Alexa Fluor 647 was injected to visualize retinal endothelial cells (ECs). Longitudinal and cross-sectional intravital retinal imaging of flowing RBCs and ECs was performed in two strains (C57BL/6 and Balb/c) by using the custom-built confocal microscope.

Results

Simultaneous tracking of the routes of many fluorescently labeled individual RBCs flowing from a large artery and vein to a single capillary in the retina of live mice was achieved, which enabled in vivo measurement of retinal RBC flow velocities in each vessel type in growing mice from 3 to 62 weeks after birth. Average RBC flow velocities were gradually increased during growing from 3 to 14 weeks by more than two times. Then the average RBC flow velocity was maintained at about 20 mm/s in artery and 16 mm/s in vein until 62 weeks.

Conclusions

Our study successfully established a custom-built high-speed 90-Hz retinal confocal microscope for measuring RBC flow velocity at the single cell level. It could be a useful tool to investigate the pathophysiology of various retinal diseases associated with blood flow impairment.

Translational Relevance

This technological method could be a valuable assessment tool to help the development of novel therapeutics for retinal diseases.

Association between erythrocyte dynamics and vessel remodelling in developmental vascular networks

Sprouting angiogenesis is an essential vascularization mechanism consisting of sprouting and remodelling. The remodelling phase is driven by rearrangements of endothelial cells (ECs) within the post-sprouting vascular plexus. Prior work has uncovered how ECs polarize and migrate in response to flow-induced wall shear stress (WSS). However, the question of how the presence of erythrocytes (widely known as red blood cells (RBCs)) and their impact on haemodynamics affect vascular remodelling remains unanswered. Here, we devise a computational framework to model cellular blood flow in developmental mouse retina. We demonstrate a previously unreported highly heterogeneous distribution of RBCs in primitive vasculature. Furthermore, we report a strong association between vessel regression and RBC hypoperfusion, and identify plasma skimming as the driving mechanism. Live imaging in a developmental zebrafish model confirms this association. Taken together, our results indicate that RBC dynamics are fundamental to establishing the regional WSS differences driving vascular remodelling via their ability to modulate effective viscosity.

Adaptive optics for high-resolution imaging

Adaptive optics (AO) is a technique that corrects for optical aberrations. It was originally proposed to correct for the blurring effect of atmospheric turbulence on images in ground-based telescopes and was instrumental in the work that resulted in the Nobel prize-winning discovery of a supermassive compact object at the centre of our galaxy. When AO is used to correct for the eye's imperfect optics, retinal changes at the cellular level can be detected, allowing us to study the operation of the visual system and to assess ocular health in the microscopic domain. By correcting for sample-induced blur in microscopy, AO has pushed the boundaries of imaging in thick tissue specimens, such as when observing neuronal processes in the brain. In this primer, we focus on the application of AO for high-resolution imaging in astronomy, vision science and microscopy. We begin with an overview of the general principles of AO and its main components, which include methods to measure the aberrations, devices for aberration correction, and how these components are linked in operation. We present results and applications from each field along with reproducibility considerations and limitations. Finally, we discuss future directions.

Label-free imaging of immune cell dynamics in the living retina using adaptive optics

Our recent work characterized the movement of single blood cells within the retinal vasculature (Joseph et al. 2019) using adaptive optics ophthalmoscopy. Here, we apply this technique to the context of acute inflammation and discover both infiltrating and tissue-resident immune cells to be visible without any labeling in the living mouse retina using near-infrared light alone. Intravital imaging of immune cells can be negatively impacted by surgical manipulation, exogenous dyes, transgenic manipulation and phototoxicity. These confounds are now overcome, using phase contrast and time-lapse videography to reveal the dynamic behavior of myeloid cells as they interact, extravasate and survey the mouse retina. Cellular motility and differential vascular responses were measured noninvasively and in vivo across hours to months at the same retinal location, from initiation to the resolution of inflammation. As comparable systems are already available for clinical research, this approach could be readily translated to human application.

Structural imaging of the retina in psychosis spectrum disorders: current status and perspectives

PURPOSE OF REVIEW

Structural changes of the retina in schizophrenia and other psychotic disorders seem plausible as these conditions are accompanied by widespread morphological abnormalities of the brain. Advances in structural retinal imaging have led to the possibility of precise quantification of individual retinal layers, using optical coherence tomography (OCT) scanners.

RECENT FINDINGS

The aggregation of information related to OCT findings in schizophrenia has resulted in three metaanalyses, which are currently described. Areas where retinal changes were reported include retinal nerve fiber layer (RNFL), ganglion cell layer complex (GCC), macular volume, and macular thickness, but findings on affected retinal segments vary to some extent across studies. Discrepancies in individual studies could be because of small samples, heterogeneity within schizophrenia (phase of the illness, illness duration, predominant symptomatology), inconsistent reporting of antipsychotic therapy, insufficient control of confounding variables (somatic comorbidities, smoking, and so on), and use of the different types of OCT scanners.

SUMMARY

Exploration of potential disturbances in retinal architecture could provide new insights into neuronal changes associated with psychosis spectrum disorders, with potential to elucidate the nature and timing of developmental, progressive, inflammatory, and degenerative aspects of neuropathology and pathophysiology, and to assist with characterizing heterogeneity and facilitating personalized treatment approaches.

Recovering the appearance of the capillary blood column from under-sampled flow data

The regular spacing of cells in capillary flow results in spurious cell trajectories if the sampling rate is too low. This makes it difficult to identify cells, even if the velocity is known. Here, we demonstrate a software method to overcome this problem and validate it using high frame rate data with known velocity, which is downsampled to produce aliasing. The method assumes high spatial sampling, constant velocity over short epochs, and an incompressible blood column. Data in successive frames are shifted along the capillary tube axis according to the flow velocity, faithfully rendering cells and plasma. The velocity estimate, required as input to this procedure, can be obtained from either a) the blind optimization of a simple heuristic, or b) a recently proposed velocimetry algorithm, which appears to extend the aliasing limit.

Origin of cell contrast in offset aperture adaptive optics ophthalmoscopy

Offset aperture and split detector imaging are variants of adaptive optics scanning ophthalmoscopy recently introduced to improve the image contrast of retinal cells. Unlike conventional confocal scanning ophthalmoscopy, these approaches collect light laterally decentered from the optical axis. A complete explanation of how these methods enhance contrast has not been described. Here, we provide an optical model with supporting in vivo data that show contrast is generated from spatial variations in the refractive index as it is in phase contrast microscopy. A prediction of this model is supported by experimental data that show contrast is optimized when the detector is placed conjugate with a deeper backscattering screen such as the retinal pigment epithelium and choroid, rather than with the layer being imaged as in conventional confocal imaging. This detection strategy provides a substantial improvement in the contrast these new methods can produce.

In vivo imaging of corneal nerves and cellular structures in mice with Gabor-domain optical coherence microscopy

Gabor-domain optical coherence microscopy (GDOCM) demonstrated in vivo corneal imaging with cellular resolution and differentiation in mice over a field of view of 1 mm². Contact and non-contact imaging was conducted on six healthy and six hyperglycemic C57BL/6J mice. Cellular resolution in the 3D GDOCM images was achieved after motion correction. Corneal nerve fibers were traced and their lengths and branches calculated. Noncontact, label-free imaging of corneal nerves has clinical utility in health and disease, and in transplant evaluation. To the authors' knowledge, this is the first report of in vivo 3D corneal imaging in mice with the capability to resolve nerve fibers using a non-contact imaging modality.

Imaging relative stasis of the blood column in human retinal capillaries

Capillary flow largely consists of alternating red cells and plasma whose speed oscillates predictably with the cardiac cycle. Superimposed on this regular background are sporadic events potentially disruptive to capillary exchange: the passage of white cells, aggregates of red cells, epochs of sparse haematocrit, or unusually slow flow. Such events are not readily differentiated with velocimetry or perfusion mapping. Here we propose a method to identify these phenomena in retinal capillaries imaged with high frame-rate adaptive optics, by calculating and representing pictorially the autocorrelation of intensity through time at each pixel during short epochs. The phenomena described above manifest as bright regions which transiently appear and propagate across an otherwise dark image. Drawing data from normal subjects and those with Type I diabetes, we demonstrate proof of concept and high sensitivity and specificity of this metric to variations in capillary contents and rate of flow in health and disease. The proposed metric offers a useful adjunct to velocimetry and perfusion mapping in the study of normal and abnormal capillary blood flow.

Imaging Retinal Activity in the Living Eye

Retinal function has long been studied with psychophysical methods in humans, whereas detailed functional studies of vision have been conducted mostly in animals owing to the invasive nature of physiological approaches. There are exceptions to this generalization, for example, the electroretinogram. This review examines exciting recent advances using in vivo retinal imaging to understand the function of retinal neurons. In some cases, the methods have existed for years and are still being optimized. In others, new methods such as optophysiology are revealing novel patterns of retinal function in animal models that have the potential to change our understanding of the functional capacity of the retina. Together, the advances in retinal imaging mark an important milestone that shifts attention away from anatomy alone and begins to probe the function of healthy and diseased eyes.

Volumetric fluorescein angiography (vFA) by oblique scanning laser ophthalmoscopy in mouse retina at 200 B-scans per second

Oblique scanning laser ophthalmoscopy (oSLO) is a recently developed technique to provide three-dimensional volumetric fluorescence imaging in retinas over a large field of view, without the need for depth sectioning. In this study, we present volumetric fluorescein angiography (vFA) at 200 B-scans per second in mouse retina in vivo by oSLO. By using a low-cost industrial CMOS camera, imaging speed was improved to 2 volumes per second, ∼10 times more than our previous results. Enabled by the volumetric imaging, we visualized hemodynamics at single capillary level in a depth-dependent manner, and provided methods to quantify capillary hematocrit, absolute capillary blood flow speed, and detection of capillary flow stagnancy and stalling at different vascular layers. The quantitative metrics for capillary hemodynamics enhanced by volumetric imaging can offer valuable insight into vision science and retinal pathologies.

Mapping flow velocity in the human retinal capillary network with pixel intensity cross correlation

We present a new method for determining cellular velocity in the smallest retinal vascular networks as visualized with adaptive optics. The method operates by comparing the intensity profile of each movie pixel with that of every other pixel, after shifting in time by one frame. The time-shifted pixel which most resembles the reference pixel is deemed to be a 'source' or 'destination' of flow information for that pixel. Velocity in the transverse direction is then calculated by dividing the spatial displacement between the two pixels by the inter-frame period. We call this method pixel intensity cross-correlation, or "PIX". Here we compare measurements derived from PIX to two other state-of-the-art algorithms (particle image velocimetry and the spatiotemporal kymograph), as well as to manually tracked cell data. The examples chosen highlight the potential of the new algorithm to substantially improve spatial and temporal resolution, resilience to noise and aliasing, and assessment of network flow properties compared with existing methods.

Cellular Control of Brain Capillary Blood Flow: In Vivo Imaging Veritas

The precise modulation of regional cerebral blood flow during neural activation is important for matching local energetic demand and supply and clearing brain metabolites. Here we discuss advances facilitated by high-resolution optical in vivo imaging techniques that for the first time have provided direct visualization of capillary blood flow and its modulation by neural activity. We focus primarily on studies of microvascular flow, mural cell control of vessel diameter, and oxygen level-dependent changes in red blood cell deformability. We also suggest methodological standards for best practices when studying microvascular perfusion, partly motivated by recent controversies about the precise location within the microvascular tree where neurovascular coupling is initiated, and the role of mural cells in the control of vasomotility.

Near infrared adaptive optics flood illumination retinal angiography

Image-based angiography is a well-adapted technique to characterize vasculature, and has been used in retinal neurovascular studies. Because the microvasculature is of particular interest, being the site of exchange between blood and tissue, a high spatio-temporal resolution is required, implying the use of adaptive optics ophthalmoscopes with a high frame rate. Creating the opportunity for decoupled stimulation and imaging of the retina makes the use of near infrared (NIR) imaging light desirable, while the need for a large field of view and a lack of distortion implies the use of a flood illumination-based setup. However, flood-illumination NIR video sequences of erythrocytes, or red blood cells (RBC), have a limited contrast compared to scanning systems and visible light. As a result, they cannot be processed via existing image-based angiography methods. We have therefore developed a new computational method relying on a spatio-temporal filtering of the sequence to isolate blood flow from noise in low-contrast sequences. Applying this computational approach enabled us to perform angiography with an adaptive optics flood illumination ophthalmoscope (AO-FIO) using NIR light, both in bright-field and dark-field modalities. Finally, we demonstrate the capabilities of our system to differentiate blood flow velocity on a retinal capillary network in vivo.

Imaging single-cell blood flow in the smallest to largest vessels in the living retina

Tissue light scatter limits the visualization of the microvascular network deep inside the living mammal. The transparency of the mammalian eye provides a noninvasive view of the microvessels of the retina, a part of the central nervous system. Despite its clarity, imperfections in the optics of the eye blur microscopic retinal capillaries, and single blood cells flowing within. This limits early evaluation of microvascular diseases that originate in capillaries. To break this barrier, we use 15 kHz adaptive optics imaging to noninvasively measure single-cell blood flow, in one of the most widely used research animals: the C57BL/6J mouse. Measured flow ranged four orders of magnitude (0.0002-1.55 µL min^-1) across the full spectrum of retinal vessel diameters (3.2-45.8 µm), without requiring surgery or contrast dye. Here, we describe the ultrafast imaging, analysis pipeline and automated measurement of millions of blood cell speeds.

Spatio-temporal dynamics of cerebral capillary segments with stalling red blood cells

Optical coherence tomography (OCT) allows label-free imaging of red blood cell (RBC) flux within capillaries with high spatio-temporal resolution. In this study, we utilized time-series OCT-angiography to demonstrate interruptions in capillary RBC flux in mouse brain in vivo. We noticed ∼7.5% of ∼200 capillaries had at least one stall in awake mice with chronic windows during a 9-min recording. At any instant, ∼0.45% of capillaries were stalled. Average stall duration was ∼15 s but could last over 1 min. Stalls were more frequent and longer lasting in acute window preparations. Further, isoflurane anesthesia in chronic preparations caused an increase in the number of stalls. In repeated imaging, the same segments had a tendency to stall again over a period of one month. In awake animals, functional stimulation decreased the observance of stalling events. Stalling segments were located distally, away from the first couple of arteriolar-side capillary branches and their average RBC and plasma velocities were lower than nonstalling capillaries within the same region. This first systematic analysis of capillary RBC stalls in the brain, enabled by rapid and continuous volumetric imaging of capillaries with OCT-angiography, will lead to future investigations of the potential role of stalling events in cerebral pathologies.

Versatile multi-detector scheme for adaptive optics scanning laser ophthalmoscopy

Adaptive optics scanning laser ophthalmoscopy (AOSLO) is a powerful tool for imaging the retina at high spatial and temporal resolution. In this paper, we present a multi-detector scheme for AOSLO which has two main configurations: pixel reassignment and offset aperture imaging. In this detection scheme, the single element detector of the standard AOSLO is replaced by a fiber bundle which couples the detected light into multiple detectors. The pixel reassignment configuration enables high resolution imaging with an increased light collection. The increase in signal-to-noise ratio (SNR) from this configuration can improve the accuracy of motion registration techniques. The offset aperture imaging configuration enhances the detection of multiply scattered light, which improves the contrast of retinal vasculature and inner retinal layers similar to methods such as nonconfocal split-detector imaging and multi-offset aperture imaging.

Noninvasive in vivo characterization of erythrocyte motion in human retinal capillaries using high-speed adaptive optics near-confocal imaging

The flow of erythrocytes in parafoveal capillaries was imaged in the living human eye with an adaptive optics near-confocal ophthalmoscope at a frame rate of 800 Hz with a low coherence near-infrared (NIR) light source. Spatiotemporal traces of the erythrocyte movement were extracted from consecutive images. Erythrocyte velocity was measured using custom software based on the Radon transform. The impact of imaging speed on velocity measurement was estimated using images of frame rates of 200, 400, and 800 Hz. The NIR light allowed for long imaging periods without visually stimulating the retina and disturbing the natural rheological state. High speed near-confocal imaging enabled direct and accurate measurement of erythrocyte velocity, and revealed a distinctively cardiac-dependent pulsatile velocity waveform of the erythrocyte flow in retinal capillaries, disclosed the impact of the leukocytes on erythrocyte motion, and provided new metrics for precise assessment of erythrocyte movement. The approach may facilitate new investigations on the pathophysiology of retinal microcirculation with applications for ocular and systemic diseases.

Preparation Steps for Measurement of Reactivity in Mouse Retinal Arterioles Ex Vivo

Vascular insufficiency and alterations in normal retinal perfusion are among the major factors for the pathogenesis of various sight-threatening ocular diseases, such as diabetic retinopathy, hypertensive retinopathy, and possibly glaucoma. Therefore, retinal microvascular preparations are pivotal tools for physiological and pharmacological studies to delineate the underlying pathophysiological mechanisms and to design therapies for the diseases. Despite the wide use of mouse models in ophthalmic research, studies on retinal vascular reactivity are scarce in this species. A major reason for this discrepancy is the challenging isolation procedures owing to the small size of these retinal blood vessels, which is ~ ≤ 30 µm in luminal diameter. To circumvent the problem of direct isolation of these retinal microvessels for functional studies, we established an isolation and preparation technique that enables ex vivo studies of mouse retinal vasoactivity under near-physiological conditions. Although the present experimental preparations will specifically refer to the mouse retinal arterioles, this methodology can readily be employed to microvessels from rats.

Wide-field absolute transverse blood flow velocity mapping in vessel centerline

We propose a wide-field absolute transverse blood flow velocity measurement method in vessel centerline based on absorption intensity fluctuation modulation effect. The difference between the light absorption capacities of red blood cells and background tissue under low-coherence illumination is utilized to realize the instantaneous and average wide-field optical angiography images. The absolute fuzzy connection algorithm is used for vessel centerline extraction from the average wide-field optical angiography. The absolute transverse velocity in the vessel centerline is then measured by a cross-correlation analysis according to instantaneous modulation depth signal. The proposed method promises to contribute to the treatment of diseases, such as those related to anemia or thrombosis.

Capillary red blood cell velocimetry by phase-resolved optical coherence tomography

We present a phase-resolved optical coherence tomography (OCT) method to extend Doppler OCT for the accurate measurement of the red blood cell (RBC) velocity in cerebral capillaries. OCT data were acquired with an M-mode scanning strategy (repeated A-scans) to account for the single-file passage of RBCs in a capillary, which were then high-pass filtered to remove the stationary component of the signal to ensure an accurate measurement of phase shift of flowing RBCs. The angular frequency of the signal from flowing RBCs was then quantified from the dynamic component of the signal and used to calculate the axial speed of flowing RBCs in capillaries. We validated our measurement by RBC passage velocimetry using the signal magnitude of the same OCT time series data.

Vision in laboratory rodents-Tools to measure it and implications for behavioral research

Mice and rats are nocturnal mammals and their vision is specialized for detection of motion and contrast in dim light conditions. These species possess a large proportion of UV-sensitive cones in their retinas and the majority of their optic nerve axons target superior colliculus rather than visual cortex. Therefore, it was a widely held belief that laboratory rodents hardly utilize vision during day-time behavior. This dogma is being questioned as accumulating evidence suggests that laboratory rodents are able to perform complex visual functions, such as perceiving subjective contours, and that declined vision may affect their performance in many behavioral tasks. For instance, genetic engineering may have unexpected consequences on vision as mouse models of Alzheimer's and Huntington's diseases have declined visual function. Rodent vision can be tested in numerous ways using operant training or reflex-based behavioral tasks, or alternatively using electrophysiological recordings. In this article, we will first provide a summary of visual system and explain its characteristics unique to rodents. Then, we present well-established techniques to test rodent vision, with an emphasis on pattern vision: visual water test, optomotor reflex test, pattern electroretinography and pattern visual evoked potentials. Finally, we highlight the importance of visual phenotyping in rodents. As the number of genetically engineered rodent models and volume of behavioral testing increase simultaneously, the possibility of visual dysfunctions needs to be addressed. Neglect in this matter potentially leads to crude biases in the field of neuroscience and beyond.

High-speed adaptive optics line scan confocal retinal imaging for human eye

Purpose

Continuous and rapid eye movement causes significant intraframe distortion in adaptive optics high resolution retinal imaging. To minimize this artifact, we developed a high speed adaptive optics line scan confocal retinal imaging system.

Methods

A high speed line camera was employed to acquire retinal image and custom adaptive optics was developed to compensate the wave aberration of the human eye’s optics. The spatial resolution and signal to noise ratio were assessed in model eye and in living human eye. The improvement of imaging fidelity was estimated by reduction of intra-frame distortion of retinal images acquired in the living human eyes with frame rates at 30 frames/second (FPS), 100 FPS, and 200 FPS.

Results

The device produced retinal image with cellular level resolution at 200 FPS with a digitization of 512×512 pixels/frame in the living human eye. Cone photoreceptors in the central fovea and rod photoreceptors near the fovea were resolved in three human subjects in normal chorioretinal health. Compared with retinal images acquired at 30 FPS, the intra-frame distortion in images taken at 200 FPS was reduced by 50.9% to 79.7%.

Conclusions

We demonstrated the feasibility of acquiring high resolution retinal images in the living human eye at a speed that minimizes retinal motion artifact. This device may facilitate research involving subjects with nystagmus or unsteady fixation due to central vision loss.