Variability and dependence on eccentricity of extrafoveolar cone photoreceptor packing metrics measured using an adaptive optics retinal camera

Abstract

 

Purpose

 

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.

 

Methods

 

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).

 

Results

 

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%.

 

Conclusions

 

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.

 

 

 

Keywords: 550 imaging/image analysis: clinical • 689 retina: distal (photoreceptors, horizontal cells, bipolar cells) • 648 photoreceptors