Assessment of photoreceptor function with ultrafast retinal densitometry

Quantification of optical lensing by cellular structures in the living human eye

Cells and other microscopic phase objects can be visualized in the living retina, non-invasively, using non-confocal light detection schemes in adaptive optics scanning light ophthalmoscopes (AOSLOs). There is not yet widespread agreement regarding the origin of image contrast, nor the best way to render multichannel images. Here, we present data to support the interpretation that variations in the intensity of non-confocal images approximate a direct linear mapping of the prismatic deflection of the scanned beam. We advance a simple geometric framework in which local 2D image gradients are used to estimate the spherocylindrical refractive power for each element of the tissue. This framework combines all available information from the non-confocal image channels simultaneously, reducing noise and directional bias. We show that image derivatives can be computed with a scalable, separable gradient operator that minimizes directional errors; this further mitigates noise and directional bias as compared with previous filtering approaches. Strategies to render the output of split-detector gradient operations have been recently described for the visualization of immune cells, blood flow, and photoreceptors; our framework encompasses these methods as rendering astigmatic refractive power. In addition to astigmatic power, we advocate the use of the mean spherical equivalent power, which appears to minimize artifacts even for highly directional micro-structures such as immune cell processes. We highlight examples of positive, negative, and astigmatic power that match expectations according to the known refractive indices and geometries of the relevant structures (for example, a blood vessel filled with plasma acts as a negatively powered cylindrical lens). The examples highlight the benefits of the proposed scheme for the visualization of diverse phase objects including rod and cone inner segments, immune cells near the inner limiting membrane, flowing blood cells, the intravascular cell-free layer, and anatomical details of the vessel wall.

Improved tracking of corneal immune cell dynamics using in vivo confocal microscopy

In vivo confocal microscopy (IVCM) is a widely used technique for imaging the cornea of the eye with a confocal scanning light ophthalmoscope. Cellular resolution and high contrast are achieved without invasive procedures, suiting the study of living humans. However, acquiring useful image data can be challenging due to the incessant motion of the eye, such that images are typically limited by noise and a restricted field of view. These factors affect the degree to which the same cells can be identified and tracked over time. To redress these shortcomings, here we present a data acquisition protocol together with the details of a free, open-source software package written in Matlab. The software package automatically registers and processes IVCM videos to significantly improve contrast, resolution, and field of view. The software also registers scans acquired at progressive time intervals from the same tissue region, producing a time-lapsed video to facilitate visualization and quantification of individual cell dynamics (e.g., motility and dendrite probing). With minimal user intervention, to date, this protocol has been employed to both cross-sectionally and longitudinally assess the dynamics of immune cells in the human corneal epithelium and stroma, using a technique termed functional in vivo confocal microscopy (Fun-IVCM) in 68 eyes from 68 participants. Using the custom software, registration of 'sequence scan' data was successful in 97% of videos acquired from the corneal epithelium and 93% for the corneal stroma. Creation of time-lapsed videos, in which the averages from single videos were registered across time points, was successful in 93% of image series for the epithelium and 75% of image series for the stroma. The reduced success rate for the stroma occurred due to practical difficulties in finding the same tissue between time points, rather than due to errors in image registration. We also present preliminary results showing that the protocol is well suited to in vivo cellular imaging in the retina with adaptive optics scanning laser ophthalmoscopy (AOSLO). Overall, the approach described here substantially improves the efficiency and consistency of time-lapsed video creation to enable non-invasive study of cell dynamics across diverse tissues in the living eye.

Adaptive optics imaging in inherited retinal diseases: A scoping review of the clinical literature

Adaptive optics (AO) imaging enables direct, objective assessments of retinal cells. Applications of AO show great promise in advancing our understanding of the etiology of inherited retinal disease (IRDs) and discovering new imaging biomarkers. This scoping review systematically identifies and summarizes clinical studies evaluating AO imaging in IRDs. Ovid MEDLINE and EMBASE were searched on February 6, 2023. Studies describing AO imaging in monogenic IRDs were included. Study screening and data extraction were performed by 2 reviewers independently. This review presents (1) a broad overview of the dominant areas of research; (2) a summary of IRD characteristics revealed by AO imaging; and (3) a discussion of methodological considerations relating to AO imaging in IRDs. From 140 studies with AO outcomes, including 2 following subretinal gene therapy treatments, 75% included fewer than 10 participants with AO imaging data. Of 100 studies that included participants' genetic diagnoses, the most common IRD genes with AO outcomes are CNGA3, CNGB3, CHM, USH2A, and ABCA4. Confocal reflectance AO scanning laser ophthalmoscopy was the most reported imaging modality, followed by flood-illuminated AO and split-detector AO. The most common outcome was cone density, reported quantitatively in 56% of studies. Future research areas include guidelines to reduce variability in the reporting of AO methodology and a focus on functional AO techniques to guide the development of therapeutic interventions.

Introduction to the Feature Issue on Adaptive Optics for Biomedical Applications

The guest editors introduce a feature issue commemorating the 25th anniversary of adaptive optics in biomedical research.

Evolution of adaptive optics retinal imaging [Invited]

This review describes the progress that has been achieved since adaptive optics (AO) was incorporated into the ophthalmoscope a quarter of a century ago, transforming our ability to image the retina at a cellular spatial scale inside the living eye. The review starts with a comprehensive tabulation of AO papers in the field and then describes the technological advances that have occurred, notably through combining AO with other imaging modalities including confocal, fluorescence, phase contrast, and optical coherence tomography. These advances have made possible many scientific discoveries from the first maps of the topography of the trichromatic cone mosaic to exquisitely sensitive measures of optical and structural changes in photoreceptors in response to light. The future evolution of this technology is poised to offer an increasing array of tools to measure and monitor in vivo retinal structure and function with improved resolution and control.