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K. Y. Li, P. Tiruveedhula, A. Roorda; Adaptive Optics Imaging and Analysis of Cone Photoreceptors Near the Fovea Center. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4770.
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© ARVO (1962-2015); The Authors (2016-present)
Not all foveal cones can be resolved using adaptive optics scanning laser ophthalmoscopy (AOSLO)1. Our goal is to improve a MEMS-based AOSLO2 so that all foveal cones can be imaged. We report on imaging results after control software upgrades and preliminary analyses of the relationship between cone density and axial length (AL) near the fovea center.
Deflection versus voltage curves, measured by an interferometer, were used to linearize the response of the deformable mirror. The control loop now employs a minimum variance reconstructor3 and an improved iterative centroiding algorithm. Correction was done over a 6 mm diameter pupil using 840 nm light. We imaged 7 eyes, with AL from 22.87 to 28.31 mm, at and immediately outside the fovea center (up to ~1 deg eccentricity). Averaged stabilized images were used to construct retinal montages, and individual cones in each montage were identified, via automated4 and manual methods, to evaluate system performance and estimate cone density. AL, corneal curvature and anterior chamber depth for each eye (Zeiss IOLMaster) were used to estimate linear cone density from angular cone density.
Robust AO performance, based on stable closed-loop operation and lack of unobservable mode buildup (piston, localized waffle, etc.), was achieved. The mean radius of the foveal region where cones were not resolved was 107 ± 31 microns (0.37 ± 0.09 deg). In 5 of the 7 eyes, certain cones within 100 microns eccentricity (0.27 to 0.47 deg) were resolved. For the other 2 eyes, cones were only resolved at eccentricities beyond 125 microns (0.44 deg) and 170 microns (0.47 deg). The smallest cones observed had center to center spacing range of 2.43 to 3.34 microns. Linear cone density was not dependent on AL for eccentricities within 260 microns. At eccentricities beyond 260 microns, linear cone density was dependent on AL (p < 0.05). Cone density decreased more than 3,400 cones/mm2 per mm increase in AL.
The improved AOSLO is able to resolve more of the fovea than earlier reports1, 2. Increases in eye size correlate with decreases in cone density but not within 260 microns of the fovea center.1. TYP Chui, et al., Invest Ophthalmol & Vis Sci, 49: 4679-4687, 2008.2. YH Zhang, el al., Optics Letters, 31: 1268-1270, 2006.3. M van Dam, et al., Appl Optics, 43: 5458-5467, 2004.4. KY Li and A Roorda, J Opt Soc Am A, 24: 1358-1363, 2007.
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