Effect of a contact lens on mouse retinal in vivo imaging: Effective focal length changes and monochromatic aberrations

A Narrowband 635 nm Autofluorescence Peak in Albino Mouse Eyes Found With Multi-Modal Imaging Reveals the Presence of Protoporphyrin IX in the Choroid

Purpose

To investigate differences in fundus autofluorescence (AF) spectra of pigmented (C57Bl/6) and albino (Balb/c) mouse retinas.

Methods

AF spectra were measured with a scanning laser ophthalmoscope (SLO) with a high-resolution spectrometer. The action spectrum of a 635 nm AF "spike" in albino mice was measured to estimate the underlying absorption spectrum. Optical coherence tomography (OCT) and OCT angiography were used to determine the most likely depth location of the source of the AF feature.

Results

Two narrowband emission peaks centered at 635 nm and ∼705 nm were observed in the AF spectra of albino (but not pigmented) mouse eyes. The dual-peak emission spectrum of the albino eye was extracted by subtracting a broadband emission that was similar in pigmented and albino mice: the two peaks correspond to emission spectra peaks of protoporphyrin IX, an obligate precursor in the biosynthesis of heme and cytochrome c. The action spectrum of the prominent 635 nm emission component corresponded with the PPIX absorption spectrum. The spatial distribution of the 635 nm emission did not correspond to that of the retinal vasculature but had a pattern more consistent with a choroidal origin.

Conclusions

Our results reveal that substantial PPIX is present in the posterior tissues of albino (Balb/c) mouse eyes, and measurable with high-resolution spectral measurements, and suggest that the absence of the narrow band PPIX emission peaks in pigmented eyes arises at least in part from screening by RPE and choroidal melanin.

Ultrafast adaptive optics for imaging the living human eye

Adaptive optics (AO) is a powerful method for correcting dynamic aberrations in numerous applications. When applied to the eye, it enables cellular-resolution retinal imaging and enhanced visual performance and stimulation. Most ophthalmic AO systems correct dynamic aberrations up to 1-2 Hz, the commonly-known cutoff frequency for correcting ocular aberrations. However, this frequency may be grossly underestimated for more clinically relevant scenarios where the medical impact of AO will be greatest. Unfortunately, little is known about the aberration dynamics in these scenarios. A major bottleneck has been the lack of sufficiently fast AO systems to measure and correct them. We develop an ultrafast ophthalmic AO system that increases AO bandwidth by ~30× and improves aberration power rejection magnitude by 500×. We demonstrate that this much faster ophthalmic AO is possible without sacrificing other system performances. We find that the discontinuous-exposure AO-control scheme runs 32% slower yet achieves 53% larger AO bandwidth than the commonly used continuous-exposure scheme. Using the ultrafast system, we characterize ocular aberration dynamics in six clinically-relevant scenarios and find their power spectra to be 10-100× larger than normal. We show that ultrafast AO substantially improves aberration correction and retinal imaging performance in these scenarios compared with conventional AO.

In vivo volumetric analysis of retinal vascular hemodynamics in mice with spatio-temporal optical coherence tomography

Significance

Microcirculation and neurovascular coupling are important parameters to study in neurological and neuro-ophthalmic conditions. As the retina shares many similarities with the cerebral cortex and is optically accessible, a special focus is directed to assessing the chorioretinal structure, microvasculature, and hemodynamics of mice, a vital animal model for vision and neuroscience research.

Aim

We aim to introduce an optical imaging tool enabling in vivo volumetric mouse retinal monitoring of vascular hemodynamics with high temporal resolution.

Approach

We translated the spatio-temporal optical coherence tomography (STOC-T) technique into the field of small animal imaging by designing a new optical system that could compensate for the mouse eye refractive error. We also developed post-processing algorithms, notably for the assessment of (i) localized hemodynamics from the analysis of pulse wave-induced Doppler artifact modulation and (ii) retinal tissue displacement from phase-sensitive measurements.

Results

We acquired high-quality, in vivo volumetric mouse retina images at a rate of 113 Hz over a lateral field of view of ∼ 500    μ m . We presented high-resolution en face images of the retinal and choroidal structure and microvasculature from various layers, after digital aberration correction. We were able to measure the pulse wave velocity in capillaries of the outer plexiform layer with a mean speed of 0.35 mm/s and identified venous and arterial pulsation frequency and phase delay. We quantified the modulation amplitudes of tissue displacement near major vessels (with peaks of 150 nm), potentially carrying information about the biomechanical properties of the retinal layers involved. Last, we identified the delays between retinal displacements due to the passing of venous and arterial pulse waves.

Conclusions

The developed STOC-T system provides insights into the hemodynamics of the mouse retina and choroid that could be beneficial in the study of neurovascular coupling and vasculature and flow speed anomalies in neurological and neuro-ophthalmic conditions.

On-axis full-field swept-source optical coherence tomography for murine retinal imaging

A full-field swept-source optical coherence tomography (FF-SS-OCT) for in vivo murine retinal imaging is demonstrated. The on-axis FF-SS-OCT system was built in a Mach-Zehnder interferometer configuration employing a tunable laser source with an adjustable sweep rate and sweep range in conjunction with a fast 2D-CMOS camera. A large field retinal (coherent) illumination was accomplished using an imaging interface comprised of a short-focal length imaging lens and a contact lens. The magnification between the camera and retina (spatial sampling) was appropriately chosen to record the microscopic structural features of the retina in the image. A pupil stop was employed in the detection path to reject unwanted backscattering from the mouse eye and other sources and limit aberrations distorting the retinal images. In vivo mouse retinal imaging was performed at a sweep rate of 150 Hz to acquire volumes unaffected by the system vibrations, which predominated at lower frequencies. Operating the FF-SS-OCT at this speed yielded an effective axial scan rate of 20 million A-scans/s and a field of view of 820 × 410 µm (24.12° × 12.06°). High-quality retinal B-scans and enface images of the retina were obtained with the SS-FF-OCT, revealing all major retinal layers and vascular plexuses.

Image metric-based multi-observation single-step deep deterministic policy gradient for sensorless adaptive optics

Sensorless adaptive optics (SAO) has been widely used across diverse fields such as astronomy, microscopy, and ophthalmology. Recent advances have proved the feasibility of using the deep deterministic policy gradient (DDPG) for image metric-based SAO, achieving fast correction speeds compared to the coordinate search Zernike mode hill climbing (ZMHC) method. In this work, we present a multi-observation single-step DDPG (MOSS-DDPG) optimization framework for SAO on a confocal scanning laser ophthalmoscope (SLO) system with particular consideration for applications in preclinical retinal imaging. MOSS-DDPG optimizes N target Zernike coefficients in a single-step manner based on 2N + 1 observations of the image sharpness metric values. Through in silico simulations, MOSS-DDPG has demonstrated the capability to quickly achieve diffraction-limited resolution performance with long short-term memory (LSTM) network implementation. In situ tests suggest that knowledge learned through simulation adapts swiftly to imperfections in the real system by transfer learning, exhibiting comparable in situ performance to the ZMHC method with a greater than tenfold reduction in the required number of iterations.

Full field-of-view virtual reality goggles for mice

Visual virtual reality (VR) systems for head-fixed mice offer advantages over real-world studies for investigating the neural circuitry underlying behavior. However, current VR approaches do not fully cover the visual field of view of mice, do not stereoscopically illuminate the binocular zone, and leave the lab frame visible. To overcome these limitations, we developed iMRSIV (Miniature Rodent Stereo Illumination VR)-VR goggles for mice. Our system is compact, separately illuminates each eye for stereo vision, and provides each eye with an ∼180° field of view, thus excluding the lab frame while accommodating saccades. Mice using iMRSIV while navigating engaged in virtual behaviors more quickly than in a current monitor-based system and displayed freezing and fleeing reactions to overhead looming stimulation. Using iMRSIV with two-photon functional imaging, we found large populations of hippocampal place cells during virtual navigation, global remapping during environment changes, and unique responses of place cell ensembles to overhead looming stimulation.

Time-Serial Evaluation of the Development and Treatment of Myopia in Mice Eyes Using OCT and ZEMAX

Myopia is a significant cause of visual impairment which may lead to many complications. However, the understanding of the mechanisms of myopia is still limited. In this paper, in order to investigate the development and the treatment of myopia, we analyzed the biological structure parameters of mice eyes, obtained from optical coherence tomography (OCT), and the optical performance of mice eyes calculated using ZEMAX software (ZEMAX Development Corporation, Kirkland, WA, USA) in which the optical model was built on the segment-by-segment optically corrected OCT 3D-images. Time-serial evaluation of three groups of mice eyes (form-deprivation myopia mice eyes, normal mice eyes, and atropine-treated myopia mice eyes) was performed. In addition to the biological structure parameters, imaging performance with the development of root-mean-square wavefront aberration at six filed angles was compared and analyzed. Results show that the biological structure parameters of the eye are closely related to the development of myopia. The peripheral defocus of the retina has a significant impact on inducing myopia, which verifies the new theory of myopia development. The delaying effect of atropine solution on myopia development is shown to verify the therapeutic effect of the medicine. This study provides technical support for the investigation of the myopia mechanism.

Adaptive optics scanning laser ophthalmoscopy and optical coherence tomography (AO-SLO-OCT) system for in vivo mouse retina imaging

Optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO) are imaging technologies invented in the 1980s that have revolutionized the field of in vivo retinal diagnostics and are now commonly used in ophthalmology clinics as well as in vision science research. Adaptive optics (AO) technology enables high-fidelity correction of ocular aberrations, resulting in improved resolution and sensitivity for both SLO and OCT systems. The potential of gathering multi-modal cellular-resolution information in a single instrument is of great interest to the ophthalmic imaging community. Although similar instruments have been developed for imaging the human retina, developing such a system for mice will benefit basic science research and should help with further dissemination of AO technology. Here, we present our work integrating OCT into an existing mouse retinal AO-SLO system, resulting in a multi-modal AO-enhanced imaging system of the living mouse eye. The new system allows either independent or simultaneous data acquisition of AO-SLO and AO-OCT, depending on the requirements of specific scientific experiments. The system allows a data acquisition speed of 200 kHz A-scans/pixel rate for OCT and SLO, respectively. It offers ∼6 µm axial resolution for AO-OCT and a ∼1 µm lateral resolution for AO-SLO-OCT imaging.

Correction of refractive distortion in whole-eye optical coherence tomography imaging of the mouse eye

Optical coherence tomography (OCT) is an imaging modality that acquires high-resolution cross-sectional images of living tissues and it has become the standard in ophthalmological diagnoses. However, most quantitative morphological measurements are based on the raw OCT images which are distorted by several mechanisms such as the refraction of probe light in the sample and the scan geometries and thus the analysis of the raw OCT images inevitably induced calculation errors. In this paper, based on Fermat's principle and the concept of inverse light tracing, image distortions due to refraction occurred at tissue boundaries in the whole-eye OCT imaging of mouse by telecentric scanning were corrected. Specially, the mathematical correction models were deducted for each interface, and the high-precision whole-eye image was recovered segment by segment. We conducted phantom and in vivo experiments on mouse and human eyes to verify the distortion correction algorithm, and several parameters of the radius of curvature, thickness of tissues and error, were calculated to quantitatively evaluate the images. Experimental results demonstrated that the method can provide accurate and reliable measurements of whole-eye parameters and thus be a valuable tool for the research and clinical diagnosis.

Volumetric data analysis enabled spatially resolved optoretinogram to measure the functional signals in the living retina

Optoretinogram, a technique in which optical coherence tomography (OCT) is used to measure retinal functions in response to a visible light stimulus, can be a potentially useful tool to quantify retinal health alterations. Existing experimental studies on animals have focused on measuring the global retinal response by transversally averaging 3D data across the retina, which minimizes the spatial resolution of the signals, and limits the signal-to-noise ratio because only central B-scans are collected and analyzed. These problems were addressed in this study by collecting volumetric data to probe functional signals and developing an improved 3D registration approach to align such series-acquired OCT volumes. These data were then divided into small blocks and subject to a spatiotemporal analysis, whose results confirmed the spatial-dependence of functional signals. By further averaging, the overall measurement accuracies for the position and the scattering signals were estimated to be approximately 30 nm and 1.1 %, respectively. With improved accuracy, this method revealed certain novel functional signals that have not been previously reported. In conclusion, this work provides a powerful tool to monitor retinal local and global functional changes in aging, diseased, or treated rodent eyes.

Optimization of Virtual Shack-Hartmann Wavefront Sensing

Virtual Shack–Hartmann wavefront sensing (vSHWS) can flexibly adjust parameters to meet different requirements without changing the system, and it is a promising means for aberration measurement. However, how to optimize its parameters to achieve the best performance is rarely discussed. In this work, the data processing procedure and methods of vSHWS were demonstrated by using a set of normal human ocular aberrations as an example. The shapes (round and square) of a virtual lenslet, the zero-padding of the sub-aperture electric field, sub-aperture number, as well as the sequences (before and after diffraction calculation), algorithms, and interval of data interpolation, were analyzed to find the optimal configuration. The effect of the above optimizations on its anti-noise performance was also studied. The Zernike coefficient errors and the root mean square of the wavefront error between the reconstructed and preset wavefronts were used for performance evaluation. The performance of the optimized vSHWS could be significantly improved compared to that of a non-optimized one, which was also verified with 20 sets of clinical human ocular aberrations. This work makes the vSHWS’s implementation clearer, and the optimization methods and the obtained results are of great significance for its applications.

Concurrent OCT and OCT angiography of retinal neurovascular degeneration in the 5XFAD Alzheimer's disease mice

Significance: As one part of the central nervous system, the retina manifests neurovascular defects in Alzheimer's disease (AD). Quantitative imaging of retinal neurovascular abnormalities may promise a new method for early diagnosis and treatment assessment of AD. Previous imaging studies of transgenic AD mouse models have been limited to the central part of the retina. Given that the pathological hallmarks of AD frequently appear in different peripheral quadrants, a comprehensive regional investigation is needed for a better understanding of the retinal degeneration associated with AD-like pathology. Aim: We aim to demonstrate concurrent optical coherence tomography (OCT) and OCT angiography (OCTA) of retinal neuronal and vascular abnormalities in the 5XFAD mouse model and to investigate region-specific retinal degeneration. Approach: A custom-built OCT system was used for retinal imaging. Retinal thickness, vessel width, and vessel density were quantitatively measured. The artery and vein (AV) were classified for differential AV analysis, and trilaminar vascular plexuses were segmented for depth-resolved density measurement. Results: It was observed that inner and outer retinal thicknesses were explicitly reduced in the dorsal and temporal quadrants, respectively, in 5XFAD mice. A significant arterial narrowing in 5XFAD mice was also observed. Moreover, overall capillary density consistently showed a decreasing trend in 5XFAD mice, but regional specificity was not identified. Conclusions: Quadrant- and layer-specific neurovascular degeneration was observed in 5XFAD mice. Concurrent OCT and OCTA promise a noninvasive method for quantitative monitoring of AD progression and treatment assessment.

Loss of the K+ channel Kv2.1 greatly reduces outward dark current and causes ionic dysregulation and degeneration in rod photoreceptors

Vertebrate retinal photoreceptors signal light by suppressing a circulating "dark current" that maintains their relative depolarization in the dark. This dark current is composed of an inward current through CNG channels and NCKX transporters in the outer segment that is balanced by outward current exiting principally from the inner segment. It has been hypothesized that Kv2.1 channels carry a predominant fraction of the outward current in rods. We examined this hypothesis by comparing whole cell, suction electrode, and electroretinographic recordings from Kv2.1 knockout (Kv2.1-/-) and wild-type (WT) mouse rods. Single cell recordings revealed flash responses with unusual kinetics, and reduced dark currents that were quantitatively consistent with the measured depolarization of the membrane resting potential in the dark. A two-compartment (outer and inner segment) physiological model based on known ionic mechanisms revealed that the abnormal Kv2.1-/- rod photoresponses arise principally from the voltage dependencies of the known conductances and the NCKX exchanger, and a highly elevated fraction of inward current carried by Ca2+ through CNG channels due to the aberrant depolarization. Kv2.1-/- rods had shorter outer segments than WT and dysmorphic mitochondria in their inner segments. Optical coherence tomography of knockout animals demonstrated a slow photoreceptor degeneration over a period of 6 mo. Overall, these findings reveal that Kv2.1 channels carry 70-80% of the non-NKX outward dark current of the mouse rod, and that the depolarization caused by the loss of Kv2.1 results in elevated Ca2+ influx through CNG channels and elevated free intracellular Ca2+, leading to progressive degeneration.

Efficient implementation of the Shack-Hartmann centroid extraction for edge computing

Adaptive optics (AO) is an established technique to measure and compensate for optical aberrations. One of its key components is the wavefront sensor (WFS), which is typically a Shack-Hartmann sensor (SH) capturing an image related to the aberrated wavefront. We propose an efficient implementation of the SH-WFS centroid extraction algorithm, tailored for edge computing. In the edge-computing paradigm, the data are elaborated close to the source (i.e., at the edge) through low-power embedded architectures, in which CPU computing elements are combined with heterogeneous accelerators (e.g., GPUs, field-programmable gate arrays). Since the control loop latency must be minimized to compensate for the wavefront aberration temporal dynamics, we propose an optimized algorithm that takes advantage of the unified CPU/GPU memory of recent low-power embedded architectures. Experimental results show that the centroid extraction latency obtained over spot images up to 700×700 pixels wide is smaller than 2 ms. Therefore, our approach meets the temporal requirements of small- to medium-sized AO systems, which are equipped with deformable mirrors having tens of actuators.

Adaptive optics two-photon microscopy enables near-diffraction-limited and functional retinal imaging in vivo

In vivo fundus imaging offers non-invasive access to neuron structures and biochemical processes in the retina. However, optical aberrations of the eye degrade the imaging resolution and prevent visualization of subcellular retinal structures. We developed an adaptive optics two-photon excitation fluorescence microscopy (AO-TPEFM) system to correct ocular aberrations based on a nonlinear fluorescent guide star and achieved subcellular resolution for in vivo fluorescence imaging of the mouse retina. With accurate wavefront sensing and rapid aberration correction, AO-TPEFM permits structural and functional imaging of the mouse retina with submicron resolution. Specifically, simultaneous functional calcium imaging of neuronal somas and dendrites was demonstrated. Moreover, the time-lapse morphological alteration and dynamics of microglia were characterized in a mouse model of retinal disorder. In addition, precise laser axotomy was achieved, and degeneration of retinal nerve fibres was studied. This high-resolution AO-TPEFM is a promising tool for non-invasive retinal imaging and can facilitate the understanding of a variety of eye diseases as well as neurodegenerative disorders in the central nervous system.

Measurement of Diurnal Variation in Rod Outer Segment Length In Vivo in Mice With the OCT Optoretinogram

Purpose

To investigate diurnal variation in the length of mouse rod outer segments in vivo.

Methods

The lengths of rod inner and outer segments (RIS, ROS) of dark-adapted albino mice maintained on a 12-hour dark:12-hour light cycle with light onset 7 AM were measured at prescribed times (6:30 AM, 11 AM, 3:30 PM) during the diurnal cycle with optical coherence tomography (OCT), taking advantage of increased visibility, after a brief bleaching exposure, of the bands corresponding to RIS/ROS boundaries and ROS tips (ROST).

Results

Deconvolution of OCT depth profiles resolved two backscatter bands located 7.4 ± 0.1 and 10.8 ± 0.2 µm (mean ± SEM) proximal to Bruch's membrane (BrM). These bands were identified with histology as arising from the apical surface of RPE and ROST, respectively. The average length of dark-adapted ROS at 6:30 AM was 17.7 ± 0.8 µm. By 11 AM, the average ROS length had decreased by 10% to 15.9 ± 0.7 µm. After 11 AM, the ROS length increased steadily at an average rate of 0.12 µm/h, returning to baseline length by 23.5 hours in the cycle.

Conclusions

The diurnal variation in ROS length measured in these experiments is consistent with prior histological investigations showing that rodent rod discs are phagocytosed by the RPE maximally over several hours around the time of normal light onset. The rate of recovery of ROS to baseline length before normal light onset is consistent with the hypothesis that disc membrane synthesis is fairly constant over the diurnal cycle.

Effects of intravitreal injection of human CD34+ bone marrow stem cells in a murine model of diabetic retinopathy

Human CD34 ⁺ stem cells are mobilized from bone marrow to sites of tissue ischemia and play an important role in tissue revascularization. This study used a murine model to test the hypothesis that intravitreal injection of human CD34 ⁺ stem cells harvested from bone marrow (BMSCs) can have protective effects in eyes with diabetic retinopathy. Streptozotocin-induced diabetic mice (C57BL/6J) were used as a model for diabetic retinopathy. Subcutaneous implantation of Alzet pump, loaded with Tacrolimus and Rapamycin, 5 days prior to intravitreal injection provided continuous systemic immunosuppression for the study duration to avoid rejection of human cells. Human CD34 ⁺ BMSCs were harvested from the mononuclear cell fraction of bone marrow from a healthy donor using magnetic beads. The CD34 ⁺ cells were labeled with enhanced green fluorescent protein (EGFP) using a lentiviral vector. The right eye of each mouse received an intravitreal injection of 50,000 EGFP-labeled CD34 ⁺ BMSCs or phosphate buffered saline (PBS). Simultaneous multimodal in vivo retinal imaging system consisting of fluorescent scanning laser ophthalmoscopy (enabling fluorescein angiography), optical coherence tomography (OCT) and OCT angiography was used to confirm the development of diabetic retinopathy and study the in vivo migration of the EGFP-labeled CD34 ⁺ BMSCs in the vitreous and retina following intravitreal injection. After imaging, the mice were euthanized, and the eyes were removed for immunohistochemistry. In addition, microarray analysis of the retina and retinal flat mount analysis of retinal vasculature were performed. The development of retinal microvascular changes consistent with diabetic retinopathy was visualized using fluorescein angiography and OCT angiography between 5 and 6 months after induction of diabetes in all diabetic mice. These retinal microvascular changes include areas of capillary nonperfusion and late leakage of fluorescein dye. Multimodal in vivo imaging and immunohistochemistry identified EGFP-labeled cells in the superficial retina and along retinal vasculature at 1 and 4 weeks following intravitreal cell injection. Microarray analysis showed changes in expression of 162 murine retinal genes following intravitreal CD34 ⁺ BMSC injection when compared to PBS-injected control. The major molecular pathways affected by intravitreal CD34 ⁺ BMSC injection in the murine retina included pathways implicated in the pathogenesis of diabetic retinopathy including Toll-like receptor, MAP kinase, oxidative stress, cellular development, assembly and organization pathways. At 4 weeks following intravitreal injection, retinal flat mount analysis showed preservation of the retinal vasculature in eyes injected with CD34 ⁺ BMSCs when compared to PBS-injected control. The study findings support the hypothesis that intravitreal injection of human CD34 ⁺ BMSCs results in retinal homing and integration of these human cells with preservation of the retinal vasculature in murine eyes with diabetic retinopathy.

Temporal speckle-averaging of optical coherence tomography volumes for in-vivo cellular resolution neuronal and vascular retinal imaging

It has been recently demonstrated that structures corresponding to the cell bodies of highly transparent cells in the retinal ganglion cell layer could be visualized noninvasively in the living human eye by optical coherence tomography (OCT) via temporal averaging. Inspired by this development, we explored the application of volumetric temporal averaging in mice, which are important models for studying human retinal diseases and therapeutic interventions. A general framework of temporal speckle-averaging (TSA) of OCT and optical coherence tomography angiography (OCTA) is presented and applied to mouse retinal volumetric data. Based on the image analysis, the eyes of mice under anesthesia exhibit only minor motions, corresponding to lateral displacements of a few micrometers and rotations of a fraction of 1 deg. Moreover, due to reduced eye movements under anesthesia, there is a negligible amount of motion artifacts within the volumes that need to be corrected to achieve volume coregistration. In addition, the relatively good optical quality of the mouse ocular media allows for cellular-resolution imaging without adaptive optics (AO), greatly simplifying the experimental system, making the proposed framework feasible for large studies. The TSA OCT and TSA OCTA results provide rich information about new structures previously not visualized in living mice with non-AO-OCT. The mechanism of TSA relies on improving signal-to-noise ratio as well as efficient suppression of speckle contrast due to temporal decorrelation of the speckle patterns, enabling full utilization of the high volumetric resolution offered by OCT and OCTA.

Non-invasive cellular-resolution retinal imaging with two-photon excited fluorescence

Two-photon excited fluorescence (TPEF) imaging of the retina is a developing technique that provides non-invasive compound-specific measurements from the retina. In this report, we demonstrate high-resolution TPEF imaging of the mouse retina using sensorless adaptive optics (SAO) and optical coherence tomography (OCT). A single near-infrared light source was used for simultaneous multi-modal imaging with OCT and TPEF. The image-based SAO could be performed using the en face OCT or the TPEF for aberration correction. Our results demonstrate OCT and TPEF for angiography. Also, we demonstrate non-invasive cellular-resolution imaging of fluorescently labelled cells and the Retinal Pigment Epithelium (RPE) mosaic.

Adaptive optics in the mouse eye: wavefront sensing based vs. image-guided aberration correction

Adaptive Optics (AO) is required to achieve diffraction limited resolution in many real-life imaging applications in biology and medicine. AO is essential to guarantee high fidelity visualization of cellular structures for retinal imaging by correcting ocular aberrations. Aberration correction for mouse retinal imaging by direct wavefront measurement has been demonstrated with great success. However, for mouse eyes, the performance of the wavefront sensor (WFS) based AO can be limited by several factors including non-common path errors, wavefront reconstruction errors, and an ill-defined reference plane. Image-based AO can avoid these issues at the cost of algorithmic execution time. Furthermore, image-based approaches can provide improvements to compactness, accessibility, and even the performance of AO systems. Here, we demonstrate the ability of image-based AO to provide comparable aberration correction and image resolution to the conventional Shack-Hartmann WFS-based AO approach. The residual wavefront error of the mouse eye was monitored during a wavefront sensorless optimization to allow comparison with classical AO. This also allowed us to improve the performance of our AO system for small animal retinal imaging.

Cataract-preventing contact lens for in vivo imaging of mouse retina

In vivo imaging of mouse retinas using two-photon or confocal laser scanning fluorescence microscopy is a powerful tool for time-lapse analyses of the dynamic movements of cell populations. However, acute and reversible opacification of the crystalline lenses of mouse eyes under anesthesia decreases the visibility of the ocular fundus. Therefore, we developed a customized contact lens for preventing cataract during continuous retinal imaging in anesthetized mice. This experimental approach will aid in the elucidation of cellular and molecular dynamics in the CNS under physiological and pathological conditions.

In vivo imaging reveals transient microglia recruitment and functional recovery of photoreceptor signaling after injury

Microglia, the resident macrophages of the central nervous system, are critical for synaptic pruning and maintenance and for mitigating injury and neurodegeneration. Determining whether microglia–neuron interactions are beneficial in specific instances has been difficult, largely because of the local and transient nature of the interactions. Using simultaneous optical coherence tomography/scanning laser ophthalmoscopy (SLO) and adaptive optics SLO retinal imaging in mice, we show interactions of microglia and photoreceptors over time scales from seconds to months during injury, degeneration, and repair. In vivo optical assessment of photoreceptor signaling in a large neuronal field encompassing the injured area allows us to relate the time course of these microglia movements to that of the tissue remodeling and functional recovery.

Microglia respond to damage and microenvironmental changes within the central nervous system by morphologically transforming and migrating to the lesion, but the real-time behavior of populations of these resident immune cells and the neurons they support have seldom been observed simultaneously. Here, we have used in vivo high-resolution optical coherence tomography (OCT) and scanning laser ophthalmoscopy with and without adaptive optics to quantify the 3D distribution and dynamics of microglia in the living retina before and after local damage to photoreceptors. Following photoreceptor injury, microglia migrated both laterally and vertically through the retina over many hours, forming a tight cluster within the area of visible damage that resolved over 2 wk. In vivo OCT optophysiological assessment revealed that the photoreceptors occupying the damaged region lost all light-driven signaling during the period of microglia recruitment. Remarkably, photoreceptors recovered function to near-baseline levels after the microglia had departed the injury locus. These results demonstrate the spatiotemporal dynamics of microglia engagement and restoration of neuronal function during tissue remodeling and highlight the need for mechanistic studies that consider the temporal and structural dynamics of neuron–microglia interactions in vivo.

A Novel Reporter Mouse Uncovers Endogenous Brn3b Expression

Brn3b (Pou4f2) is a class-4 POU domain transcription factor known to play central roles in the development of different neuronal populations of the Central Nervous System, including retinal ganglion cells (RGCs), the neurons that connect the retina with the visual centers of the brain. Here, we have used CRISPR-based genetic engineering to generate a Brn3b-mCherry reporter mouse without altering the endogenous expression of Brn3b. In our mouse line, mCherry faithfully recapitulates normal Brn3b expression in the retina, the optic tracts, the midbrain tectum, and the trigeminal ganglia. The high sensitivity of mCherry also revealed novel expression of Brn3b in the neuroectodermal cells of the optic stalk during early stages of eye development. Importantly, the fluorescent intensity of Brn3b-mCherry in our reporter mice allows for noninvasive live imaging of RGCs using Scanning Laser Ophthalmoscopy (SLO), providing a novel tool for longitudinal monitoring of RGCs.

Scattering-Angle-Resolved Optical Coherence Tomography of a Hypoxic Mouse Retina Model

Several studies have noted a correlation between retinal degeneration and traumatic encephalopathy (TE) making the retina a leading candidate for detection and assessment. Scattering-angle-resolved optical coherence tomography (SAR-OCT) is a candidate imaging modality to detect sub-resolution changes in retinal microstructure. SAR-OCT images of murine retinas that experience a hypoxic insult-euthanasia by isoflurane overdose-are presented. A total of 4 SAR-OCT measurement parameters are reported in 6 longitudinal experiments: blood flow volume fraction, total retinal thickness, reflectance index, and scattering angle. As each mouse expires, blood flow volume fraction decreases, total retinal thickness increases, reflectance index decreases, and scattering angle diversity increases. Contribution of the retinal vasculature to scattering angle diversity is discussed. Results of this study suggest the utility of SAR-OCT to measure TE using scattering angle diversity contrast in the retina.

Aperture phase modulation with adaptive optics: a novel approach for speckle reduction and structure extraction in optical coherence tomography

Speckle is an inevitable consequence of the use of coherent light in imaging and acts as noise that corrupts image formation in most applications. Optical coherence tomographic imaging, as a technique employing coherence time gating, suffers from speckle. We present here a novel method of suppressing speckle noise intrinsically compatible with adaptive optics (AO) for confocal coherent imaging: modulation of the phase in the system pupil aperture with a segmented deformable mirror (DM) to introduce minor perturbations in the point spread function. This approach creates uncorrelated speckle patterns in a series of images, enabling averaging to suppress speckle noise while maintaining structural detail. A method is presented that efficiently determines the optimal range of modulation of DM segments relative to their AO-optimized position so that speckle noise is reduced while image resolution and signal strength are preserved. The method is active and independent of sample properties. Its effectiveness and efficiency are quantified and demonstrated by both ex vivo non-biological and in vivo biological applications.

Sensorless adaptive optics multimodal en-face small animal retinal imaging

Vision researchers often use small animals due to the availability of many transgenic strains that model human diseases or express biomarkers. Adaptive optics (AO) enables non-invasive single-cell imaging in a living animal but often results in high system complexity. Sensorless AO (SAO) can provide depth-resolved aberration correction with low system complexity. We present a multi-modal sensorless AO en face retina imaging system that includes optical coherence tomography (OCT), OCT-angiography, confocal scanning laser ophthalmoscopy (SLO), and fluorescence detection. We present a compact lens-based imaging system design that allows for a 50-degree maximum field of view (FOV), which can be reduced to the region of interest to perform SAO with the modality of choice. The system performance was demonstrated on wild type mice (C57BL/6J), and transgenic mice with GFP labeled cells. SAO SLO was used for imaging microglia (Cx3cr1-GFP) over ~1 hour, where dynamics of the microglia branches were clearly observed. Our results also include volumetric cellular imaging of microglia throughout the inner retina.

Design Considerations for Murine Retinal Imaging Using Scattering Angle Resolved Optical Coherence Tomography

Optical coherence tomography (OCT), an optical imaging approach enabling cross-sectional analysis of turbid samples, is routinely used for retinal imaging in human and animal models of diseases affecting the retina. Scattering angle resolved (SAR-)OCT has previously been demonstrated as offering additional contrast in human studies, but no SAR-OCT system has been reported in detail for imaging the retinas of mice. An optical model of a mouse eye was designed and extended for validity at wavelengths of light around 1310 nm; this model was then utilized to develop a SAR-OCT design for murine retinal imaging. A Monte Carlo technique simulates light scattering from the retina, and the simulation results are confirmed with SAR-OCT images. Various images from the SAR-OCT system are presented and utility of the system is described. SAR-OCT is demonstrated as a viable and robust imaging platform to extend utility of retinal OCT imaging by incorporating scattering data into investigative ophthalmologic analysis.