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Z. Zhong, H. Song, X. Qi, A.E. Elsner, S.A. Burns; Spatially Resolved Measurements of Retinal Scattering Using an Adaptive Optics Scanning Laser Ophthalmoscope . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4069.
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© ARVO (1962-2015); The Authors (2016-present)
A confocal, adaptive optics SLO provides high resolution images of retinal structures such as cone photoreceptors, and the images most likely arise from nearly singly scattered light. We designed our AOSLO to allow us to compare the properties of these readily imaged structures to the distribution of scattered light, since areas of increased retinal scattering often arise from pathological processes in the retina.
We modified the Indiana Adaptive Optics SLO (AOSLO) to allow precise control the confocal apertures in the plane of retina. Apertures were 0.6x, 1.2x, 1.7x, 12x, and 120x the Airy disc. We also used a blocker to occlude the center of the point spread function (PSF), but allow the tails to return. Apertures were interchanged using an aperture wheel translated by a pair of high precision linear translation stages, to capture the off axis light. Images were digitized at 512 x 512 pixels and 1.3 deg, using 830 nm light. Adaptive optics control was maintained using a BMC MEMS deformable mirror and Shack Hartmann sensor operating in closed loop at 8 Hz.
Excellent images of the cones were obtained. The contrast of the cones decreased with increasing aperture size. This occurred for two reasons. First, increasing the confocal aperture increases the depth of field and second, the larger apertures accept multiply scattered light. Thus, the large aperture images are much brighter (more than 20x) than the small ones. As aperture size increased, we can simultaneously image the cones and other low contrast structures. With the center of the PSF blocked, it was possible to see regions of diffuse scattering that moved with the eye. These regions most likely represent areas of increased scattering from the vicinity of Bruch’s membrane. Sizes depended on the subject but were 10 to 20 microns in size or larger. By translating we generated a mixed image, to compare the locations of the photoreceptors to the retinal scattering.
While the distribution of photoreceptors is readily measured, other retinal structures are measured less readily. While we have not identified the scattering structures, we have demonstrated that there is considerable information available by measuring the more widely scattered light. By controlling the position of apertures, and performing postprocessing, it is possible to develop spatially resolved maps of retinal light scattering. The small retinal structures presented in the scattered light images suggests that the scattering is arising from forward scattering of light on the way into the retina, and the larger apertures allow the light collected from the eye to reach the detector.
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