“…In the case of the analysis of human retinal vessels, it has been reported that a healthy eye has an FD of around 1.7 (Family, Masters, & Platt, 1989;Mainster, 1990;Popovic et al, 2018). However, it has also been shown that, due to its underlying dependence on the structural properties of the image, the FD is sensitive to a number of other factors from both biological origin e.g., age, cataracts, and lens opacity (Cheung et al, 2012), changes in blood pressure due different origins (Sng et al, 2010;Zhu et al, 2014), existing diabetic condition , and cognitive dysfunctions (Taylor et al, 2015), as well as numerical origin e.g., size and location of the region of interest Huang et al, 2015) or the specific stages in the pre-processing procedure (Che Azemin et al, 2016). Due to this overall 'instability' of the FD, which can be significant in some cases, one should not rely a quantitative analysis for diagnosis purposes solely on the value of the FD (Huang et al, 2015).…”