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F. Romero–Borja, K. Venkateswaran, A. Roorda; Reflective Properties of Retinal Features Elucidated with AOSLO . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2798.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Characterization of the adaptive optics laser scanning ophthalmoscope (AOSLO), and its ability to optically section retinal tissue, require an understanding of the light reflection mechanisms responsible for the reflected signal from different retinal features. The purpose of this study was to determine the predominant reflection mechanism for different retinal features and to discuss their implications for retinal imaging. Methods: Axial resolution measurements were done in vivo in three subjects based on scattered light from blood vessels. The AOSLO measured and compensated aberrations of the eye while imaging retinal regions of 1.5 X 1.4 degrees, at 30 frames/second. The imaging wavelength was 660 nm and the pupil size was 5.9 mm. Different retinal planes were imaged by applying defocus with the deformable mirror. The AO loop was closed as the defocus was applied to minimize the induction of new aberrations. The average intensity as a function of defocus was used to compute the axial resolution (full–width at half–maximum) of a retinal feature. This procedure was repeated for five different pinhole sizes in the range of 50 to 200 micrometers. Plots of the axial resolution versus pinhole size were compared with an artificial eye and also with theoretical models, which predict different axial resolution for diffuse and specular reflectors. Results: Axial resolution values from the in vivo measurements were as low as 66 µm and 195 µm for confocal pinholes of 50 µm and 200 µm. These values lie between those predicted by the model for the diffuse and specular reflector, which, for these pinhole sizes, were 83 and 286 µm, and 36 and 121 µm, respectively. On the other hand, axial resolutions measured from diffuse and specular artificial eye models were in good agreement with the theoretical model. The predicted values were only slightly worse when the residual aberrations, after AO correction, were taken into account. Additionally, the in vivo measurements confirm the predicted improvement in the axial resolution of the AOSLO by reducing the pinhole size, at the price of decreasing the levels of the detected light. Conclusions: Reflections from blood vessels have specular components, which give rise to increased axial resolution beyond that predicted by the diffuse model. This study not only clarifies what kinds of light reflection mechanisms are present in the imaging process but it also allows the instrument characterization to be more realistic and to account for possible artifacts in the image processing.
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