Effect of image compression and resolution on retinal vascular caliber.

PURPOSE: Changes in retinal vascular caliber measured from digital color fundus photographs have been independently associated with systemic outcomes in epidemiologic studies, but the effect of image resolution and compression on vascular measurements has not been previously evaluated. METHODS: To explore image compression, 40 natively digital fundus images were selected with good photo quality, high spatial resolution, and no previous image compression. Using Adobe Photoshop, these images were compressed at progressively higher levels up to 147:1, and then retinal vascular caliber was measured at each level using semiautomated software. To examine resolution, 40 fundus photographs acquired on high-resolution film were scanned with settings corresponding to 10, 7, 5, 3, and 1 megapixel fundus cameras. After adjusting for scale factor, vascular caliber was measured at each level of resolution. Data were analyzed by comparing the calculated central retinal arteriole equivalent (CRAE) and the central retinal venular equivalent (CRVE) of the original and altered images, using repeated measures ANOVA. RESULTS: CRAE became significantly wider with increasing levels of compression at the 25:1 threshold (~1 µm wider, P < 0.001) and was ~5 µm wider with 147:1 compression. CRVE also increased, but less than CRAE. Using 7 (megapixel)-MP resolution as the standard, CRVE was significantly narrower at the 5-MP simulation (~2 µm, P < 0.001) and was ~12 µm narrower at the 1-MP simulation. CRAE also decreased, but less than CRVE. CONCLUSIONS: Increasing digital image file compression and decreasing fundus image spatial resolution led to skewed measurements of the retinal vascular caliber.

Suboptimal image focus broadens retinal vessel caliber measurement.

PURPOSE: Studies have used central retinal arteriolar (CRAE) and central retinal venular (CRVE) calibers, measured from images produced with computerized image analysis, to detect risk factors for systemic diseases. The authors explored suboptimal image focus as a possible contributing factor to artificially larger vascular caliber measurements. METHODS: From the reading center image collections, 30 digital retinal images were selected for optimum quality. Image analysis software was used to derive nine progressively blurred versions of the originals. IVAN measurement software was used to measure CRAE and CRVE in the original and the blurred series derived from them. To check the adequacy of the simulation, progressively defocused series of images were taken of several volunteers. RESULTS: For CRAE, each level of simulated blurring produced a statically significant increase in apparent vessel caliber from the original (P<0.01, Wilcoxon signed rank test). For an average CRAE of 160 µm, mean broadening with minimal/moderate/severe blurring was 3 µm/12 µm/33 µm. For CRVE, every blurring level beyond the first was found to be significant (P<0.01). From an average CRVE of 200 µm, mean broadening ranged from 0 to 11 µm with minimal to severe blurring. Analysis of the progressively defocused series taken of volunteers yielded similar results overall. CONCLUSIONS: Suboptimal focus can result in erroneously larger vessel caliber measurements. Slight blurring has a minimal effect, but more severe blurring has a progressively greater effect.