Optimization of confocal scanning laser ophthalmoscope design

Spiral scanning improves subject fixation in widefield retinal imaging

Point scanning retinal imaging modalities, including confocal scanning light ophthalmoscopy (cSLO) and optical coherence tomography, suffer from fixational motion artifacts. Fixation targets, though effective at reducing eye motion, are infeasible in some applications (e.g., handheld devices) due to their bulk and complexity. Here, we report on a cSLO device that scans the retina in a spiral pattern under pseudo-visible illumination, thus collecting image data while simultaneously projecting, into the subject's vision, the image of a bullseye, which acts as a virtual fixation target. An imaging study of 14 young adult volunteers was conducted to compare the fixational performance of this technique to that of raster scanning, with and without a discrete inline fixation target. Image registration was used to quantify subject eye motion; a strip-wise registration method was used for raster scans, and a novel, to the best of our knowledge, ring-based method was used for spiral scans. Results indicate a statistically significant reduction in eye motion by the use of spiral scanning as compared to raster scanning without a fixation target.

Principles of microscopy for ophthalmologists

This short review begins with the theories of Airy, Rayleigh and Abbe on microscope resolution. Next, the principal developments in microscopy in the last half-century are examined for relevance to ophthalmology: confocal microscopy, photoactivation light microscopy (PALM), stochastic optical reconstruction microscopy (STORM), stimulated emission depletion (STED), structured illumination (SI), 2-photon and multiphoton excitation microscopy with a focused beam. Except for confocal, these are difficult to apply to the eye in vivo, as are the interference methods available in microscopes. However, interferometry in the form of coherence tomography is now a major ophthalmic method which has diverged from microscopy. Multiphoton excitation microscopy with an unfocussed beam is a new, low-damage microscope method so-far not exploited in ophthalmoscopy. The Mesolens, which throws off the historic limitations in microscopy set by the human eye, is described as a possible future aid to ophthalmology of the anterior eye.

Chemical Insights into Topical Agents in Intraocular Pressure Management: From Glaucoma Etiopathology to Therapeutic Approaches

Glaucoma encompasses a group of optic neuropathies characterized by complex and often elusive etiopathology, involvihttng neurodegeneration of the optic nerve in conjunction with abnormal intraocular pressure (IOP). Currently, there is no cure for glaucoma, and treatment strategies primarily aim to halt disease progression by managing IOP. This review delves into the etiopathology, diagnostic methods, and treatment approaches for glaucoma, with a special focus on IOP management. We discuss a range of active pharmaceutical ingredients used in glaucoma therapy, emphasizing their chemical structure, pharmacological action, therapeutic effectiveness, and safety/tolerability profiles. Notably, most of these therapeutic agents are administered as topical formulations, a critical aspect considering patient compliance and drug delivery efficiency. The classes of glaucoma therapeutics covered in this review include prostaglandin analogs, beta blockers, alpha agonists, carbonic anhydrase inhibitors, Rho kinase inhibitors, and miotic (cholinergic) agents. This comprehensive overview highlights the importance of topical administration in glaucoma treatment, offering insights into the current state and future directions of pharmacological management in glaucoma.

Hybrid spiral scanning in a double-clad fiber-based handheld confocal scanning light ophthalmoscope

High-speed, accessible, and robust in vivo imaging of the human retina is critical for screening of retinal pathologies, such as diabetic retinopathy, age-related macular degeneration, and others. Scanning light ophthalmoscopy (SLO) is a retinal imaging modality that produces digital, en face images of the human retina with superior image gradability rates when compared to the current standard of care in screening for these diseases, namely the flood-illumination handheld fundus camera (HFC). However, current-generation commercial SLO systems are mostly tabletop devices, limiting their accessibility and utility in screening applications. Moreover, most existing SLO systems use raster scan patterns, which are both inefficient and lead to undesired subject gaze drift when used with visible or pseudo-visible illumination. Non-raster scan patterns, especially spiral scanning as described herein, promise advantages in both scan efficiency and reduced subject eye motion. In this work, we introduce a novel "hybrid spiral" scan pattern and the associated hardware design and real-time image reconstruction techniques necessary for its implementation in an SLO system. Building upon this core hybrid spiral scanning SLO (HSS-SLO) technology, we go on to present a complete handheld HSS-SLO system, featuring a fiber-coupled portable patient interface which leverages a dual-clad fiber (DCF) to form a single-path optical topology, thus ensuring mechanically robust co-alignment of illumination and collection apertures, a necessity for a handheld system. The feasibility of HSS-SLO for handheld, in vivo imaging is demonstrated by imaging eight human volunteers.

In-vivo visualization of the photoreceptors using Spectralis High Magnification Module imaging in central serous chorioretinopathy

Purpose

To visualize photoreceptors using the Spectralis High Magnification Module (HMM) in a case of central serous chorioretinopathy (CSCR) and to correlate the findings with those of optical coherence tomography (OCT) and optical coherence tomography angiography (OCT-A).

Observations

A 35-year-old Caucasian male presenting with chronic CSCR in the left eye was examined using HMM, OCT and OCT-A. The photoreceptors mosaic was assessed both in diseased and apparently uninvolved areas. A partial topographic correlation between the loss of photoreceptors on HMM images and an altered reflectivity of the photoreceptor layer on en-face OCT was noted. Interestingly, a correlation between the photoreceptor damage on HMM and choriocapillaris flow-void areas on OCT-A was seen.

Conclusions and Importance

HMM is a non-invasive imaging modality, allowing the in-vivo visualization of photoreceptor damage in a diseased retina. A focal abnormal perfusion of the choriocapillaris might influence the integrity of the overlying photoreceptors in CSCR.

Computer-assisted photoreceptor assessment on Heidelberg Engineering Spectralis™ High Magnification Module™ images

Purpose

To evaluate reliability and repeatability of computer-assisted measurements of cone photoreceptor metrics on Heidelberg Engineering Spectralis™ High Magnification Module (HMM™) Automatic Real-time Tracking (ART™) images.

Methods

We analyzed HMM™ images in three separate study arms. Computer-assisted cone identification software was validated using an open-access adaptive optics (AO) dataset. We compared results of the first arm to data from AO and histology. We evaluated intersession repeatability of our computer-assisted cone analysis in the second arm. We assessed the capability of HMM™ to visualize cones in the presence of pathology in the third arm.

Results

We included 10 healthy subjects in the first arm of our study, 5 additional healthy participants in the second arm and 5 patients in the third arm. In total, we analyzed 225 regions of interest on HMM™ images. We were able to automatically identify cone photoreceptors and assess corresponding metrics at all eccentricities between 2 and 9° from the fovea. Cone density significantly declined with increasing eccentricity (p = 4.890E-26, Friedman test). With increasing eccentricity, we found a significant increase in intercell distance (p = 2.196E-25, Friedman test) and nearest neighbor distance (p = 1.997E-25, Friedman test). Cone hexagonality ranged between 71 and 85%. We found excellent automated intersession repeatability of cone density counts and spacing measurements. In pathology, we were also able to repeatedly visualize photoreceptors.

Conclusion

Computer-assisted cone photoreceptor analysis on Spectralis™ HMM™ images is feasible, and most cone metrics show excellent repeatability. HMM™ imaging may be useful for photoreceptor analysis as progression marker in outer retinal disease.

Noninvasive monitoring of suprachoroidal, subretinal, and intravitreal implants using confocal scanning laser ophthalmoscope (cSLO) and optical coherence tomography (OCT)

To address the need for noninvasive monitoring of injectable preformed drug delivery implants in the eye, we developed noninvasive methods to monitor such implants in different locations within the eye. Cylindrical polymeric poly(lactide-co-glycolide) or metal implants were injected into isolated bovine eyes at suprachoroidal, subretinal, and intravitreal locations and imaged noninvasively using the cSLO and OCT modes of a Heidelberg Spectralis HRA + OCT instrument after adjusting for the corneal curvature. Length and diameter of implants were obtained using cSLO images for all three locations, and the volume was calculated. Additionally, implant volume for suprachoroidal and subretinal location was estimated by integrating the cross-sectional bleb area over the implant length in multiple OCT images or using the maximum thickness of the implant based on thickness map along with length in cSLO image. Simultaneous cSLO and OCT imaging identified implants in different regions of the eye. Image-based measurements of implant dimensions mostly correlated well with the values prior to injection using blade micrometer. The accuracy (82-112%) and precision (1-19%) for noninvasive measurement of length was better than the diameter (accuracy 69-130%; precision 3-38%) using cSLO image for both types of implants. The accuracy for the measurement of volume of both types of implants from all three intraocular locations was better with cSLO imaging (42-152%) compared to those obtained using OCT cross-sectional bleb area integration (117-556%) or cSLO and thickness map (32-279%) methods. Suprachoroidal, subretinal, and intravitreal implants can be monitored for length, diameter, and volume using cSLO and OCT imaging. Such measurements may be useful in noninvasively monitoring implant degradation and drug release in the eye.

Retinal Imaging Using a Confocal Scanning Laser Ophthalmoscope-Based High-Magnification Module

PURPOSE

To evaluate the usefulness of a high-magnification module (HMM) lens to visualize retinal photoreceptors, retinal nerve fiber layer (RNFL), and superficial retinal vasculature in physiologic and pathologic retinal conditions.

DESIGN

Observational descriptive study.

PARTICIPANTS

Thirty-two participants with normal and pathologic retina examination results.

METHODS

Normal and pathologic maculae were imaged in vivo using still and video HMM lens modes, with fixation and contrast adjustments to enhance visualization. The HMM images were classified qualitatively based on structures identified as either good (photoreceptors seen), average (photoreceptor mosaic cannot be visualized clearly, retinal vessels and other retinal changes can be seen), or poor (no identifiable structures). Selected eyes were imaged with fundus photography, OCT, OCT angiography, indocyanine green angiography, and fluorescein angiography for comparison with the pathologic maculae.

MAIN OUTCOMES MEASURES

Description of HMM module-obtained macula images.

RESULTS

From 32 eyes imaged (16 normal and 16 pathologic retinas), 12 of 16 normal and 11 of 16 pathologic retinas demonstrated at least average image quality, in which retinal vasculature and landmarks could be visualized. The mosaic pattern of hexagonal shapes representing photoreceptors could not be resolved in most pathologic retinas. For the retinas in which the photoreceptor mosaics were visualized (12 of 16 normal and 2 of 16 pathologic retinas), parafoveal mosaic patterns appeared denser with better image quality for all participants compared with foveal photoreceptors. Difficulty in resolving the photoreceptors in the umbo, fovea, and perifovea was encountered, similar to what has been reported with adaptive optics devices. The RNFL was seen as arcuate hyperreflective bundles. Flow was observed in the macular microvasculature. Poorly resolved photoreceptors and scattered hyperreflective foci were correlated with changes in the retinal pigment epithelium in eyes with age-related macular degeneration or central serous chorioretinopathy. Macular striae were seen in eyes with epiretinal membrane.

CONCLUSIONS

In most eyes, regardless of whether retinal pathologic features were present, it was challenging to obtain average quality (or better) images. In the few participants with good-quality imaging, the parafoveal photoreceptor mosaic, vascular flow, and various features of pathologic eyes could be visualized.

High-resolution, ultrafast, wide-field retinal eye-tracking for enhanced quantification of fixational and saccadic motion

We introduce a novel, noninvasive retinal eye-tracking system capable of detecting eye displacements with an angular resolution of 0.039 arcmin and a maximum velocity of 300°/s across an 8° span. Our system is designed based on a confocal retinal imaging module similar to a scanning laser ophthalmoscope. It utilizes a 2D MEMS scanner ensuring high image frame acquisition frequencies up to 1.24 kHz. In contrast with leading eye-tracking technology, we measure the eye displacements via the collection of the observed spatial excursions for all the times corresponding a full acquisition cycle, thus obviating the need for both a baseline reference frame and absolute spatial calibration. Using this approach, we demonstrate the precise measurement of eye movements with magnitudes exceeding the spatial extent of a single frame, which is not possible using existing image-based retinal trackers. We describe our retinal tracker, tracking algorithms and assess the performance of our system by using programmed artificial eye movements. We also demonstrate the clinical capabilities of our system with in vivo subjects by detecting microsaccades with angular extents as small as 0.028°. The rich kinematic ocular data provided by our system with its exquisite degree of accuracy and extended dynamic range opens new and exciting avenues in retinal imaging and clinical neuroscience. Several subtle features of ocular motion such as saccadic dysfunction, fixation instability and abnormal smooth pursuit can be readily extracted and inferred from the measured retinal trajectories thus offering a promising tool for identifying biomarkers of neurodegenerative diseases associated with these ocular symptoms.

Two-photon microperimetry: sensitivity of human photoreceptors to infrared light

Microperimetry is a subjective ophthalmologic test used to assess retinal function at various specific and focal locations of the visual field. Historically, visible light has been described as ranging from 400 to 720 nm. However, we previously demonstrated that infra-red light can initiate visual transduction in rod photoreceptors by a mechanism of two-photon absorption by visual pigments. Here we introduce a newly designed and constructed two-photon microperimeter. We provide for the first time evidence of the presence of a nonlinear process occurring in the human retina based on psychophysical tests using newly developed instrumentation. Since infra-red light penetrates the aged front of the eye better than visible light, it has the potential for improved functional diagnostics in patients with age-related visual disorders.

Optimal wavelengths for subdiffuse scanning laser oximetry of the human retina

Retinal blood vessel oxygenation is considered to be an important marker for numerous eye diseases. Oxygenation is typically assessed by imaging the retinal vessels at different wavelengths using multispectral imaging techniques, where the choice of wavelengths will affect the achievable measurement accuracy. Here, we present a detailed analysis of the error propagation of measurement noise in retinal oximetry, to identify optimal wavelengths that will yield the lowest uncertainty in saturation estimation for a given measurement noise level. In our analysis, we also investigate the effect of hemoglobin packing in discrete blood vessels (pigment packaging), which may result in a nonnegligible bias in saturation estimation if unaccounted for under specific geometrical conditions, such as subdiffuse sampling of smaller blood vessels located deeper within the retina. Our analyses show that using 470, 506, and 592 nm, a fairly accurate estimation of the whole oxygen saturation regime [0 1] can be realized, even in the presence of the pigment packing effect. To validate the analysis, we developed a scanning laser ophthalmoscope to produce high contrast images with a maximum pixel rate of 60 kHz and a maximum 30-deg imaging field of view. Confocal reflectance measurements were then conducted on a tissue-mimicking scattering phantom with optical properties similar to retinal tissue including narrow channels filled with absorbing dyes to mimic blood vessels. By imaging at three optimal wavelengths, the saturation of the dye combination was calculated. The experimental values show good agreement with our theoretical derivations.

Involuntary eye motion correction in retinal optical coherence tomography: Hardware or software solution?

In this paper, we review state-of-the-art techniques to correct eye motion artifacts in Optical Coherence Tomography (OCT) imaging. The methods for eye motion artifact reduction can be categorized into two major classes: (1) hardware-based techniques and (2) software-based techniques. In the first class, additional hardware is mounted onto the OCT scanner to gather information about the eye motion patterns during OCT data acquisition. This information is later processed and applied to the OCT data for creating an anatomically correct representation of the retina, either in an offline or online manner. In software based techniques, the motion patterns are approximated either by comparing the acquired data to a reference image, or by considering some prior assumptions about the nature of the eye motion. Careful investigations done on the most common methods in the field provides invaluable insight regarding future directions of the research in this area. The challenge in hardware-based techniques lies in the implementation aspects of particular devices. However, the results of these techniques are superior to those obtained from software-based techniques because they are capable of capturing secondary data related to eye motion during OCT acquisition. Software-based techniques on the other hand, achieve moderate success and their performance is highly dependent on the quality of the OCT data in terms of the amount of motion artifacts contained in them. However, they are still relevant to the field since they are the sole class of techniques with the ability to be applied to legacy data acquired using systems that do not have extra hardware to track eye motion.

Enhanced visualization of peripheral retinal vasculature with wavefront sensorless adaptive optics optical coherence tomography angiography in diabetic patients

Optical coherence tomography angiography (OCTA) is a promising technique for non-invasive visualization of vessel networks in the human eye. We debut a system capable of acquiring wide field-of-view (>70°) OCT angiograms without mosaicking. Additionally, we report on enhancing the visualization of peripheral microvasculature using wavefront sensorless adaptive optics (WSAO). We employed a fast WSAO algorithm that enabled wavefront correction in <2  s by iterating the mirror shape at the speed of OCT B-scans rather than volumes. Also, we contrasted ∼7° field-of-view OCTA angiograms acquired in the periphery with and without WSAO correction. On average, WSAO improved the sharpness of microvasculature by 65% in healthy eyes and 38% in diseased eyes. Preliminary observations demonstrated that the location of 7° images could be identified directly from the wide field-of-view angiogram. A pilot study on a normal subject and patients with diabetic retinopathy showed the impact of utilizing WSAO for OCTA when visualizing peripheral vasculature pathologies.

Medical Management of Glaucoma in the 21st Century from a Canadian Perspective

Glaucoma is a medical term describing a group of progressive optic neuropathies characterized by degeneration of retinal ganglion cells and retinal nerve fibre layer and resulting in changes in the optic nerve head. Glaucoma is a leading cause of irreversible vision loss worldwide. With the aging population it is expected that the prevalence of glaucoma will continue to increase. Despite recent advances in imaging and visual field testing techniques that allow establishment of earlier diagnosis and treatment initiation, significant numbers of glaucoma patients are undiagnosed and present late in the course of their disease. This can lead to irreversible vision loss, reduced quality of life, and a higher socioeconomic burden. Selection of therapeutic approaches for glaucoma should be based on careful ocular examination, patient medical history, presence of comorbidities, and awareness of concomitant systemic therapies. Therapy should also be individualized to patients' needs and preferences. Recent developments in this therapeutic field require revisiting treatment algorithms and integration of traditional and novel approaches in order to ensure optimal visual outcomes. This article provides an overview of recent developments and practice trends in the medical management of glaucoma in Canada. A discussion of the surgical management is beyond the scope of this paper.

Optical models of the human eye

Optical models of the human eye have been used in visual science for purposes such as providing a framework for explaining optical phenomena in vision, for predicting how refraction and aberrations are affected by change in ocular biometry and as computational tools for exploring the limitations imposed on vision by the optical system of the eye. We address the issue of what is understood by optical model eyes, discussing the 'encyclopaedia' and 'toy train' approaches to modelling. An extensive list of purposes of models is provided. We discuss many of the theoretical types of optical models (also schematic eyes) of varying anatomical accuracy, including single, three and four refracting surface variants. We cover the models with lens structure in the form of nested shells and gradient index. Many optical eye models give accurate predictions only for small angles and small fields of view. If aberrations and image quality are important to consider, such 'paraxial' model eyes must be replaced by 'finite model' eyes incorporating features such as aspheric surfaces, tilts and decentrations, wavelength-dependent media and curved retinas. Many optical model eyes are population averages and must become adaptable to account for age, gender, ethnicity, refractive error and accommodation. They can also be customised for the individual when extensive ocular biometry and optical performance data are available. We consider which optical model should be used for a particular purpose, adhering to the principle that the best model is the simplest fit for the task. We provide a glimpse into the future of optical models of the human eye. This review is interwoven with historical developments, highlighting the important people who have contributed so richly to our understanding of visual optics.

Adaptive optics ophthalmoscopy

This review starts with a brief history and description of adaptive optics (AO) technology, followed by a showcase of the latest capabilities of AO systems for imaging the human retina and an extensive review of the literature on where AO is being used clinically. The review concludes with a discussion on future directions and guidance on usage and interpretation of images from AO systems for the eye.

True color scanning laser ophthalmoscopy and optical coherence tomography handheld probe

Scanning laser ophthalmoscopes (SLOs) are able to achieve superior contrast and axial sectioning capability compared to fundus photography. However, SLOs typically use monochromatic illumination and are thus unable to extract color information of the retina. Previous color SLO imaging techniques utilized multiple lasers or narrow band sources for illumination, which allowed for multiple color but not "true color" imaging as done in fundus photography. We describe the first "true color" SLO, handheld color SLO, and combined color SLO integrated with a spectral domain optical coherence tomography (OCT) system. To achieve accurate color imaging, the SLO was calibrated with a color test target and utilized an achromatizing lens when imaging the retina to correct for the eye's longitudinal chromatic aberration. Color SLO and OCT images from volunteers were then acquired simultaneously with a combined power under the ANSI limit. Images from this system were then compared with those from commercially available SLOs featuring multiple narrow-band color imaging.

Modern technologies for retinal scanning and imaging: an introduction for the biomedical engineer

This review article is meant to help biomedical engineers and nonphysical scientists better understand the principles of, and the main trends in modern scanning and imaging modalities used in ophthalmology. It is intended to ease the communication between physicists, medical doctors and engineers, and hopefully encourage “classical” biomedical engineers to generate new ideas and to initiate projects in an area which has traditionally been dominated by optical physics. Most of the methods involved are applicable to other areas of biomedical optics and optoelectronics, such as microscopic imaging, spectroscopy, spectral imaging, opto-acoustic tomography, fluorescence imaging etc., all of which are with potential biomedical application. Although all described methods are novel and important, the emphasis of this review has been placed on three technologies introduced in the 1990’s and still undergoing vigorous development: Confocal Scanning Laser Ophthalmoscopy, Optical Coherence Tomography, and polarization-sensitive retinal scanning.

Handheld simultaneous scanning laser ophthalmoscopy and optical coherence tomography system

Scanning laser ophthalmoscopy (SLO) and optical coherence tomography (OCT) are widely used retinal imaging modalities that can assist in the diagnosis of retinal pathologies. The combination of SLO and OCT provides a more comprehensive imaging system and a method to register OCT images to produce motion corrected retinal volumes. While high quality, bench-top SLO-OCT systems have been discussed in the literature and are available commercially, there are currently no handheld designs. We describe the first design and fabrication of a handheld SLO/spectral domain OCT probe. SLO and OCT images were acquired simultaneously with a combined power under the ANSI limit. High signal-to-noise ratio SLO and OCT images were acquired simultaneously from a normal subject with visible motion artifacts. Fully automated motion estimation methods were performed in post-processing to correct for the inter- and intra-frame motion in SLO images and their concurrently acquired OCT volumes. The resulting set of reconstructed SLO images and the OCT volume were without visible motion artifacts. At a reduced field of view, the SLO resolved parafoveal cones without adaptive optics at a retinal eccentricity of 11° in subjects with good ocular optics. This system may be especially useful for imaging young children and subjects with less stable fixation.