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J. L. Reimers, S. Gangaputra, B. Esser, T. Harding, D. Thayer, D. Peng, L. Hubbard, R. Danis; Green Channel vs. Color Retinal Images for Grading Diabetic Retinopathy in DCCT/EDIC. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2285.
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To compare diabetic retinopathy (DR) severity levels from green channel (GrCh) fundus images (simulating red free) with full color (Red/Green/Blue) images.
From the Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications Study (DCCT/EDIC) we selected digital image sets (stereo 7-standard field) of 100 eyes spanning the full ETDRS scale. From color images (RGB), tonally optimized as in AREDS2, we extracted the GrCh for monochrome display. (Comparison of red free (RF) with extracted GrCh images in other studies showed similar brightness/contrast). Reading center graders did independent duplicate grading/adjudication of ETDRS level. For RGB sets, the routine study grade was used; for GrCh sets, a masked grading was performed months later. Separately, a Reading Center photographer measured flash intensity, as experienced by subjects, with a peak light meter for both color and RF photography at various flash settings.
Color and green channel gradings agreed exactly on ETDRS level in 67 eyes, +1 level in 90 eyes, and + 2 levels in 97 eyes. Analysis for agreement yielded unweighted Kappa = 0.61 (95% CI, 0.51-0.72) and weighted K = 0.83. ("substantial" and "almost perfect" per Landis/Koch). After post hoc comparison of discrepant RGB and GrCh sets, another grader judged that subtle lesions of DR (microaneurysms, IRMA, small new vessels) were better seen in GrCh. Measured flash intensity for red-free imaging was reduced 75% from color imaging, a percentage consistent across flash settings.
Red free rather than color imaging for documenting DR offers potential advantages. Reduction of flash intensity by ¾ decreases patient discomfort. Reduction of digital image file size by 2/3 facilitates storage/transmission. Evaluating DR severity from simulated red free (green channel) is comparable to using color images - perhaps slightly more sensitive for some lesions. This finding may have particular relevance to DR screening/telemedicine.
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