A New Image Analysis Technique for the Calculation of Optical Density in Retinal Oximetry

Abstract

Abstract: : Purpose: Optical density (OD) is defined as log₁₀(1/transmission), with transmission equal to transmitted light/illumination. In the case of reflectance oximetry utilizing an 8 bit digital image (0 = black and 255 = white), the illumination has a level of 255. The purpose of this study is to determine the effect of an improved unconventional pigment insensitive method of optical density calculation on reproducibility in retinal oximetry. Methods: : All procedures were approved by an IRB, and informed consent obtained. Five oximetry images were obtained in 17 normal healthy adults. Images were analyzed using a conventional OD calculation, and an innovative OD calculation; replacing the illumination level of 255 with the average reflectance of tissue surrounding blood vessels. OD, therefore, becomes log₁₀(tissue/blood vessel). The coefficient of variation (CV) was calculated for each OD method, and compared by paired student’s t test. Results: CV of the old and new artery OD calculations were 12.0% and 12.9% respectively (p = 0.76). For the vein, the CVs were 12.6% and 14.7% (p = 0.59). Conclusions: Replacing the 255 constant term with a varying tissue reflectance level does not significantly alter variability in the calculation optical density in retinal oximetry analysis.

Keywords: imaging/image analysis: clinical • blood supply • hypoxia