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A. I. McNaught, D. J. Mordant, I. Alabboud, P. A. Ritchie, G. D. Muyo, A. R. Harvey; Development of a Model Eye to Validate Oximetric Measurements in the Human Retinal Vasculature Using Hyperspectral Imaging. Invest. Ophthalmol. Vis. Sci. 2008;49(13):921.
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© ARVO (1962-2015); The Authors (2016-present)
Quantitative oximetry of the retinal vasculature using hyperspectral imaging requires a high degree of accuracy to detect oximetric changes in the retinal circulation.Hyperspectral imaging for the measurement of retinal blood oxygenation is dependent upon the level of uncertainty in the inferred oxygenation values which arise from the complex imaging conditions in the retina: contributions to recorded spectral images of blood vessels arise from many different light paths within the retina and choroid. We aimed to improve the understanding of the physics underlying the formation of spectral images of retinal blood vessels so as to enable more accurate oximetry.
We report the use of an artificial eye that closely mimics the optical structure of a human eye. Human blood is contained within glass capillaries of known diameter, to simulate the retinal vasculature. The capillaries are immersed within a vitreous simulant, and displaced from 'Spectralon' surfaces that simulate the optical scattering of the retinal pigment epithelium. The model eye geometry, and reflectivity, of this RPE simulant can be varied to enable an exploration of the effect of the multiple light paths on oxygenation values. These are calculated from a numerical fit to an analytical model of light propagation within the retina.
Arterial and venous blood from human subjects were analyzed using a GEM Premiere 4000 CO-Oximeter to establish measured oxygen saturation, and then imaged in the model eye using a hyperspectral retinal camera. There were significant discrepancies of 20% between the oxygenations inferred from the hyperspectral images, and the measured oxygenations. The standard deviations of oxygenation along a capillary,at 1%, were a factor of 5 lower than those that we have measured in a human eye.
The lower uncertainty in the inferred oxygenation in the artificial eye is due to the more uniform texture of the 'Spectralon' RPE compared to the retinal structure, and emphasises that reducing this uncertainty in human retinal oximetry requires processing which compensates for retinal structure. This discrepancy in absolute blood oxygenation has been corrected by recalibration and improvements to the optical model.
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