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. 2020 Nov 5;10:19222. doi: 10.1038/s41598-020-75784-1

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Quantification of retinal photographic arteriolar and venular metrics. Measurement of retinal photographic vascular metrics are generated from retinal photographs by a semi-automated computer-assisted program (Singapore I Vessel Assessment [SIVA], version 4.0, National University of Singapore, Singapore. https://retinaresearch.d2.comp.nus.edu.sg/users/sign_in). (A) Vessel covers are measurement lines used to estimate the retinal vessel calibers. Shown in the image are vessel covers laid on an arteriole for estimation of its caliber by the SIVA program. (B) All vessels within zones B and C were traced for calculation of fractal dimension by the SIVA program using the ‘box-counting’ method. (C) A tortuous venule traced and measured by the SIVA program. Retinal vascular tortuosity was defined as the integral of the curvature square along the path of the vessel, normalized by the total path length. (D) A branching angle θ as defined by the first angle subtended between 2 daughter vessels at each vascular bifurcation measured by the SIVA program.