Objective assessment of corneal transparency in the clinical setting with standard SD-OCT devices

TL;DR

This paper presents an automated algorithm to quantify corneal transparency using standard spectral-domain OCT images, establishing normative values and analyzing transparency related to tissue properties without significant correlation to age or thickness.

Contribution

The study introduces a novel automated method for measuring corneal transparency from clinical OCT images, providing normative data and addressing artifacts for improved accuracy.

Findings

Median photon mean-free path of 570 μm in healthy corneas

Median transmitted coherent light fraction of 51% in normal corneas

No significant correlation between transparency and age or thickness

Abstract

PURPOSE: To develop an automated algorithm allowing extraction of quantitative corneal transparency parameters from clinical spectral-domain OCT images. To establish a representative dataset of normative transparency values from healthy corneas. METHODS: SD-OCT images of 83 normal corneas (ages 22-50 years) from a standard clinical device (RTVue-XR Avanti, Optovue Inc.) were processed. A pre-processing procedure is applied first, including a derivative approach and a PCA-based correction mask, to eliminate common central artifacts (i.e., apex-centered column saturation artifact and posterior stromal artifact) and enable standardized analysis. The mean intensity stromal-depth profile is then extracted over a 6-mm-wide corneal area and analyzed according to our previously developed method deriving quantitative transparency parameters related to the physics of light propagation in tissues, notably tissular heterogeneity (Birge ratio; $B_r$), followed by the photon mean-free path ($l_s$) in homogeneous tissues (i.e., $B_r \sim 1$). RESULTS: After confirming stromal homogeneity ($B_r < 10$, IDR: 1.9-5.1), we measured a median $l_s$ of 570 $\mu$m (IDR: 270-2400 $\mu$m). Considering corneal thicknesses, this may be translated into a median fraction of transmitted (coherent) light $T_{coh(stroma)}$ of 51$\%$ (IDR: 22-83$\%$). No statistically significant correlation between transparency and age or thickness was found. CONCLUSIONS: Our algorithm provides robust and quantitative measurement of corneal transparency from standard clinical SD-OCT images. It yields lower transparency values than previously reported, which may be attributed to our method being exclusively sensitive to spatially coherent light. Excluding images with central artifacts wider than 300 $\mu$m also raises our median $T_{coh(stroma)}$ to 70$\%$ (IDR: 34-87$\%$).