Objective Characterisation of Corneal Transparency by Analysis of Clinical SD-OCT Images

We describe an automated algorithm allowing extraction of quantitative corneal transparency parameters with clinical spectral-domain optical coherence tomography (SD-OCT). Our algorithm employs a novel pre-processing procedure to standardise SD-OCT image analysis and to numerically correct common instrumental artifacts before extracting mean intensity stromal-depth (z) profiles over a 6-mm-wide corneal area. The z-profiles are analysed using our previously developed objective method deriving quantitative transparency parameters which are directly related to the physics of light propagation in tissues. Tissular heterogeneity is quantified by the Birge ratio, Br; for homogeneous tissues (i.e., Br~1), the photon mean-free path (ls) may be determined. Images of 83 normal corneas (ages 22-50 years) from a standard SD-OCT device (RTVue-XR Avanti, Optovue Inc.) were processed to establish a normative dataset of transparency values. After confirming stromal homogeneity (Br<10), we measured a median ls of 570 µm (interdecile range: 270-2400 µm). Considering corneal thicknesses, this may be translated into a median fraction of transmitted (coherent) light Tcoh(stroma) of 51% (interdecile range: 22-83%). Excluding images with central saturation artifact raised our median Tcoh(stroma) to 73% (interdecile range: 34-84%). These transparency values are slightly lower than previously reported, which we attribute to the detection configuration of SD-OCT with a relatively small and selective acceptance angle. No statistically significant correlation between transparency and age or thickness was found. Our algorithm provides robust and quantitative measurements of corneal transparency from standard SD-OCT images with sufficient quality and addresses the demand for such an objective means in the clinical setting.