Light path-length distributions within the retina.

A Monte Carlo simulation of light propagation through the retina has been developed to understand the path-length distributions within the retinal vessel. For full-field illumination, the path-length distribution within the vessel comprises directly backscattered light and light that has passed once or twice through the vessel. The origins of these light path-length distributions can be better understood by investigating different combinations of single-point illumination and detection positions. Perhaps the most significant observation is that illumination at the edges of the vessel, rather than over the whole field of view, and detection directly above the vessel capture only the light that has taken a single pass through the vessel. This path-length distribution is tightly constrained around the diameter of the vessel and can potentially provide enhancements for oxygen saturation imaging. The method could be practically implemented using an offset-pinhole confocal imaging system or structured light illumination.

Validation of human whole blood oximetry, using a hyperspectral fundus camera with a model eye.

PURPOSE: To assess the accuracy of human blood oximetry measurements in a model eye with a hyperspectral fundus camera. METHODS: Seven human whole blood samples (two arterial, five venous) were obtained, the oxygen saturations measured with a CO oximeter, and the samples inserted into quartz tubes with internal diameters of 100 and 150 µm. The tubes (n = 20; ten 100 µm and ten 150 µm) were placed within a model eye in front of a background reflectance surface with reflectivities of 20%, 60%, and 99%. Spectral images at wavelengths between 500 and 650 nm were acquired with a hyperspectral fundus camera and analyzed with an oximetric model to calculate the oxygen saturation of blood within the tubes. The calculated oxygen saturations were compared with the measured oxygen saturations. The effects of the background reflectivity and tube size on the accuracy of the calculated oxygen saturations were evaluated. RESULTS: Background reflectivity and tube size had no significant effect on the mean oxygen saturation difference (P = 0.18 and P = 0.99, respectively; repeated-measures, two-way ANOVA). The mean differences (SD) between the measured and calculated oxygen saturations in segments of the 100 and 150 µm tubes overlying the 20%, 60%, and 99% background reflectivities were (100 µm) -4.0% (13.4%), -6.4% (9.9%), and -5.5% (10.2%) and (150 µm) -5.3% (10.8%), -5.2% (10.7%), and -5.2% (10.9%), respectively. CONCLUSIONS: There was reasonable agreement between the measured oxygen saturation values and those calculated by the oximetry model. The oximetry model could be used to determine the functional health of the retina.

Quantitative spectrophotometry of scattering media via frequency-domain and constant-intensity measurements.

The attenuation of light by scattering and absorbing media is nonlinearly dependent upon the absorption coefficient, since detected light has experienced many different flight times. The frequency response of such a sample to modulated light is also nonlinear. We derive an expression for the attenuation that includes both the absorption coefficient and the modulation frequency. Its form is a power series whose coefficients are the cumulants of the temporal point-spread function (TPSF). Recasting this expression in terms of intensity leads to a similar expression with the cumulants replaced by the moments of the TPSF. A means of exploiting this relationship to produce estimates of the absolute concentration of the absorbing species is suggested.