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N. M. Putnam, D. X. Hammer, A. Roorda; Optical Properties of Foveal Cones: Consequences for Imaging. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4202. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To understand imaging limits of the adaptive optics scanning laser ophthalmoscope (AOSLO), we developed a model of the interference between neighboring cones observed in AOSLO images of the foveola.
Although foveal cones can be resolved under optimal conditions using an incoherent source in an AO equipped ophthalmoscope, interference artifacts obscure the smallest cones in AOSLO images. In addition to their small size, the difficulty in resolving foveal cones might be due, in part, to their unique structure. Foveal cones are more rod-like, with minimal taper at the inner segment (IS)/outer segment (OS) junction compared with peripheral cones. The resulting reflectivity from that interface is reduced, with an average reflectance ratio between the IS/OS junction and the OS layers, measured from OCT images, of 1:3.5 in the foveola and 1:1.5 at 1 deg. In our model of the cone mosaic imaged with AOSLO, each cone acts as a point source with contributions from both the IS/OS junction and the OS. In an AOSLO, each image is effectively a sum of two coherent images, where the contribution of the second image with different relative phases between cones is weighted according to the reflectance ratio. We assumed a Gaussian blur and modeled Incoherent, Coherent, and AOSLO images.
With a Gaussian blur at the theoretical resolution of the AOSLO, most cones in the foveola were well resolved in an incoherent image. In the coherent image interference effects were seen, particularly in the foveola. These interference artifacts were reduced in a single simulated AOSLO image, but not eliminated. A sum of two or more AOSLO images approached the incoherent image.
Our model is in good agreement with actual AOSLO images of the foveal cone mosaic. Given this finding, the next step to mitigating interference artifacts is to add a series of images with different phase relationships between the cones. Three proposed strategies to accomplish phase differences between images are through imaging at different times, multiple wavelength imaging or by generating dynamic phase changes in cones using visible light stimulation.
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