Quadrant Non-Confocal AOSLO Imaging of Retinal Vasculature

Abstract

Purpose : Collecting non-confocal, reflected light in adaptive optics SLO imaging has been effective at enhancing the appearance of low-reflectance structures of the retina, such as the inner segments of cones, walls of large blood vessels, and small capillaries. The apparent edges of the structures are orthogonal to the direction of displacement of the non-confocal aperture. In this work we use four separate apertures in the focal plane and demonstrate that the vascular features in any orientation can be visualized by appropriate weighting the four detection signals.

Methods : The quad-detection capability was added to an existing adaptive optics SLO system which has a resolution of 4µm, frame rate of 17Hz, wavelength of 790nm, and an average illumination power of 60µW. Human subjects were consented under an IRB-approved protocol. Short recordings of approximately 10-20 seconds were taken at multiple fundus locations near the fovea and axially focused in the inner-most vascular plexus of retina. Vessels of various calibers, from small capillaries to larger arteries and veins, were imaged. In post-processing, the pixel data from the 4 non-confocal channels were weighted according to the equation a_n=cos([2n-1]π/4-θ_view) and then summed and normalized to the average value at each pixel. The user-chosen θ_view designates the contrast direction. Figure 1 is a small schematic of the orientation of the apertures with the contrast direction overlaid.

Results : Three healthy controls and 14 patients with retinopathies (3 diabetic retinopathy and 11 sickle cell retinopathy) were imaged with the system. Erythrocyte blockages, termed a “rouleau,” were often found in capillaries of SCR subjects. An example of an such a blockage is shown in Figure 2, with two different contrast directions rendered. The arbitrary contrast direction is advantageous over split-detection systems which have shown similar vascular features but may miss some vessels and pathologies due to the fixed perspective. The video-rate capability also allows us to observe dynamic features such as intermittent blockages and blood flow in microaneurysms, which are not visible with current OCTA imaging.

Conclusions : We have successfully demonstrated an AOSLO system with 4 non-confocal detectors capable of arbitrarily selecting the direction for edge contrast and used it in investigating small-scale and dynamic features of the retinal vasculature in healthy controls and patients.

This is a 2020 Imaging in the Eye Conference abstract.