# Local Versus Global Binarization Techniques After Frangi Filtering for Optical Coherence Tomography Angiography Based Retinal Vessel Density Assessment in Diabetic Retinopathy

## TL;DR

This study compares local and global binarization methods for analyzing retinal vessel density in diabetic retinopathy using OCTA, finding that local methods perform better.

## Contribution

The study introduces a comparison of local and global binarization techniques after Frangi filtering for OCTA-based vessel density assessment in diabetic retinopathy.

## Key findings

- Local binarization methods showed significant differences in vessel density between PDR and control/no DR groups.
- Global Otsu thresholding was the only global method to detect significant differences between PDR and control.
- Nasal and inferior quadrants showed the most robust differences in vessel density.

## Abstract

Background/Objectives: Optical coherence tomography angiography (OCTA) enables noninvasive quantitative assessment of the retinal microvasculature and is widely used in diabetic retinopathy (DR). However, OCTA-derived metrics are highly dependent on post-processing techniques, particularly vessel binarization. This study aimed to compare local and global binarization methods applied after Frangi filtering for vessel enhancement in parafoveal vessel density analysis. Methods: This cross-sectional study included 69 participants: 17 healthy controls and 52 diabetic patients, classified as the following: no DR (n = 14), non-proliferative DR (NPDR, n = 18), or proliferative DR (PDR, n = 20). All subjects underwent comprehensive ophthalmological examination and OCTA imaging of the superficial capillary plexus using a Topcon OCTA system. Images were processed using a custom MATLAB protocol. Following Frangi filtering, five binarization methods were applied: three local (Phansalkar, local Otsu, adaptive mean) and two global (global mean and global Otsu). Parafoveal vessel density was quantified within the four inner quadrants of the ETDRS grid. Results: Statistically significant differences in vessel density were consistently observed between PDR group and both the control and no DR groups across all local binarization methods. Among global methods, only global Otsu thresholding detected a significant difference between PDR and control. The most robust differences were predominantly identified in the nasal and inferior quadrants. Conclusions: Local adaptive binarization methods demonstrated superior sensitivity and structural preservation for parafoveal vessel density analysis in DR. Global methods showed limited discriminative capability. These findings support the preferential use of local adaptive techniques for reliable OCTA-based vascular assessment in diabetic retinopathy.