Fractal Analysis of Retinal Vasculature in Normal Subjects on Ultra-wide Field Fluorescein Angiography

Aim: To evaluate the fractal feature of the retinal vasculature of normal eyes on a stereographic projected and montaged ultra-wide field (UWF) fluorescein angiography (FA).

Methods: Prospective, observational, cross-sectional study. Totally 59 eyes of 31 normal subjects were imaged using the Optos 200Tx. Images obtained at different gaze angles stereographically projected and montaged. The early-phase UWF FA frames were processed to segment the retinal vasculature and the results were exported as binary masks. The fractal dimension (FD) was calculated using the box-counting method.

Results: The global FD for the entire retina was 1.6±0.04, with no difference between males and females (1.59±0.04 1.61±0.04, =0.084) or between right and left eyes (1.6±0.04 1.6±0.05, =0.61). FD was non-uniformly distributed among four quadrants (<0.001) and decreased as the distance from the fovea increased (<0.001). A negative association was observed between FD and age (=-0.37, =0.006), and this relationship was observed in the posterior and mid-peripheral retina (<0.05) but absent in far-periphery (>0.05).

Conclusion: Fractal geometry is non-uniformly distributed across the retina in normal eyes and decreases from the fovea to the far-periphery. Subjects with an older age tend to have a smaller FD, however, the FD in the far-periphery does not appear to be influenced by age.