Alireza Akhlagh Moayed

Swelling of the Human Cornea Revealed by High-Speed, Ultrahigh-Resolution Optical Coherence Tomography

PURPOSE To evaluate the change in thickness of the anterior, stromal, and posterior corneal laminae in response to hypoxia-induced corneal swelling, by means of ultrahigh-resolution optical coherence tomography (UHR-OCT). METHODS A UHR-OCT system, operating in the 1060-nm range, was used to acquire in vivo cross-sectional images of human cornea with a 3.2x10-microm (axial x lateral) resolution in corneal tissue. Corneal edema was induced by inserting a thick, positive-powered, soft contact lens, over which the eye was closed and patched for 3 hours. Tomograms were acquired from eight non-contact-lens wearers. Baseline images were obtained before inducing corneal edema, immediately after removal of the patch and the lens, and then every 15 minutes for approximately 2 hours. All images were postprocessed with a segmentation algorithm to identify the laminae visible in the image. The apical thickness of the laminae (epithelium [EPI], epithelial-Bowmans membrane [Ep-BM] complex, stroma, and endothelial-Descemets membrane [En-DM] complex) were determined at each time interval. RESULTS There was an interaction between time after removal of the hypoxic stimulus and deswelling of the layers (RM-ANOVA; P<0.001). The epithelial and stromal thickness reduced significantly with time (P=0.001; P<0.001, respectively), whereas the Ep-BM and En-DM complexes did not (P>0.50). All layers except the En-DM complex exhibited a biphasic pattern of recovery. CONCLUSIONS UHR-OCT showed regional differences in swelling due to hypoxic provocation. On removal of the hypoxic stimulus, the rate of recovery varied between layers, and all layers except the En-DM complex exhibited a biphasic recovery.

Limiting factors to the OCT axial resolution for in-vivo imaging of human and rodent retina in the 1060nm wavelength range

A computational model was developed to evaluate the limitations to the highest axial resolution, achievable with ultrahigh resolution optical coherence tomography (UHROCT) in the 1060 nm wavelength region for in-vivo imaging of the human and rodent retina. The model considers parameters such as the wavelength dependent water absorption, the average length of the human and rodent eyes, and the power limitations for the imaging beam as defined in the ANSI standard. A custom-built light source with re-shaped spectrum was used to verify experimentally the results from the computational model. Axial OCT resolution of 4.2 microm and 7.7 microm was measured from a mirror reflection with the custom light source by propagating the imaging beam through water cells with 5 mm and 25 mm thickness, corresponding to the average axial length of the rodent and human eye respectively. Assuming an average refractive index of 1.38 for retinal tissue, the expected axial OCT resolution in the rodent and human retina is 3 microm and 5.7 microm respectively. Retinal tomograms acquired in-vivo from the rat eye with the modified light source show clear visualization of all intraretinal layers, as well as a network of capillaries (approximately 10 microm in diameter) in the inner- and outer plexiform layers of the retina.

In vivo volumetric imaging of the human corneo-scleral limbus with spectral domain OCT

The limbus is the structurally rich transitional region of tissue between the cornea on one side, and the sclera and conjunctiva on the other. This zone, among other things, contains nerves passing to the cornea, blood and lymph vasculature for oxygen and nutrient delivery and for waste, CO2 removal and drainage of the aqueous humour. In addition, the limbus contains stem cells responsible for the existence and healing of the corneal epithelium. Here we present 3D images of the healthy human limbus, acquired in vivo with a spectral domain optical coherence tomography system operating at 1060nm. Cross-sectional and volumetric images were acquired from temporal and nasal locations in the human limbus with ~3µm x 18µm (axial x lateral) resolution in biological tissue at the rate of 92,000 A-scans/s. The imaging enabled detailed mapping of the corneo-scleral tissue morphology, and visualization of structural details such as the Vogt palisades, the blood and lymph vasculature including the Schlemm’s canal and the trabecular meshwork, as well as corneal nerve fiber bundles. Non-invasive, volumetric, high resolution imaging reveals fine details of the normal human limbal structure, and promises to provide invaluable information about its changes in health and disease as well as during and after corneal surgery.

In vivo imaging of intrinsic optical signals in chicken retina with functional optical coherence tomography.

Visually evoked intrinsic optical signals (IOSs) were measured in vivo for the first time to our knowledge from all retina layers of the chicken retina with a combined functional optical coherence tomography and electroretinography (ERG) system. IOS traces were recorded from a small volume in the retina with 3.5 μm axial resolution and 7 ms time resolution. Comparison of the IOS and ERG traces shows a correlation between the positive and negative IOS measured from different retinal layers and the timing of the a and b waves in the ERG recording.

In Vivo Assessment of Thickness and Reflectivity in a Rat Outer Retinal Degeneration Model with Ultrahigh Resolution Optical Coherence Tomography

PURPOSE To provide in vivo quantitative assessment of sodium iodate-induced retinal damage in a rat model of outer retinal degeneration using ultrahigh resolution optical coherence tomography (UHR-OCT). METHODS Outer retinal degeneration was induced in four female Long Evans rats via tail vein injection of sodium iodate (40 mg/kg). Changes in the thickness and optical reflectivity of individual retinal layers were extracted using a semi-automatic segmentation algorithm and were assessed in vivo at 6 hours, days 1, 3, and 7, and up to 3 months post injection with UHR-OCT. Hematoxylin and eosin (H&E) histology was used to confirm the morphologic changes observed in the UHR-OCT images. RESULTS UHR-OCT tomograms showed progressive structural damage in the rat retina over time, such as swelling, thinning, complete disintegration of individual retinal layers, and clustering of highly reflective cellular debris. Photoreceptor swelling was observed 6 hours after injection of sodium iodate, followed by progressive structural decomposition of the outer retina. At 3 months post injection, the outer retina was completely disintegrated, and the inner nuclear layer (INL) was in direct contact with the choroid. Changes in the reflectivity of individual retinal layers were observed over time and correlated well with the morphologic changes. CONCLUSIONS UHR-OCT permits in vivo, noninvasive, longitudinal, quantitative assessment of the progressive changes in retinal morphology and optical reflectivity in a sodium iodate rodent model of outer retinal degeneration.

In vivo volumetric imaging of chicken retina with ultrahigh-resolution spectral domain optical coherence tomography

The chicken retina is an established animal model for myopia and light-associated growth studies. It has a unique morphology: it is afoveate and avascular; oxygen and nutrition to the inner retina is delivered by a vascular tissue (pecten) that protrudes into the vitreous. Here we present, to the best of our knowledge, the first in vivo, volumetric high-resolution images of the chicken retina. Images were acquired with an ultrahigh-resolution optical coherence tomography (UHROCT) system with 3.5 µm axial resolution in the retina, at the rate of 47,000 A-scans/s. Spatial variations in the thickness of the nerve fiber and ganglion cell layers were mapped by segmenting and measuring the layer thickness with a semi-automatic segmentation algorithm. Volumetric visualization of the morphology and morphometric analysis of the chicken retina could aid significantly studies with chicken retinal models of ophthalmic diseases.

Noninvasive imaging of the early effect of sodium iodate toxicity in a rat model of outer retina degeneration with spectral domain optical coherence tomography

Abstract. An ultrahigh resolution spectral domain optical coherence tomography (SD-OCT) system is used to observe for the first time in vivo the early effect of sodium iodate (NaIO3) toxicity on retinal morphology. Retinal degeneration is induced in rats via tail vein injection of NaIO3 and structural changes in the outer retina are assessed longitudinally at baseline and 1, 2, 3, 6, 8, and 10 h, and 12 post drug administration with OCT, H&E histology, and IgG immunochemistry. Disruption of the structural integrity and changes in the optical reflectivity of the photoreceptor inner (IS) and outer segment (OS) layers are observed as early as 1 h post NaIO3 injection. A new layer is observed in the OCT tomograms to form between the retinal pigmented epithelium and the photoreceptors OS a few hours post NaIO3 injection. The dynamics and the low optical reflectivity of this layer, as well as cell swelling and disruption of the blood-retina barrier observed in the histological and immunohistochemistry cross-sections suggest that the layer corresponds to temporary fluid accumulation in the retina. Results from this study demonstrate the effectiveness of OCT technology for monitoring dynamic changes in the retinal morphology and provide better understanding of the early stages of outer retina degeneration induced by NaIO3 toxicity.

Correlation of visually evoked intrinsic optical signals and electroretinograms recorded from chicken retina with a combined functional optical coherence tomography and electroretinography system

Visually evoked fast intrinsic optical signals (IOSs) were recorded for the first time in vivo from all layers of healthy chicken retina by using a combined functional optical coherence tomography (fOCT) and electroretinography (ERG) system. The fast IOSs were observed to develop within ∼5 ms from the on-set of the visual stimulus, whereas slow IOSs were measured up to 1 s later. The visually evoked IOSs and ERG traces were recorded simultaneously, and a clear correlation was observed between them. The ability to measure visually evoked fast IOSs non-invasively and in vivo from individual retinal layers could significantly improve the understanding of the complex communication between different retinal cell types in healthy and diseased retinas.

Combined optical coherence tomography and electroretinography system for in vivo simultaneous morphological and functional imaging of the rodent retina.

A combined ultrahigh resolution optical coherence tomography (UHROCT) and a electroretinography (ERG) system is presented for simultaneous imaging of the retinal structure and physiological response to light stimulation in the rodent eye. The 1060-nm UHROCT system provides approximately 3x5 microm (axialxlateral) resolution in the rat retina and time resolution of 22 micros. A custom-designed light stimulator integrated into the UHROCT imaging probe provides light stimuli with user-selected color, duration, and intensity. The performance of the combined system is demonstrated in vivo in healthy rats, and in a rat model of drug-induced outer retinal degeneration. Experimental results show correlation between the observed structural and physiological changes in the healthy and degenerated retina.

In-vivo human retina imaging with 5μm axial resolution, at 92000 A-scans/s with 1μm spectral domain OCT system

We have outfitted a 1060nm Spectral Domain Optical Coherence Tomography system with a prototype, high speed infrared linear array camera and a custom spectrally reshaped superluminescent diode to achieve 5μm axial resolution at 91,911 A-scans/s image acquisition rate in-vivo in the human retina. 4dB loss of sensitivity was observed as a result of the reduced integration time (7μs) of the fast camera as compared to similar commercially available cameras with 14μs integration time and 47kHz readout rate. Fewer motion artefacts were observed in the retinal images acquired with the fast camera, while the higher axial resolution along with deeper penetration allowed for improved visualization of fine morphological details such as retinal and choroidal capillaries and the deep choroidal structure.