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.

Assessing blood vessel perfusion and vital signs through retinal imaging photoplethysmography

One solution to the global challenge of increasing ocular disease is a cost-effective technique for rapid screening and assessment. Current ophthalmic imaging techniques, e.g. scanning and ocular blood flow systems, are expensive, complex to operate and utilize invasive contrast agents during assessment. The work presented here demonstrates a simple retinal imaging photoplethysmography (iPPG) system with the potential to provide screening, diagnosis, monitoring and assessment that is non-invasive, painless and radiationless. Time series of individual retinal blood vessel images, captured with an eye fundus camera, are processed using standard filtering, amplitude demodulation and principle component analysis (PCA) methods to determine the values of the heart rate (HR) and respiration rate (RR), which are in compliance with simultaneously obtained measurements using commercial pulse oximetry. It also seems possible that some information on the dynamic changes in oxygen saturation levels (SpO₂) in a retinal blood vessel may also be obtained. As a consequence, the retinal iPPG modality system demonstrates a potential avenue for rapid ophthalmic screening, and even early diagnosis, against ocular disease without the need for fluorescent or contrast agents.

Erratum: Non-invasive assessment of human cone photoreceptor function: erratum

[This corrects the article on p. 5098 in vol. 8, PMID: 29188106.].

Enhanced retinal vasculature imaging with a rapidly configurable aperture

In adaptive optics scanning laser ophthalmoscope (AOSLO) systems, capturing multiply scattered light can increase the contrast of the retinal microvasculature structure, cone inner segments, and retinal ganglion cells. Current systems generally use either a split detector or offset aperture approach to collect this light. We tested the ability of a spatial light modulator (SLM) as a rapidly configurable aperture to use more complex shapes to enhance the contrast of retinal structure. Particularly, we varied the orientation of a split detector aperture and explored the use of a more complex shape, the half annulus, to enhance the contrast of the retinal vasculature. We used the new approach to investigate the influence of scattering distance and orientation on vascular imaging.

Ultra-high contrast retinal display system for single photoreceptor psychophysics

Due to the enormous dynamic range of human photoreceptors in response to light, studying their visual function in the intact retina challenges the stimulation hardware, specifically with regard to the displayable luminance contrast. The adaptive optics scanning laser ophthalmoscope (AOSLO) is an optical platform that focuses light to extremely small retinal extents, approaching the size of single photoreceptor cells. However, the current light modulation techniques produce spurious visible backgrounds which fundamentally limit experimental options. To remove unwanted background light and to improve contrast for high dynamic range visual stimulation in an AOSLO, we cascaded two commercial fiber-coupled acousto-optic modulators (AOMs) and measured their combined optical contrast. By compensating for zero-point differences in the individual AOMs, we demonstrate a multiplicative extinction ratio in the cascade that was in accordance with the extinction ratios of both single AOMs. When latency differences in the AOM response functions were individually corrected, single switch events as short as 50 ns with radiant power contrasts up to 1:10¹⁰ were achieved. This is the highest visual contrast reported for any display system so far. We show psychophysically that this contrast ratio is sufficient to stimulate single foveal photoreceptor cells with small and bright enough visible targets that do not contain a detectable background. Background-free stimulation will enable photoreceptor testing with custom adaptation lights. Furthermore, a larger dynamic range in displayable light levels can drive photoreceptor responses in cones as well as in rods.

Non-invasive assessment of human cone photoreceptor function

Vision begins when light isomerizes the photopigments within photoreceptors. Noninvasive cellular-scale observation of the structure of the human photoreceptor mosaic is made possible through the use of adaptive optics (AO) enhanced ophthalmoscopes, but establishing noninvasive objective measures of photoreceptor function on a similar scale has been more difficult. AO ophthalmoscope images acquired with near-infrared light show that individual cone photoreceptor reflectance can change in response to a visible stimulus. Here we show that the intrinsic response depends on stimulus wavelength and intensity, and that its action spectrum is well-matched to the spectral sensitivity of cone-mediated vision. Our results demonstrate that the cone reflectance response is mediated by photoisomerization, thus making it a direct measure of photoreceptor function.

Computer-aided classification of sickle cell retinopathy using quantitative features in optical coherence tomography angiography

As a new optical coherence tomography (OCT) imaging modality, there is no standardized quantitative interpretation of OCT angiography (OCTA) characteristics of sickle cell retinopathy (SCR). This study is to demonstrate computer-aided SCR classification using quantitative OCTA features, i.e., blood vessel tortuosity (BVT), blood vessel diameter (BVD), vessel perimeter index (VPI), foveal avascular zone (FAZ) area, FAZ contour irregularity, parafoveal avascular density (PAD). It was observed that combined features show improved classification performance, compared to single feature. Three classifiers, including support vector machine (SVM), k-nearest neighbor (KNN) algorithm, and discriminant analysis, were evaluated. Sensitivity, specificity, and accuracy were quantified to assess the performance of each classifier. For SCR vs. control classification, all three classifiers performed well with an average accuracy of 95% using the six quantitative OCTA features. For mild vs. severe stage retinopathy classification, SVM shows better (97% accuracy) performance, compared to KNN algorithm (95% accuracy) and discriminant analysis (88% accuracy).

Quantitative characteristics of sickle cell retinopathy in optical coherence tomography angiography

Early detection is an essential step for effective intervention of sickle cell retinopathy (SCR). Emerging optical coherence tomography angiography (OCTA) provides excellent three-dimensional (3D) resolution to enable label-free, noninvasive visualization of retinal vascular structures, promising improved sensitivity in detecting SCR. However, quantitative analysis of SCR characteristics in OCTA images is yet to be established. In this study, we conducted comprehensive analysis of six OCTA parameters, including blood vessel tortuosity, vessel diameter, vessel perimeter index (VPI), area of foveal avascular zone (FAZ), contour irregularity of FAZ and parafoveal avascular density. Compared to traditional retinal thickness analysis, five of these six OCTA parameters show improved sensitivity for SCR detection than retinal thickness. It is observed that the most sensitive parameters were the contour irregularity of FAZ in the superficial layer and avascular density in temporal regions, while the area of FAZ, tortuosity and mean diameter of the vessel were moderately sensitive.

Adaptive optics optical coherence tomography angiography for morphometric analysis of choriocapillaris [Invited]

Histological studies have shown that morphometric changes at the microscopic level of choriocapillaris (CC) occur with aging and disease onset, and therefore may be sensitive biomarkers of outer retinal health. However, visualizing CC at this level in the living human eye is challenging because its microvascular is tightly interconnected and weakly reflecting. In this study, we address these challenges by developing and validating a method based on adaptive optics optical coherence tomography with angiography (AO-OCTA) that provides the necessary 3D resolution and image contrast to visualize and quantify these microscopic details. The complex network of anastomotic CC capillaries was successfully imaged in nine healthy subjects (26 to 68 years of age) and at seven retinal eccentricities across the macula. Using these images, four fundamental morphometric parameters of CC were characterized: retinal pigment epithelium-to-CC depth separation (17.5 ± 2.1 µm), capillary diameter (17.4 ± 2.3 µm), normalized capillary density (0.53 ± 0.08), and capillary length per unit area (50.4 ± 9.5 mm^-1). AO-OCTA results were consistent with histologic studies and, unlike OCTA, showed clear delineation of CC capillaries, a requirement for measuring three of the four morphometric parameters. Success in younger and older eyes establishes a path for testing aging and disease effects in larger populations. To the best of our knowledge, this is the first quantitative morphometry of choriocapillaris at the level of individual capillaries in the living human retina.

Computational fluid dynamics assisted characterization of parafoveal hemodynamics in normal and diabetic eyes using adaptive optics scanning laser ophthalmoscopy

Diabetic retinopathy (DR) is the leading cause of visual loss in working-age adults worldwide. Previous studies have found hemodynamic changes in the diabetic eyes, which precede clinically evident pathological alterations of the retinal microvasculature. There is a pressing need for new methods to allow greater understanding of these early hemodynamic changes that occur in DR. In this study, we propose a noninvasive method for the assessment of hemodynamics around the fovea (a region of the eye of paramount importance for vision). The proposed methodology combines adaptive optics scanning laser ophthalmoscopy and computational fluid dynamics modeling. We compare results obtained with this technique with in vivo measurements of blood flow based on blood cell aggregation tracking. Our results suggest that parafoveal hemodynamics, such as capillary velocity, wall shear stress, and capillary perfusion pressure can be noninvasively and reliably characterized with this method in both healthy and diabetic retinopathy patients.

Erratum: Perceived image quality with simulated segmented bifocal corrections: publisher's note

[This corrects the article on p. 4388 in vol. 7.].

Perceived image quality with simulated segmented bifocal corrections

Bifocal contact or intraocular lenses use the principle of simultaneous vision to correct for presbyopia. A modified two-channel simultaneous vision simulator provided with an amplitude transmission spatial light modulator was used to optically simulate 14 segmented bifocal patterns (+ 3 diopters addition) with different far/near pupillary distributions of equal energy. Five subjects with paralyzed accommodation evaluated image quality and subjective preference through the segmented bifocal corrections. There are strong and systematic perceptual differences across the patterns, subjects and observation distances: 48% of the conditions evaluated were significantly preferred or rejected. Optical simulations (in terms of through-focus Strehl ratio from Hartmann-Shack aberrometry) accurately predicted the pattern producing the highest perceived quality in 4 out of 5 patients, both for far and near vision. These perceptual differences found arise primarily from optical grounds, but have an important neural component.

Photoreceptor disc shedding in the living human eye

Cone photoreceptors undergo a daily cycle of renewal and shedding of membranous discs in their outer segments (OS), the portion responsible for light capture. These physiological processes are fundamental to maintaining photoreceptor health, and their dysfunction is associated with numerous retinal diseases. While both processes have been extensively studied in animal models and postmortem eyes, little is known about them in the living eye, in particular human. In this study, we report discovery of the optical signature associated with disc shedding using a method based on adaptive optics optical coherence tomography (AO-OCT) in conjunction with post-processing methods to track and monitor individual cone cells in 4D. The optical signature of disc shedding is characterized by an abrupt transient loss in the cone outer segment tip (COST) reflection followed by its return that is axially displaced anteriorly. Using this signature, we measured the temporal and spatial properties of shedding events in three normal subjects. Average duration of the shedding event was 8.8 ± 13.4 minutes, and average length loss of the OS was 2.1 μm (7.0% of OS length). Prevalence of cone shedding was highest in the morning (14.3%) followed by the afternoon (5.7%) and evening (4.0%), with load distributed across the imaged patch. To the best of our knowledge these are the first images of photoreceptor disc shedding in the living retina.

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

Measuring blood cell dynamics within the capillaries of the living eye provides crucial information regarding the health of the microvascular network. To date, the study of single blood cell movement in this network has been obscured by optical aberrations, hindered by weak optical contrast, and often required injection of exogenous fluorescent dyes to perform measurements. Here we present a new strategy to non-invasively image single blood cells in the living mouse eye without contrast agents. Eye aberrations were corrected with an adaptive optics camera coupled with a fast, 15 kHz scanned beam orthogonal to a capillary of interest. Blood cells were imaged as they flowed past a near infrared imaging beam to which the eye is relatively insensitive. Optical contrast of cells was optimized using differential scatter of blood cells in the split-detector imaging configuration. Combined, these strategies provide label-free, non-invasive imaging of blood cells in the retina as they travel in single file in capillaries, enabling determination of cell flux, morphology, class, velocity, and rheology at the single cell level.

Optical coherence tomography angiography of stimulus evoked hemodynamic responses in individual retinal layers

Blood flow changes are highly related to neural activities in the retina. It has been reported that neural activity increases when flickering light stimulation of the retina is used. It is known that blood flow changes with flickering light stimulation can be altered in patients with vascular disease and that measurement of flicker-induced vasodilatation is an easily applied tool for monitoring functional microvascular alterations. However, details of distortions in retinal neurovascular coupling associated with major eye diseases are not well understood due to the limitation of existing techniques. In this study, flickering light stimulation was applied to mouse retinas to investigate stimulus evoked hemodynamic responses in individual retinal layers. A spectral domain optical coherence tomography (OCT) angiography imaging system was developed to provide dynamic mapping of hemodynamic responses in the ganglion cell layer, inner plexiform layer, outer plexiform layer and choroid layer before, during and after flickering light stimulation. Experimental results showed hemodynamic responses with different magnitudes and time courses in individual retinal layers. We anticipate that the dynamic OCT angiography of stimulus evoked hemodynamic responses can greatly foster the study of neurovascular coupling mechanisms in the retina, promising new biomarkers for retinal disease detection and diagnosis.

Detection of retinal blood vessel changes in multiple sclerosis with optical coherence tomography

Although retinal vasculitis is common in multiple sclerosis (MS), it is not known if MS is associated with quantitative abnormalities in retinal blood vessels (BVs). Optical coherence tomography (OCT) is suitable for examining the integrity of the anterior visual pathways in MS. In this paper we have compared the size and number of retinal blood vessels in patients with MS, with and without a history of optic neuritis (ON), and control subjects from the cross-sectional retinal images from OCT. Blood vessel diameter (BVD), blood vessel number (BVN), and retinal nerve fiber layer thickness (RNFLT) were extracted from OCT images collected from around the optic nerves of 129 eyes (24 control, 24 MS + ON, 81 MS-ON) of 71 subjects. Associations between blood vessel metrics, MS diagnosis, MS disability, ON, and RNFLT were evaluated using generalized estimating equation (GEE) models. MS eyes had a lower total BVD and BVN than control eyes. The effect was more pronounced with increased MS disability, and persisted in multivariate models adjusting for RNFLT and ON history. Twenty-nine percent (29%) of MS subjects had fewer retinal blood vessels than all control subjects. MS diagnosis, disability, and ON history were not associated with average blood vessel size. The relationship between MS and lower total BVD/BVN is not accounted for by RNFLT or ON. Further study is needed to determine the relationship between OCT blood vessel metrics and qualitative retinal blood vessel abnormalities in MS.

Modal content of living human cone photoreceptors

Decades of experimental and theoretical investigations have established that photoreceptors capture light based on the principles of optical waveguiding. Yet considerable uncertainty remains, even for the most basic prediction as to whether photoreceptors support more than a single waveguide mode. To test for modal behavior in human cone photoreceptors in the near infrared, we took advantage of adaptive-optics optical coherence tomography (AO-OCT, λc = 785 nm) to noninvasively image in three dimensions the reflectance profile of cones. Modal content of reflections generated at the cone inner segment and outer segment junction (IS/OS) and cone outer segment tip (COST) was examined over a range of cone diameters in 1,802 cones from 0.6° to 10° retinal eccentricity. Second moment analysis in conjunction with theoretical predictions indicate cone IS and OS have optical properties consistent of waveguides, which depend on segment diameter and refractive index. Cone IS was found to support a single mode near the fovea (≤3°) and multiple modes further away (>4°). In contrast, no evidence of multiple modes was found in the cone OSs. The IS/OS and COST reflections share a common optical aperture, are most circular near the fovea, show no orientation preference, and are temporally stable. We tested mode predictions of a conventional step-index fiber model and found that in order to fit our AO-OCT results required a lower estimate of the IS refractive index and introduction of an IS focusing/tapering effect.

Imaging translucent cell bodies in the living mouse retina without contrast agents

The transparency of most retinal cell classes typically precludes imaging them in the living eye; unless invasive methods are used that deploy extrinsic contrast agents. Using an adaptive optics scanning light ophthalmoscope (AOSLO) and capitalizing on the large numerical aperture of the mouse eye, we enhanced the contrast from otherwise transparent cells by subtracting the left from the right half of the light distribution in the detector plane. With this approach, it is possible to image the distal processes of photoreceptors, their more proximal cell bodies and the mosaic of horizontal cells in the living mouse retina.

Binocular visual performance and summation after correcting higher order aberrations

Although the ocular higher order aberrations degrade the retinal image substantially, most studies have investigated their effect on vision only under monocular conditions. Here, we have investigated the impact of binocular higher order aberration correction on visual performance and binocular summation by constructing a binocular adaptive optics (AO) vision simulator. Binocular monochromatic aberration correction using AO improved visual acuity and contrast sensitivity significantly. The improvement however, differed from that achieved under monocular viewing. At high spatial frequency (24 c/deg), the monocular benefit in contrast sensitivity was significantly larger than the benefit achieved binocularly. In addition, binocular summation for higher spatial frequencies was the largest in the presence of subject's native higher order aberrations and was reduced when these aberrations were corrected. This study thus demonstrates the vast potential of binocular AO vision testing in understanding the impact of ocular optics on habitual binocular vision.

Modeling and minimizing interference from corneal birefringence in retinal birefringence scanning for foveal fixation detection

Utilizing the measured corneal birefringence from a data set of 150 eyes of 75 human subjects, an algorithm and related computer program, based on Müller-Stokes matrix calculus, were developed in MATLAB for assessing the influence of corneal birefringence on retinal birefringence scanning (RBS) and for converging upon an optical/mechanical design using wave plates ("wave-plate-enhanced RBS") that allows foveal fixation detection essentially independently of corneal birefringence. The RBS computer model, and in particular the optimization algorithm, were verified with experimental human data using an available monocular RBS-based eye fixation monitor. Fixation detection using wave-plate-enhanced RBS is adaptable to less cooperative subjects, including young children at risk for developing amblyopia.

Method for deriving visual field boundaries from OCT scans of patients with retinitis pigmentosa

The location of the loss of the inner segment (IS)/outer segment (OS) border, as seen with frequency domain optical coherence tomography (fdOCT), was determined on fdOCT scans from patients with retinitis pigmentosa. A comparison to visual field loss supported the hypothesis, based upon previous work, that the point at which the IS/OS border disappears provides a structural marker for the edge of the visual field. Repeat fdOCT measures showed good within day reproducibility, while data obtained on average 22.5 months later showed signs of progression. The IS/OS contour shows promise as a measure for following changes in patients undergoing treatment.

Method for comparing visual field defects to local RNFL and RGC damage seen on frequency domain OCT in patients with glaucoma

To relate structural (anatomical) and functional (behavioral) measures of glaucomatous damage, a method is described for comparing visual field defects to local retinal ganglion cell (RGC) and retinal nerve fiber layer (RNFL) thicknesses. Thickness maps of the RNFL and the RGC + inner plexiform layer, obtained with frequency domain optical coherence tomography (fdOCT), were transformed into probability maps by comparing them to a normative group. As demonstrated in patients with glaucomatous damage to the macula, the probability map associated with the patient's visual field can be directly compared to the fdOCT probability maps by taking into consideration the displacement of the RGCs.

Optical properties of the mouse eye

The Shack-Hartmann wavefront sensor (SHWS) spots upon which ocular aberration measurements depend have poor quality in mice due to light reflected from multiple retinal layers. We have designed and implemented a SHWS that can favor light from a specific retinal layer and measured monochromatic aberrations in 20 eyes from 10 anesthetized C57BL/6J mice. Using this instrument, we show that mice are myopic, not hyperopic as is frequently reported. We have also measured longitudinal chromatic aberration (LCA) of the mouse eye and found that it follows predictions of the water-filled schematic mouse eye. Results indicate that the optical quality of the mouse eye assessed by measurement of its aberrations is remarkably good, better for retinal imaging than the human eye. The dilated mouse eye has a much larger numerical aperture (NA) than that of the dilated human eye (0.5 NA vs. 0.2 NA), but it has a similar amount of root mean square (RMS) higher order aberrations compared to the dilated human eye. These measurements predict that adaptive optics based on this method of wavefront sensing will provide improvements in retinal image quality and potentially two times higher lateral resolution than that in the human eye.