We read with great interest the article “Validation of concentric rings method as a topographic measure of retinal nonperfusion in ultra-widefield fluorescein angiography” by Nicholson and associates. The authors proposed a method to quantify avascular areas in patients with central retinal vein occlusion. The proposed method employs concentric rings and the size of the ring is determined based on disc diameter. As the use of such imaging technologies becomes more and more common, standardized analysis methods are of particular importance. Although the method proposed by the authors is simple and easy to use in a clinical setting, the program for correcting error from flattening a 3-D image to a 2-D image is not commercially available yet. Minor modification may make the measurement possible with currently available images.

When creating a plane image from a spherical surface such as the retina, projection error is inevitable, as discussed by the authors and others. As shown in the previous study conducted by one of the authors using a model eye, images created by Optos 200TX (Optos Plc, Dumfermline, Scotland) are stretched in the horizontal direction with respect to the vertical direction and are magnified quadratically toward the periphery. Thus, using concentric rings with even intervals will underestimate the ischemic area in the periphery, particularly in the horizontal direction. Instead, concentric ellipses with uneven intervals would reduce the error and can be applied to original images without gaze steering.

We entirely agree with the authors that the correction factor should be made available within the ultra-widefield imaging software.