Abstract

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 de-veloped 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, B_r; for homogeneous tissues (i.e., B_r~1), the photon mean-free path (l_s) 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 (B_r<10), we measured a median l_s of 570 µm (interdecile range: 270–2400 µm). Consid-ering corneal thicknesses, this may be translated into a median fraction of transmitted (coherent) light T_coh(stroma) of 51% (interdecile range: 22–83%). Excluding images with central saturation artifact raised our median T_coh(stroma) to 73% (inter-decile 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 meas-urements of corneal transparency from standard SD-OCT images with sufficient quality and addresses the demand for such an objective means in the clinical setting.

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