Effect of using different fundus cameras and image resolutions on automatic measurements of retinal vascular parameters.

Morphological measurements of retinal vessel parameters are commonly obtained from fundus images using semi-automatic software packages. Since image quality, and image processing methods are potential sources of variation of such measurements, the aim of this study was to determine quantitatively the effect of using different fundus cameras and image resolutions on measurements of retinal vascular parameters. Fifty-four digital fundus images of 27 healthy subjects were acquired using two non-mydriatic cameras, Topcon TRC NW6S and Canon CR-2. Central retinal artery and vein equivalent (CRAE, CRVE), arteriolar and venular tortuosity (TortA and TortV) and fractal dimension (FD) were calculated using VAMPIRE software. First, VAMPIRE measurements were compared between Topcon images (3008 × 2000 pixels) and Canon images after resizing manually (CR2r, 3008 × 2000 px). The effect of different resolutions was studied using Canon images at full resolution (4147 × 2764 px, 100%) and then scaled at 83.8%, 75% and 50% in JPEG format using the dedicated Canon software. When comparing images acquired by CR2r and Topcon at the same resolution, all parameters were significantly affected. When using images with different resolutions from the same camera, significant increases of CRAE and CRVE and significant decreases of FD were found for decreasing resolutions. In conclusion, vascular measurements from images from different fundus cameras or at different resolutions are not, in general, directly comparable.