Multiple Scattering Analysis of Retinal Blood Oximetry

A recently developed instrument, the photographic eye oximeter (PEO), has been used to make extensive measurements of retinal blood oxygen saturation in rabbits. A model for the reflection of light from the ocular fundus is developed to account for the experimental results. This model is based on the multiple scattering of photons by red blood cells (rbc's) and enables three components of detected light to be distinguished, the coherently transmitted, the diffusely scattered into the detector cone, and the diffusely scattered out of the detector cone. The small diameter of these vessels, the wavelength bands selected for measurement, the large total scattering cross section, and the sharply forward peaked scattering pattern of the red blood cells causes the detected signal from the fundus to be composed primarily of light that has diffusely traversed the vessel twice, reflecting from a background layer after a first traverse. The light is attenuated by a saturation dependent molecular absorption. Equations are derived to enable the blood oxygen saturation to be calculated from photometrically measured quantities. A theoretical calibration line for the PEO is calculated and is shown to be in very good agreement with the experimental line, especially at high saturations.