Confocal microscopes unlike their conventional counterparts have the ability to optically ‘section’ thick specimens. However the resolution and optical sectioning can be severely degraded by system or specimen-induced aberrations. The use of high aperture lenses further exacerbates the difficulties. We will describe an adaptive optics solution to this fundamental problem.

Adaptive optics in microscopy

The human retina is a uniquely accessible tissue. Tools like scanning laser ophthalmoscopy and spectral domain-optical coherence tomography provide clinicians with remarkably clear pictures of the living retina. Although the anterior optics of the eye permit such non-invasive visualization of the retina and associated pathology, the same optics induce significant aberrations that obviate cellular-resolution imaging in most cases. Adaptive optics (AO) imaging systems use active optical elements to compensate for aberrations in the optical path between the object and the camera. When applied to the human eye, AO allows direct visualization of individual rod and cone photoreceptor cells, retinal pigment epithelium cells, and white blood cells. AO imaging has changed the way vision scientists and ophthalmologists see the retina, helping to clarify our understanding of retinal structure, function, and the etiology of various retinal pathologies. Here, we review some of the advances that were made possible with AO imaging of the human retina and discuss applications and future prospects for clinical imaging.

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Adaptive Optics Retinal Imaging: Emerging Clinical Applications

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.

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Adaptive optics ophthalmoscopy

Adaptive optics imaging of the human retina.

https://escholarship.org/content/qt43k6z8gp/qt43k6z8gp.pdf?t=onkgvd

Vision science and adaptive optics, the state of the field.

Wavefront sensor noise and fidelity place a fundamental limit on achievable image quality in current adaptive optics ophthalmoscopes. Additionally, the wavefront sensor ‘beacon’ can interfere with visual experiments. We demonstrate real-time (25 Hz), wavefront sensorless adaptive optics imaging in the living human eye with image quality rivaling that of wavefront sensor based control in the same system. A stochastic parallel gradient descent algorithm directly optimized the mean intensity in retinal image frames acquired with a confocal adaptive optics scanning laser ophthalmoscope (AOSLO). When imaging through natural, undilated pupils, both control methods resulted in comparable mean image intensities. However, when imaging through dilated pupils, image intensity was generally higher following wavefront sensor-based control. Despite the typically reduced intensity, image contrast was higher, on average, with sensorless control. Wavefront sensorless control is a viable option for imaging the living human eye and future refinements of this technique may result in even greater optical gains.

Wavefront sensorless adaptive optics ophthalmoscopy in the human eye

Glaucoma is a multifaceted group of eye diseases that results in the degeneration of retinal ganglion cell axons and the death of retinal ganglion cells (RGCs). The mechanisms of glaucomatous damage (e.g., mechanical, vascular, and glial) are not fully understood. Although several of these factors likely contribute to RGC death, substantial evidence suggests that the initial site of axonal injury likely occurs at the level of the lamina cribrosa in the optic nerve head (ONH).1–11 In humans and nonhuman primates, the lamina cribrosa is a three-dimensional porous structure consisting of flexible beams of collagenous tissue that support and nourish the RGC axons passing through it from the retina to the brain.12 Increases in intraocular pressure (IOP) impart stress and strain on the lamina,13–15 resulting in a posterior bowing and stretching of the load-bearing laminar beams12,16 and potential increases in laminar pore area and elongation (or more elliptically shaped pores). Stretching and deformation of the laminar beams (and associated pores) could sheer or damage encompassed axons and laminar capillaries, thereby hampering axonal transport, blood flow and the diffusion of nutrients,17 and/or the neurotrophic support provided to the axons by alterations in glial cells.18

ONH tissues have been examined in human glaucomatous eyes and nonhuman primate models of experimental glaucoma. Postmortem histologic studies have shown significant changes in prelaminar and laminar tissues within the ONH in early experimental glaucoma.16,19,20 Additional ex vivo studies in glaucomatous human and nonhuman primate eyes have described early changes in laminar morphology and position,16,21 laminar pore geometry,22 and the composition and architecture of laminar connective tissues.20,23 In light of this work, there is a growing consensus that in vivo studies are necessary to validate ex vivo results and examine longitudinal changes during glaucoma.24,25

The lamina cribrosa has been examined in vivo. Fontana et al.26 imaged anterior laminar pores in living human glaucomatous eyes using a modified confocal scanning laser ophthalmoscope (SLO) and showed that pore area and elongation were larger in eyes with more severe glaucoma. However, images were taken using a wide field of view (20°) and were not of high resolution because of the presence of ocular aberrations. Therefore, the size and total number of pores that could be resolved and the number of subjects in which laminar pores could be successfully imaged were limited. Vilupuru et al.27 used a confocal adaptive optics scanning laser ophthalmoscope (AOSLO), an instrument that noninvasively provides cellular-level images through a correction of the eye's aberrations, to improve the resolution and contrast of laminar images in vivo in nonhuman primates with experimental glaucoma. This study quantified laminar pore geometries in three diseased monkey eyes, each at a single time point. However, changes in pore geometry with disease were not investigated. Recent studies have combined adaptive optics with optical coherence tomography (OCT) to image the human lamina cribrosa in three spatial dimensions,28,29 but a quantitative analysis of laminar geometry and pore structure has not been reported.

We have assessed the repeatability of imaging and quantifying laminar pores in vivo in two normal macaque and three normal human eyes using an AOSLO at different time points. Statistical tests were performed to investigate whether significant differences were present in any analyzed laminar pore parameters within and across imaging sessions. This study provides an important first step in understanding the feasibility of using in vivo AOSLO imaging to assess laminar structure in normal eyes, human glaucomatous eyes, and nonhuman primate eyes with experimentally induced glaucoma.

/pdf/reproducibility-of-measuring-lamina-cribrosa-pore-geometry-1ds4hq405x.pdf

Reproducibility of measuring lamina cribrosa pore geometry in human and nonhuman primates with in vivo adaptive optics imaging.

We present a direct slope-based correction algorithm to simultaneously control two deformable mirrors (DMs) in a woofer-tweeter adaptive optics system. A global response matrix was derived from the response matrices of each deformable mirror and the voltages for both deformable mirrors were calculated simultaneously. This control algorithm was tested and compared with a 2-step sequential control method in five normal human eyes using an adaptive optics scanning laser ophthalmoscope. The mean residual total root-mean-square (RMS) wavefront errors across subjects after adaptive optics (AO) correction were 0.128 ± 0.025 μm and 0.107 ± 0.033 μm for simultaneous and 2-step control, respectively (7.75-mm pupil). The mean intensity of reflectance images acquired after AO convergence was slightly higher for 2-step control. Radially-averaged power spectra calculated from registered reflectance images were nearly identical for all subjects using simultaneous or 2-step control. The correction performance of our new simultaneous dual DM control algorithm is comparable to 2-step control, but is more efficient. This method can be applied to any woofer-tweeter AO system.

A correction algorithm to simultaneously control dual deformable mirrors in a woofer-tweeter adaptive optics system

Embodiments of the invention generally provide apparatuses and methods utilized in optics, and more specifically to apparatuses and methods for adaptive optics correction and imaging. Real-time wavefront sensorless adaptive optics correction and imaging is used with the living human eye to produce optical quality rivaling that of wavefront sensor based control in the similar systems. Using an optimization algorithm that is based on an image quality metric, the apparatus and method optimize the optical quality in ocular image frames acquired with an adaptive optics system.

Chaohong Li

Papers

Wavefront sensorless adaptive optics ophthalmoscopy in the human eye

Reproducibility of measuring lamina cribrosa pore geometry in human and nonhuman primates with in vivo adaptive optics imaging.

A correction algorithm to simultaneously control dual deformable mirrors in a woofer-tweeter adaptive optics system

Wavefront sensorless adaptive correction of the wave aberration for an eye

Wavefront sensorless confocal adaptive optics scanning laser ophthalmoscopy in the human eye