Cone photoreceptor definition on adaptive optics retinal imaging

Abstract

Aims: To quantitatively analyse cone photoreceptor matrices on images captured on an adaptive optics (AO) camera and assess their correlation to well-established parameters in the retinal histology literature.

Methods: High resolution retinal images were acquired from 10 healthy subjects, aged 20-35 years old, using an AO camera (rtx1, Imagine Eyes, France). Left eye images were captured at 5° of retinal eccentricity, temporal to the fovea for consistency. In three subjects, images were also acquired at 0, 2, 3, 5 and 7° retinal eccentricities. Cone photoreceptor density was calculated following manual and automated counting. Inter-photoreceptor distance was also calculated. Voronoi domain and power spectrum analyses were performed for all images.

Results: At 5° eccentricity, the cone density (cones/mm(2) mean±SD) was 15.3±1.4×10(3) (automated) and 13.9±1.0×10(3) (manual) and the mean inter-photoreceptor distance was 8.6±0.4 μm. Cone density decreased and inter-photoreceptor distance increased with increasing retinal eccentricity from 2 to 7°. A regular hexagonal cone photoreceptor mosaic pattern was seen at 2, 3 and 5° of retinal eccentricity.

Conclusions: Imaging data acquired from the AO camera match cone density, intercone distance and show the known features of cone photoreceptor distribution in the pericentral retina as reported by histology, namely, decreasing density values from 2 to 7° of eccentricity and the hexagonal packing arrangement. This confirms that AO flood imaging provides reliable estimates of pericentral cone photoreceptor distribution in normal subjects.

Keywords: Adaptive Optics; Cone Photoreceptor; Retinal Imaging.

Figure 1

Infrared fundus image of subject B's left eye. B1, B2, B3 and B4 on the image mark the points at 0, 3, 5 and 7°, respectively, at which the adaptive optics retinal images were acquired. Scale bar is 292 μ, equivalent to 1°.

Figure 2

(A) Adaptive optics (AO) retinal image montages of subjects A, B and C from 0 to 7°. This image shows the decreasing cone photoreceptor density with increasing retinal eccentricity. In images A, B and C (1, 2, 3 and 4) correspond to AO imaging at 0, 3, 5 and 7° retinal eccentricities. The cones are clearly visible in the 3, 5 and 7° images, but not at 0°, and this is due to the resolution limit of the rtx1 AO camera being 4 μ and therefore is not able to resolve the highest density of cone packing at the foveola. (B) Magnified areas of the red box from figure 2A of AO images of subjects A, B and C at 0, 3, 5 and 7° retinal eccentricities. The magnified AO images of A2 through A3 to A4, and similarly for B2 to B4 and C2 to C4 clearly show the cone photoreceptors with decreasing density at increasing retinal eccentricity as well as the loss of their packing regularity in A4, B4 and C4. The cone photoreceptors in images A1, B1 and C1 are not discernible due to the highest cone packing density at 0° which is beyond the device's resolution. (C) Voronoi tessellation of subject A's retinal image at 5° retinal eccentricity.

Figure 2

(A) Adaptive optics (AO) retinal image montages of subjects A, B and C from 0 to 7°. This image shows the decreasing cone photoreceptor density with increasing retinal eccentricity. In images A, B and C (1, 2, 3 and 4) correspond to AO imaging at 0, 3, 5 and 7° retinal eccentricities. The cones are clearly visible in the 3, 5 and 7° images, but not at 0°, and this is due to the resolution limit of the rtx1 AO camera being 4 μ and therefore is not able to resolve the highest density of cone packing at the foveola. (B) Magnified areas of the red box from figure 2A of AO images of subjects A, B and C at 0, 3, 5 and 7° retinal eccentricities. The magnified AO images of A2 through A3 to A4, and similarly for B2 to B4 and C2 to C4 clearly show the cone photoreceptors with decreasing density at increasing retinal eccentricity as well as the loss of their packing regularity in A4, B4 and C4. The cone photoreceptors in images A1, B1 and C1 are not discernible due to the highest cone packing density at 0° which is beyond the device's resolution. (C) Voronoi tessellation of subject A's retinal image at 5° retinal eccentricity.

Figure 3

(A) Spatial frequency technique for processing of adaptive optics images. (B) Voronoi cell analysis for inter-photoreceptor distance approximation.

Figure 4

Fast Fourier transformation at 2, 3, 5 and 7° retinal eccentricities of subjects A, B and C.

Figure 5

Cone density versus retinal eccentricity for manual and automated count dataset of the three subjects at 2, 3, 5 and 7° and mean of 10 subjects at 5°—plotted on graph with histology data from Curcio et al, Jonas et al and adaptive optics scanning laser ophthalmoscope data from Song et al. Exponential pattern was noted.