Department of Ophthalmology and Visual Sciences, University of Wisconsin-Madison, Madison, WI 53717, USA.
Invest Ophthalmol Vis Sci. 2012 Aug 1;53(9):5117-23. doi: 10.1167/iovs.12-9643.
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.
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.
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.
Increasing digital image file compression and decreasing fundus image spatial resolution led to skewed measurements of the retinal vascular caliber.
从数字眼底彩色照片中测量的视网膜血管口径的变化已在流行病学研究中与系统结果独立相关，但以前尚未评估图像分辨率和压缩对血管测量的影响。
为了探索图像压缩，选择了 40 张具有良好照片质量、高空间分辨率且无先前图像压缩的原始数字眼底图像。使用 Adobe Photoshop，将这些图像以逐步更高的水平进行压缩，最高可达 147:1，然后使用半自动软件在每个水平测量视网膜血管口径。为了检查分辨率，用与 10、7、5、3 和 1 百万像素眼底相机相对应的设置扫描 40 张在高分辨率胶片上拍摄的眼底照片。在调整比例因子后，在每个分辨率水平测量血管口径。通过比较原始和改变后的图像的计算中央视网膜小动脉等效（CRAE）和中央视网膜小静脉等效（CRVE），使用重复测量方差分析来分析数据。
在 25:1 的阈值处，随着压缩水平的增加，CRAE 明显变宽（1 μm，P <0.001），在 147:1 压缩时变宽约 5 μm。CRVE 也增加了，但不如 CRAE 增加得多。使用 7（百万像素）-MP 分辨率作为标准，在 5-MP 模拟时 CRVE 明显变窄（2 μm，P <0.001），在 1-MP 模拟时变窄约 12 μm。CRAE 也减少了，但不如 CRVE 减少得多。
数字图像文件压缩的增加和眼底图像空间分辨率的降低导致视网膜血管口径的测量结果偏斜。
