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Julie Jacob, Fanny Poulon, Valérie Krivosic, Michel Paques, Bénédicte Dupas, Ali Erginay, Ramin Tadayoni, Alain Gaudric, Pascale G Massin; Variability and dependence on eccentricity of extrafoveolar cone photoreceptor packing metrics measured using an adaptive optics retinal camera. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2629.
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© ARVO (1962-2015); The Authors (2016-present)
To analyze the density, spacing and packing arrangement of the cone mosaic as a function of retinal eccentricity in adaptive optics (AO) images, and determine the variability and repeatability of the measuring technique.
Retinal images were acquired from 20 healthy subjects, aged 20-38 years, using an AO flood-illumination camera (rtx1, Imagine Eyes, France). Five images were taken along the horizontal axis to obtain a 12x4 deg field. Cones were detected in 50x50 µm regions located at 2, 2.5, 3, 4 and 5 deg nasally and temporally from the foveolar center using automated software (AOdetect, Imagine Eyes, France). Cone density, inter-cone spacing and percentage of hexagonal voronoi cells were calculated. The density and spacing values were computed using both retinal units (cones/mm2 and µm) and visual units (cones/deg2 and arcmin). The whole procedure was repeated twice in each retinal area. Intra-individual repeatability and inter-individual variability were assessed by computing respectively Bland-Altman coefficients of repeatability (BACR) and coefficients of variation (CoV).
Cone density at 2 deg was 26.9 ±4.2 x103 cones/mm2 (22.0 ±3.3 x102 cones/deg2) and decreased progressively to 16.6 ±2.4 x103 cones/mm2 (13.4 ±1.5 x102 cones/deg2) (mean ±SD) at 5 deg of eccentricity. Inter-cone distance was 6.8 ±0.7 µm (1.41± 0.12 arcmin) at 2 deg and increased to 8.6 ±0.6 µm (1.81± 0.12 arcmin) at 5 deg of eccentricity. The proportion of hexagonal voronoi cells was 49% at all locations. Average BACR for density was 3.70 x103 cones/mm2 (2.86 x102 cones/deg2), with repeatability tending to be lower closer to the foveal center. The average BACR for inter-cone distance was 0.7 µm (0.15 arcmin). The CoV was generally lower for the visual metrics, in comparison with the retinal metrics of density and spacing: this difference in CoV ranged between 0 and 5%.
The AO flood-illumination camera enabled reproducible measurements of cone packing metrics across the extrafoveal retina, starting from an eccentricity of 2 deg. Mean values and dependence on eccentricity were compatible with previous AOSLO and histology data. Visual metrics of cone density and spacing showed less inter-individual variability than retinal metrics due to their lower dependence on the eye’s axial length.
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