Three-dimensional adaptive optics ultrahigh-resolution optical coherence tomography using a liquid crystal spatial light modulator

A liquid crystal programmable phase modulator (PPM) is used as correcting device in an adaptive optics system for three-dimensional ultrahigh-resolution optical coherence tomography (UHR OCT). The feasibility of the PPM to correct high order aberrations even when using polychromatic light is studied, showing potential for future clinical use. Volumetric UHR OCT of the living retina, obtained with up 25,000A-scans/s and high resolution enables visualization of retinal features that might correspond to groups of terminal bars of photoreceptors at the external limiting membrane.     

Adaptive optics imaging of the retinal microvasculature

The eye has long been recognised as the window to pathological processes occurring in the brain and other organs. By imaging the vasculature of the retina we have improved the scientific understanding and clinical best practice for a diverse range of conditions, ranging from diabetes, to stroke, to dementia. Mounting evidence suggests that damage to the smallest and most delicate vessels in the body, the capillaries, is the first sign in many vasculopathies. These are the most critical vessels involved in the exchange of metabolites with tissue. Accurate assessment of retinal capillary structure and function would therefore be of great benefit across a broad range of disciplines in medical science; however, their small size does not make this an easy task. This has led to the development of high-resolution adaptive optics imaging methods to non-invasively explore retinal microvascular networks in living human eyes. This review describes the present state of the art in the field, the scientific breakthroughs that have been made possible in the understanding of vessel structure and function in health and disease, and future directions for this emerging technology.     

Adaptive optics scanning laser ophthalmoscopy in fundus imaging, a review and update

Adaptive optics scanning laser ophthalmoscopy (AO-SLO) has been a promising technique in funds imaging with growing popularity. This review firstly gives a brief history of adaptive optics (AO) and AO-SLO. Then it compares AO-SLO with conventional imaging methods (fundus fluorescein angiography, fundus autofluorescence, indocyanine green angiography and optical coherence tomography) and other AO techniques (adaptive optics flood-illumination ophthalmoscopy and adaptive optics optical coherence tomography). Furthermore, an update of current research situation in AO-SLO is made based on different fundus structures as photoreceptors (cones and rods), fundus vessels, retinal pigment epithelium layer, retinal nerve fiber layer, ganglion cell layer and lamina cribrosa. Finally, this review indicates possible research directions of AO-SLO in future.     

In Vivo High-Resolution Retinal Imaging Using Adaptive Optics

Retinal imaging with conventional methods is only able to overcome the lowest order of aberration, defocus and astigmatism. The human eye is fraught with higher order of aberrations. Since we are forced to use the human optical system in retinal imaging, the images are degraded. In addition, all of these distortions are constantly changing due to head/eye movement and change in accommodation. Adaptive optics is a promising technology introduced in the field of ophthalmology to measure and compensate for these aberrations. High-resolution obtained by adaptive optics enables us to view and image the retinal photoreceptors, retina pigment epithelium, and identification of cone subclasses in vivo. In this review we will be discussing the basic technology of adaptive optics and hardware requirement in addition to clinical applications of such technology.     

Adaptive optics and the eye (super resolution OCT)

The combination of adaptive optics (AO) and optical coherence tomography (OCT) was first reported 8 years ago and has undergone tremendous technological advances since then. The technical benefits of adding AO to OCT (increased lateral resolution, smaller speckle, and enhanced sensitivity) increase the imaging capability of OCT in ways that make it well suited for three-dimensional (3D) cellular imaging in the retina. Today, AO-OCT systems provide ultrahigh 3D resolution (3 × 3 × 3 μm3) and ultrahigh speed (up to an order of magnitude faster than commercial OCT). AO-OCT systems have been used to capture volume images of retinal structures, previously only visible with histology, and are being used for studying clinical conditions. Here, we present representative examples of cellular structures that can be visualized with AO-OCT. We overview three studies from our laboratory that used ultrahigh-resolution AO-OCT to measure the cross-sectional profiles of individual bundles in the retinal nerve fiber layer; the diameters of foveal capillaries that define the terminal rim of the foveal avascular zone; and the spacing and length of individual cone photoreceptor outer segments as close as 0.5° from the fovea center.     

Axial length and cone density as assessed with adaptive optics in myopia

Aim:

To assess the variations in cone mosaic in myopia and its correlation with axial length (AL).

Subjects and Methods:

Twenty-five healthy myopic volunteers underwent assessment of photoreceptors using adaptive optics retinal camera at 2° and 3° from the foveal center in four quadrants superior, inferior, temporal and nasal. Data was analyzed using SPSS version 17 (IBM). Multivariable regression analysis was conducted to study the relation between cone density and AL, quadrant around the fovea and eccentricity from the fovea.

Results:

The mean cone density was significantly lower as the eccentricity increased from 2° from the fovea to 3° (18,560 ± 5455–16,404 ± 4494/mm² respectively). There was also a statistically significant difference between four quadrants around the fovea. The correlation of cone density and spacing with AL showed that there was a significant inverse relation of AL with the cone density.

Conclusion:

In myopic patients with good visual acuity cone density around the fovea depends on the quadrant, distance from the fovea as well as the AL. The strength of the relation of AL with cone density depends on the quadrant and distance.     

Spectral imaging of the retina

Introduction

The work described here involved the use of a modified fundus camera to obtain sequential hyperspectral images of the retina in 14 normal volunteers and in 1 illustrative patient with a retinal vascular occlusion.

Methods

The paper describes analysis techniques, which allow oximetry within retinal vessels; these results are presented as retinal oximetry maps.

Results

Using spectral images, with wavelengths between 556 and 650 nm, the mean oxygen saturation (OS) value in temporal retinal arterioles in normal volunteers was 104.3 (±16.7), and in normal temporal retinal venules was 34.8 (±17.8). These values are comparable to those quoted in the literature, although, the venular saturations are slightly lower than those values found by other authors; explanations are offered for these differences.

Discussion

The described imaging and analysis techniques produce a clinically useful map of retinal oximetric values. The results from normal volunteers and from one illustrative patient are presented. Further developments, including the recent development of a ‘snapshot'' spectral camera, promises enhanced non-invasive retinal vessel oximetry mapping.     

基于自适应光学的视网膜成像技术应用进展

自适应光学(AO)是一种通过减少波前畸变来降低光学像差影响,从而改进光学系统性能的技术。基于AO的视网膜成像技术减少了屈光系统光学像差的产生,从而提高了视网膜成像的分辨率和质量,这种活体视网膜细胞水平的成像技术,具有巨大的眼科应用潜力。AO与眼底相机、扫描激光检眼镜、光学相干断层扫描技术及光学相干断层血管造影结合,可以应用于健康人群眼底视锥细胞分布情况、形态特征和功能作用观察,并在更精细的血管层面了解视网膜血管形态和灌注情况; 可以对病变眼底中细胞数量和形态、筛板以及视网膜微血管系统和神经组织等微观结构进行定性和定量分析,从而作为糖尿病视网膜病变、年龄相关性黄斑变性、青光眼、遗传性视网膜疾病等眼病早期诊断、进展检测和治疗效果随访观察的新手段。     

Adaptive Optics Imaging in Retinal Vasculitis

Purpose: To study the sheathing of retinal vasculitis in various systemic autoimmune diseases using adaptive optics imaging (AOI).

Methods: Prospective, observational case series with six patients: Behçet disease (n = 1); systemic lupus erythematosus (n = 1); idiopathic retinal vasculitis (n = 2); granulomatosis with polyangiitis (n = 1); and Takayasu aorta arteritis (n = 1). Fundus photograph (FP), fundus fluorescein angiography (FFA) were done in all cases at presentation. Using the Image J software, perivascular sheathing and wall-to-wall diameter of the vessel involved were measured on AOI at time of presentation and on follow-up.

Results: AOI was able to pick the pipe-stem sheathing in SLE and IRV(I) and parallel sheathing in rest, which correlated with FP and FFA. Moreover, the decrease and a complete resolution in the sheathing were also noted by AOI on follow-up.

Conclusions: AOI can be used as an additional investigative tool for diagnosis and to monitor the disease course during the treatment.     

自适应光学系统在青光眼视网膜毛细血管中的观察

目的以自适应光学系统观测青光眼患者的视网膜毛细血管特征。方法用37单元活体人眼视网膜高分辨率成像系统，对18岁以上原发性开角型青光眼患者和健康对照者的视网膜毛细血管，以中心小凹向下旁开1°的位置进行成像，比较其成像结果和数据，并进行统计学检验。结果健康对照组清晰成像者5例，原发性开角型青光眼组4例。健康对照组的平均毛细血管直径（5.39μm）大于原发性开角型青光眼组（3.9μm），差异有统计学意义（P=0．047）。两组的血管总长度及血管条数未见明显差别。结论黄斑区视网膜毛细血管在原发性开角型青光眼患者中变细，这可能是血管自我调节异常的一种表现。     

Imaging single cells in the living retina

A quarter century ago, we were limited to a macroscopic view of the retina inside the living eye. Since then, new imaging technologies, including confocal scanning laser ophthalmoscopy, optical coherence tomography, and adaptive optics fundus imaging, transformed the eye into a microscope in which individual cells can now be resolved noninvasively. These technologies have enabled a wide range of studies of the retina that were previously impossible.     

Characterization of a transgenic mouse line lacking photoreceptor development within the ventral retina

A unique transgenic mouse line was generated by incorporating a minigene that contained a cone-specific human cone transducin alpha-subunit (GNAT2) promoter, an attenuated diphtheria toxin A (DTA) gene, and an enhancer element from human interphotoreceptor retinoid-binding protein (IRBP) gene. This transgenic mouse line is designated h-GNAT2pro-DTA. During postnatal retinal development, both transgenic and non-transgenic retinas showed similar morphology and thickness at P1. Between ages P8 and P30, all retinal layers became recognizable in non-transgenic and also in transgenic dorsal retinas. However, in the ventral retina of the transgenic mice the photoreceptor layers did not develop. This aberration occurred as a result of abnormal cellular development, rather than as a consequence of retinal degeneration. In adult transgenic animals, approximately 44% of the retina located dorsally appeared morphologically normal, whereas 32% of the retina located ventrally was completely lacking photoreceptor development. The 24% mid-retinal region exhibited transitional morphology containing malformed photoreceptors. At P360 or older, the thickness of retina layers was reduced in both dorsal and ventral regions. The abnormality observed in transgenic retinas involved mainly the photoreceptors; the other retinal cell types were all present in both dorsal and ventral retinas. Since the DTA gene was only expressed in cone cells, the absence of cone photoreceptors in the transgenic retina was to be expected. However, what was unexpected was the concomitant absence of rod photoreceptors in the ventral retina, suggesting that the presence of cones may be important for the development of rods. This new transgenic line can lead to better understanding of photoreceptor development, and may serve as a new animal model for studying photoreceptor-related retinal diseases.     

A novel cone visual cycle in the cone-dominated retina

The visual processing of humans is primarily reliant upon the sensitivity of cone photoreceptors to light during daylight conditions. This underscores the importance of understanding how cone photoreceptors maintain the ability to detect light. The vertebrate retina consists of a combination of both rod and cone photoreceptors. Subsequent to light exposure, both rod and cone photoreceptors are dependent upon the recycling of vitamin A to regenerate photopigments, the proteins responsible for detecting light. Metabolic processing of vitamin A in support of rod photopigment renewal, the so-called "rod visual cycle", is well established. However, the metabolic processing of vitamin A in support of cone photopigment renewal remains a challenge for characterization in the recently discovered "cone visual cycle". In this review we summarize the research that has defined the rod visual cycle and our current concept of the novel cone visual cycle. Here, we highlight the research that supports the existence of a functional cone-specific visual cycle: the identification of novel enzymatic activities that contribute to retinoid recycling, the observation of vitamin A recycling in cone-dominated retinas, and the localization of some of these activities to the Müller cell. In the opinions of the authors, additional research on the possible interactions between these two visual cycles in the duplex retina is needed to understand visual detection in the human retina.     

Imaging the pediatric retina: An overview

Recent decade has seen a shift in the causes of childhood blinding diseases from anterior segment to retinal disease in both developed and developing countries. The common retinal disorders are retinopathy of prematurity and vitreoretinal infections in neonates, congenital anomalies in infants, and vascular retinopathies including type 1 diabetes, tumors, and inherited retinal diseases in children (up to 12 years). Retinal imaging helps in diagnosis, management, follow up and prognostication in all these disorders. These imaging modalities include fundus photography, fluorescein angiography, ultrasonography, retinal vascular and structural studies, and electrodiagnosis. Over the decades there has been tremendous advances both in design (compact, multifunctional, tele-consult capable) and technology (wide- and ultra-wide field and noninvasive retinal angiography). These new advances have application in most of the pediatric retinal diseases though at most times the designs of new devices have remained confined to use in adults. Poor patient cooperation and insufficient attention span in children demand careful crafting of the devices. The newer attempts of hand-held retinal diagnostic devices are welcome additions in this direction. While much has been done, there is still much to do in the coming years. One of the compelling and immediate needs is the pediatric version of optical coherence tomography angiography. These needs and demands would increase many folds in future. A sound policy could be the simultaneous development of adult and pediatric version of all ophthalmic diagnostic devices, coupled with capacity building of trained medical personnel.     

Impact of Reference Center Choice on Adaptive Optics Imaging Cone Mosaic Analysis

PurposeFoveal center marking is a key step in retinal image analysis. We investigated the discordance between the adaptive optics (AO) montage center (AMC) and the foveal pit center (FPC) and its implications for cone mosaic analysis using a commercial flood-illumination AO camera.MethodsThirty eyes of 30 individuals (including 15 healthy and 15 patients with rod–cone dystrophy) were included. Spectral-domain optical coherence tomography was used to determine the FPC, and flood-illumination AO imaging was performed with overlapping image frames to create an AO montage. The AMC was determined by averaging the (0,0) coordinates in the four paracentral overlapping AO image frames. Cone mosaic measurements at various retinal eccentricities were compared between corresponding retinal loci relative to the AMC or FPC.ResultsAMCs were located temporally to the FPCs in 14 of 15 eyes in both groups. The average AMC–FPC discordance was 0.85° among healthy controls and 0.33° among patients with rod-cone dystrophy (P < 0.05). The distance of the AMC from the FPC was a significant determinant of the cone density (β estimate = 218 cells/deg²/deg; 95% confidence interval [CI], 107–330; P < 0.001) and inter-cone distance (β estimate = 0.28 arcmin/deg; 95% CI, 0.15–0.40; P < 0.001), after adjustment for age, sex, axial length, spherical equivalent, eccentricity, and disease status.ConclusionsThere is a marked mismatch between the AMC and FPC in healthy eyes that may be modified by disease process such as rod–cone dystrophy. We recommend users of AO imaging systems carefully align the AO montage with a foveal anatomical landmark, such as the FPC, to ensure precise and reproducible localization of the eccentricities and regions of interest for cone mosaic analysis.