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Andres Guevara-Torres, Christina Schwarz, David R Williams, Jesse B Schallek; Retinal cell refractive model describes the source of the contrast in split-detector ophthalmoscopy. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3435.
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© ARVO (1962-2015); The Authors (2016-present)
Split-detector and offset aperture imaging are two recent approaches in ophthalmoscopy that enable visualization of translucent cells such as ganglion cells in the living eye. However, the mechanism producing optical contrast is not fully understood. Here we develop and test an optical model based on changes in refractive index in combination with deeper backscattering layers that gives rise to the asymmetric contrast that characterizes split-detector and offset aperture images of retinal cells.
The model treats retinal cell bodies at the illumination focal plane as tiny spherical lenses. When a focused beam is scanned across a cell, the illuminating beam is deviated from the optical axis in the direction consistent with the optical power of that cell. Beams passing through the left and right of the cell will be steered into opposite directions. In each case, the deviated beams strike layers beneath the cell that scatter the light back toward the detector. Lateral displacement of the detector from the optical axis, which is used in split-detector and offset methods then favors light passing through one side of the cell over the other. This was computationally modelled in MATLAB using Fourier optics. Predictions from this model were verified through simulation as well as experimental tests in the living macaque and mouse retinas using adaptive optics scanning light ophthalmoscopes with split-detector and offset capabilities.
Computer simulations based on cell shapes and refractive indices produced images with the same asymmetry observed empirically, one side of the cell appearing light and the other dark. The model also predicted the contrast polarity seen on the convex edges of red blood cells surrounded by plasma. Further instructed by a prediction of this model, we experimentally observed an increase in image quality when the detector is displaced axially towards deeper reflective layers (photoreceptors-choroid) and a decrease in quality when the detector was displaced by the same amount towards the vitreous.
Both simulations and experimental data from living eyes support this refined optical model of the source of contrast in offset aperture and split-detector images. The model not only offers a new framework to explain the source of contrast, but also prescribes a method to optimize image quality and contrast in this new imaging modality.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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