Reflective mirror-based line-scan adaptive optics OCT for imaging retinal structure and function

Parafoveal cone function in choroideremia assessed with adaptive optics optoretinography

Choroideremia (CHM) is an X-linked retinal degeneration leading to loss of the photoreceptors, retinal pigment epithelium (RPE), and choroid. Adaptive optics optoretinography is an emerging technique for noninvasive, objective assessment of photoreceptor function. Here, we investigate parafoveal cone function in CHM using adaptive optics optoretinography and compare with cone structure and clinical assessments of vision. Parafoveal cone mosaics of 10 CHM and four normal-sighted participants were imaged with an adaptive optics scanning light ophthalmoscope. While acquiring video sequences, a 2 s 550Δ10 nm, 450 nW/deg2 stimulus was presented. Videos were registered and the intensity of each cone in each frame was extracted, normalized, standardized, and aggregated to generate the population optoretinogram (ORG) over time. A gamma-pdf was fit to the ORG and the peak was extracted as ORG amplitude. CHM ORG amplitudes were compared to normal and were correlated with bound cone density, ellipsoid zone to RPE/Bruch's membrane (EZ-to-RPE/BrM) distance, and foveal sensitivity using Pearson correlation analysis. ORG amplitude was significantly reduced in CHM compared to normal (0.22 ± 0.15 vs. 1.34 ± 0.31). In addition, CHM ORG amplitude was positively correlated with cone density, EZ-to-RPE/BrM distance, and foveal sensitivity. Our results demonstrate promise for using ORG as a biomarker of photoreceptor function.

High-speed, phase contrast retinal and blood flow imaging using an adaptive optics partially confocal multi-line ophthalmoscope

High-speed, phase contrast retinal and blood flow imaging using an adaptive optics partially confocal multi-line ophthalmosocope (AO-pcMLO) is described. It allows for simultaneous confocal and phase contrast imaging with various directional multi-line illumination by using a single 2D camera and a digital micromirror device (DMD). Both vertical and horizontal line illumination directions were tested, for photoreceptor and vascular imaging. The phase contrast imaging provided improved visualization of retinal structures such as cone inner segments, vessel walls and red blood cells with images being acquired at frame rates up to 500 Hz. Blood flow velocities of small vessels (<40 µm in diameter) were measured using kymographs for capillaries and cross-correlation between subsequent images for arterioles or venules. Cardiac-related pulsatile patterns were observed with normal resting heart-beat rate, and instantaneous blood flow velocities from 0.7 to 20 mm/s were measured.

Real-time line-field optical coherence tomography for cellular resolution imaging of biological tissue

A real-time line-field optical coherence tomography (LF-OCT) system is demonstrated with image acquisition rates of up to 5000 B-frames or 2.5 million A-lines per second for 500 A-lines per B-frame. The system uses a high-speed low-cost camera to achieve continuous data transfer rates required for real-time imaging, allowing the evaluation of future applications in clinical or intraoperative environments. The light source is an 840 nm super-luminescent diode. Leveraging parallel computing with GPU and high speed CoaXPress data transfer interface, we were able to acquire, process, and display OCT data with low latency. The studied system uses anamorphic beam shaping in the detector arm, optimizing the field of view and sensitivity for imaging biological tissue at cellular resolution. The lateral and axial resolution measured in air were 1.7 µm and 6.3 µm, respectively. Experimental results demonstrate real-time inspection of the trabecular meshwork and Schlemm's canal on ex vivo corneoscleral wedges and real-time imaging of endothelial cells of human subjects in vivo.

Powell lens-based line-field spectral domain optical coherence tomography system for cellular resolution imaging of biological tissue

A Powell lens is used in a line-field spectral domain OCT (PL-LF-SD-OCT) system to generate a line-shaped imaging beam with almost uniform distribution of the optical power in the line direction. This design overcomes the severe sensitivity loss (∼10 dB) observed along the line length direction (B-scan) in LF-OCT systems based on cylindrical lens line generators. The PL-LF-SD-OCT system offers almost isotropic spatial resolution (Δx and Δy ∼2 µm, Δz ∼1.8 µm) in free space and sensitivity of ∼87 dB for 2.5 mW imaging power at 2,000 fps imaging rate with only ∼1.6 dB sensitivity loss along the line length. Images acquired with the PL-LF-SD-OCT system allow for visualization of the cellular and sub-cellular structure of biological tissues.

Twenty-five years of clinical applications using adaptive optics ophthalmoscopy [Invited]

Twenty-five years ago, adaptive optics (AO) was combined with fundus photography, thereby initiating a new era in the field of ophthalmic imaging. Since that time, clinical applications of AO ophthalmoscopy to investigate visual system structure and function in both health and disease abound. To date, AO ophthalmoscopy has enabled visualization of most cell types in the retina, offered insight into retinal and systemic disease pathogenesis, and been integrated into clinical trials. This article reviews clinical applications of AO ophthalmoscopy and addresses remaining challenges for AO ophthalmoscopy to become fully integrated into standard ophthalmic care.

Characterizing cone spectral classification by optoretinography

Light propagation in photoreceptor outer segments is affected by photopigment absorption and the phototransduction amplification cascade. Photopigment absorption has been studied using retinal densitometry, while recently, optoretinography (ORG) has provided an avenue to probe changes in outer segment optical path length due to phototransduction. With adaptive optics (AO), both densitometry and ORG have been used for cone spectral classification based on the differential bleaching signatures of the three cone types. Here, we characterize cone classification by ORG, implemented in an AO line-scan optical coherence tomography (OCT), and compare it against densitometry. The cone mosaics of five color normal subjects were classified using ORG showing high probability (∼0.99), low error (<0.22%), high test-retest reliability (∼97%), and short imaging durations (< 1 hour). Of these, the cone spectral assignments in two subjects were compared against AO-scanning laser opthalmoscope densitometry. High agreement (mean: 91%) was observed between the two modalities in these two subjects, with measurements conducted 6-7 years apart. Overall, ORG benefits from higher sensitivity and dynamic range to probe cone photopigments compared to densitometry, and thus provides greater fidelity for cone spectral classification.

Retinal imaging using adaptive optics optical coherence tomography with fast and accurate real-time tracking

One of the main obstacles in high-resolution 3-D retinal imaging is eye motion, which causes blur and distortion artifacts that require extensive post-processing to be corrected. Here, an adaptive optics optical coherence tomography (AOOCT) system with real-time active eye motion correction is presented. Correction of ocular aberrations and of retinal motion is provided by an adaptive optics scanning laser ophthalmoscope (AOSLO) that is optically and electronically combined with the AOOCT system. We describe the system design and quantify its performance. The AOOCT system features an independent focus adjustment that allows focusing on different retinal layers while maintaining the AOSLO focus on the photoreceptor mosaic for high fidelity active motion correction. The use of a high-quality reference frame for eye tracking increases revisitation accuracy between successive imaging sessions, allowing to collect several volumes from the same area. This system enables spatially targeted retinal imaging as well as volume averaging over multiple imaging sessions with minimal correction of motion in post processing.

Coarse-scale optoretinography (CoORG) with extended field-of-view for normative characterization

Optoretinography (ORG) has the potential to be an effective biomarker for light-evoked retinal activity owing to its sensitive, objective, and precise localization of retinal function and dysfunction. Many ORG implementations have used adaptive optics (AO) to localize activity on a cellular scale. However, the use of AO restricts field-of-view (FOV) to the isoplanatic angle, necessitating the montaging of multiple regions-of-interest to cover an extended field. In addition, subjects with lens opacities, increased eye movements and decreased mobility pose challenges for effective AO operation. Here, we developed a coarse-scale ORG (CoORG) system without AO, which accommodates FOVs up to 5.5 deg. in a single acquisition. The system is based on a line-scan spectral domain OCT with volume rates of up to 32 Hz (16,000 B-frames per second). For acquiring ORGs, 5.5 deg. wide OCT volumes were recorded after dark adaptation and two different stimulus bleaches. The stimulus-evoked optical phase change was calculated from the reflections encasing the cone outer segments and its variation was assessed vs. eccentricity in 12 healthy subjects. The general behavior of ΔOPL vs. time mimicked published reports. High trial-to-trial repeatability was observed across subjects and with eccentricity. Comparison of ORG between CoORG and AO-OCT based ORG at 1.5°, 2.5°, and 3.5° eccentricity showed an excellent agreement in the same 2 subjects. The amplitude of the ORG response decreased with increasing eccentricity. The variation of ORG characteristics between subjects and versus eccentricity was well explained by the photon density of the stimulus on the retina and the outer segment length. Overall, the high repeatability and rapid acquisition over an extended field enabled the normative characterization of the cone ORG response in healthy eyes, and provides a promising avenue for translating ORG for widespread clinical application.

Ultrahigh-speed multimodal adaptive optics system for microscopic structural and functional imaging of the human retina

We describe the design and performance of a multimodal and multifunctional adaptive optics (AO) system that combines scanning laser ophthalmoscopy (SLO) and optical coherence tomography (OCT) for simultaneous retinal imaging at 13.4 Hz. The high-speed AO-OCT channel uses a 3.4 MHz Fourier-domain mode-locked (FDML) swept source. The system achieves exquisite resolution and sensitivity for pan-macular and transretinal visualization of retinal cells and structures while providing a functional assessment of the cone photoreceptors. The ultra-high speed also enables wide-field scans for clinical usability and angiography for vascular visualization. The FDA FDML-AO system is a powerful platform for studying various retinal and neurological diseases for vision science research, retina physiology investigation, and biomarker development.

Visible Light Optical Coherence Tomography Reveals the Relationship of the Myoid and Ellipsoid to Band 2 in Humans

Purpose

We employ visible light optical coherence tomography (OCT) to investigate the relationship between the myoid, ellipsoid, and band 2 in the living human retina. Rather than refute existing theories, we aim to reveal new bands and better delineate the structures at hand.

Methods

An upgraded spectral/Fourier domain visible light OCT prototype, with 1.0-µm axial resolution, imaged 13 eyes of 13 young adult human subjects (23–40 years old) without a history of ocular pathology. The external limiting membrane (band 1) and band 2 edges were segmented. Reflectivity was examined along the inner segment (IS), defined as extending from band 1 to the band 2 center, and within band 2 itself.

Results

Images highlight a nearly continuously resolved extrafoveal internal limiting membrane, the peripheral single-cell thick ganglion cell layer, and the peripheral photoreceptor axonal fiber layer, a peripheral division of band 2 into bands 2a and 2b, and a reflectivity-based division of the IS into “m” and “e” zones.

Discussion

Topography and transverse intensity variations of the outermost band 2b suggest an association with rods. The “m” and “e” zone border is consistent with the myoid–ellipsoid boundary, even recapitulating the well-documented distribution of mitochondria throughout the IS at the foveal center. Theories of outer retinal reflectivity in OCT must adequately explain these observations.

Translational Relevance

Findings support that band 2 does partially overlap with the ellipsoid in transversally averaged OCT images due to photoreceptor IS length dispersion but argue that the inner ellipsoid must be inner to band 2, as suggested by prior quantitative measurements.

The Development and Clinical Application of Innovative Optical Ophthalmic Imaging Techniques

The field of ophthalmic imaging has grown substantially over the last years. Massive improvements in image processing and computer hardware have allowed the emergence of multiple imaging techniques of the eye that can transform patient care. The purpose of this review is to describe the most recent advances in eye imaging and explain how new technologies and imaging methods can be utilized in a clinical setting. The introduction of optical coherence tomography (OCT) was a revolution in eye imaging and has since become the standard of care for a plethora of conditions. Its most recent iterations, OCT angiography, and visible light OCT, as well as imaging modalities, such as fluorescent lifetime imaging ophthalmoscopy, would allow a more thorough evaluation of patients and provide additional information on disease processes. Toward that goal, the application of adaptive optics (AO) and full-field scanning to a variety of eye imaging techniques has further allowed the histologic study of single cells in the retina and anterior segment. Toward the goal of remote eye care and more accessible eye imaging, methods such as handheld OCT devices and imaging through smartphones, have emerged. Finally, incorporating artificial intelligence (AI) in eye images has the potential to become a new milestone for eye imaging while also contributing in social aspects of eye care.

Application of Adaptive Optics in Ophthalmology

The eye, the photoreceptive organ used to perceive the external environment, is of great importance to humans. It has been proven that some diseases in humans are accompanied by fundus changes; therefore, the health status of people may be interpreted from retinal images. However, the human eye is not a perfect refractive system for the existence of ocular aberrations. These aberrations not only affect the ability of human visual discrimination and recognition, but restrict the observation of the fine structures of human eye and reduce the possibility of exploring the mechanisms of eye disease. Adaptive optics (AO) is a technique that corrects optical wavefront aberrations. Once integrated into ophthalmoscopes, AO enables retinal imaging at the cellular level. This paper illustrates the principle of AO in correcting wavefront aberrations in human eyes, and then reviews the applications and advances of AO in ophthalmology, including the adaptive optics fundus camera (AO-FC), the adaptive optics scanning laser ophthalmoscope (AO-SLO), the adaptive optics optical coherence tomography (AO-OCT), and their combined multimodal imaging technologies. The future development trend of AO in ophthalmology is also prospected.