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Sveinn Hakon Hardarson, Benedikt Atli Jonsson, Robert Arnar Karlsson, Asbjorg Geirsdottir, David Bragason, Thor Eysteinsson, Olof Birna Olafsdottir, Jona Valgerdur Kristjansdottir, Einar Stefansson; Image Quality Affects Measurements of Retinal Vessel Oxygen Saturation. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3316.
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Non-invasive measurements of oxygen saturation in retinal vessels are based on retinal images, taken at two wavelengths of light. The purpose of the study was to test if and how image quality affects measurements of retinal vessel oxygen saturation.
The retinal oximeter (Oxymap ehf., Iceland) simultaneously acquires images of the retina at 570nm and 600nm. Software automatically measures light absorbance of retinal vessels and calculates oxygen saturation. A newly developed software tool automatically grades the images on the scale of 0 to 1 according to quality of the images. The quality grade is composed of assessment of focus and contrast. Oximetry images of 108 healthy individuals (age 18-77) and 17 individuals, scheduled for cataract surgery were analyzed for quality.
In the heatlhy individuals, measured venous oxygen saturation decreased with worsening image quality (p<0.0001, R2=0.17) but measured arteriolar oxygen saturation was not significantly affected (p=0.79). A linear model indicated that the relationship between quality and measured venous saturation was not due to age: VenousSaturation=26-0.043*Age+37*ImageQualityGrade, p=0.29 for age and p=0.0008 for image quality. Further analysis showed that the effect of image quality was due to contrast (p=0.0002) rather than focus (p=0.36). In individuals scheduled for cataract surgery, image quality correlated positively with measured oxygen saturation in both arterioles (p=0.0079, R2=0.38) and venules (p=0.0034, R2=0.45).
Poor image quality leads to lower measured venous oxygen saturation and, in more extreme cases, also affects measurements of arterioles. This may explain why measured oxygen saturation has previously been found to decrease with age. The new tool for image quality assessment allows quality control of retinal oximetry measurements and may potentially be used to apply correction factors.
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