Evolution of adaptive optics retinal imaging [Invited]

Abstract

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.

Fig. 1.

Adaptive optics papers published per year between 1997 and 2021. Only peer-reviewed articles that either applied a closed loop AO system to the eye or were intended to augment such a system were included. Review papers, conference proceedings, abstracts, articles that used wavefront sensors without real-time wavefront correction, and papers that offer computational alternatives to AO were excluded. The publication list was based on a search of the Dimensions database (Digital Science & Research Solutions Ltd, Cambridge, MA), using a search string that captured 100% of the publications meeting our inclusion criteria of a test set of 7 scientists, clinicians, and engineers actively engaged in the field. This string was then used to capture the entire field as represented in the Dimensions database. Papers that did not meet our criteria were deleted manually. (A) The growth of publications over this quarter century was largely driven by applications of the technology with developments in instrumentation and AO theory for the eye holding relatively steady. (B) The growth of retinal imaging, vision, and papers that involve both applications over the past 25 years. (C) The growth in predominantly clinical publications, basic science publications, and publications involving both over the past 25 years. (D) The growth in publications over the past 25 years that combine AO with various additional imaging modalities including reflectance scanning laser ophthalmoscopy, flood reflectance, OCT, fluorescence, and phase contrast. “Other” refers to publications that use other modalities including those that combine multiple modalities in a single instrument.

Fig. 2.

A 25-year timeline of selected technical and scientific advances associated with AO applied to the eye.

Fig. 3.

First published images of the photoreceptor mosaic obtained with an AO ophthalmoscope from David Williams’ group at the University of Rochester [5] marking the transition from ophthalmoscopy to in vivo retinal microscopy. The flood instrument had a 37-actuator Xinetics deformable mirror and operated at a closed loop rate of ∼0.01 Hz). (A) Without AO compensation. (B) With AO compensation. Scale bar = 25 microns.

Fig. 4.

Confocal AOSLO images of a human retina. (A) Foveal cone mosaic. (B) mosaic of rods and cones in the peripheral retina. A similar version of this figure is published by Carroll et al. [58]. Scale bar = 50 microns. Original image courtesy of Joe Carroll and Alfredo Dubra.

Fig. 5.

Images obtained using different forms of phase contrast imaging with AOSLO. (A) Single frame image of human arterioles and capillaries showing individual red blood cells. Cells in the larger vessels are distorted due to being sampled at slightly different times during the progressive scan [44]. Scale bar = 50 microns. (B) Averaged AOSLO image using an offset aperture showing individual mural cells, presumably pericytes lining the wall of the arteriole [73]. Scale bar = 25 microns. (C) Image of cone inner segments using split detection [65]. Scale bar = 25 microns. (D) Image of ganglion cells in a non-human primate using multi-offset imaging [66]. Scale bar = 25 microns.

Fig. 6.

(A) AO autofluorescence image of the human RPE mosaic, excitation 532 nm (adapted from Granger et al. [75]). Scale bar = 50 microns. (B) AO Rhodamine fluorescence image of monkey retinal ganglion cells, Scale bar = 5 microns. (C) AO two-photon image of the monkey rod and cone mosaic (courtesy of Christina Schwarz). Scale bar = 50 microns. (D) AO fluorescence lifetime image of the monkey photoreceptor mosaic, where color variations reveal the cones which have shorter fluorescence lifetimes than the rod regions between them. 730 nm excitation Scale bar = 25 microns (adapted from Walters et al. [76]).

Fig. 7.

AO-OCT imaging reveals cellular details across the full thickness of the retina of the living human eye. (A) B-scan cross section with depth layers labeled. (B-J) En face images are shown selected from the AO-OCT volume and labeled by retinal depth as denoted in the cross-sectional slice on the left. Image was acquired in the parafovea. Key: RNF, retinal nerve fiber; GCL, ganglion cell layer; IPL, inner plexiform layer; INL, inner nuclear layer; ONL, outer nuclear layer; outer plexiform layer; IS/OS, inner segment - outer segment junction; RPE, retinal pigment epithelium. Scale bar = 100 microns. Figure adapted with permission from Miller and Kurokawa [106].

Fig. 8.

(A) Trichromatic cone mosaic from Roorda and Williams [140]. (B) Plots of color percepts elicited by small spot stimulation of targeted retinal cones from Sabesan et al. [17]. The annular ring indicates the fraction of color percepts elicited when that cone was stimulated. Scale bar for both panels = 5 arcminutes.

Fig. 9.

Modified from Carroll et al. [144]. Cone mosaic image of two individuals with different gene mutations that cause color blindness. Scale bar for both panels = 20 micrometers.

Fig. 10.

The physiological response of a cone cell to light produces nanometer changes in its OS optical path length (ΔOPL) that can be measured and tracked with AO-OCT imaging. (A) Schematic shows the inner and outer segments of a cone cell and the underlying RPE cell that ensheathes it. The cone cell is stimulated with a visible flash during imaging and a phase change results between the two bright reflections at opposing ends of the cone OS, which are labeled IS/OS and COST. Plot shows the normalized sensitivity functions of the three cone types that are sensitive to short- (S), medium- (M), and long- (L) wavelength light [185]. Vertical dashed line depicts peak of example red stimulus. (B) Phase response of cones is biphasic with a rapid, shallow decrease in ΔOPL (inset) followed by a sustained increase whose amplitude varies with cone type (S, M, and L). The dashed gray line at 0 s represents the 5-ms stimulus flash. Individual traces of cone cells are randomly colored. (C) En face intensity image shows the cone mosaic in the parafovea and (D) the same mosaic is color coded on the basis of cone classification (S = blue; M = green; L = red). Scale bar for panels C and D = 50 microns. Figure adapted with permission from Zhang et al. [143].