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 standardize 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 analyzed 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% (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 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.
KEYWORDS: Transparency, Cornea, Optical coherence tomography, Image segmentation, Principal component analysis, Algorithm development, In vivo imaging, Statistical analysis, Signal to noise ratio, Signal processing
We present an automated data analysis procedure for clinical SD-OCT images, capable of correcting hyperreflective artifacts due to the instrument. Quantitative parameters related to corneal transparency are extracted from n=85 normal corneas.
Lack of corneal transparency is a major cause of blindness worldwide. However, means to assess corneal transparency are limited and in current clinical and eye-bank practice usually involve a subjective and qualitative observation of opacities, sometimes with comparison against an arbitrary grading scale, by means of slit-lamp biomicroscopy. To address this unmet need, we have developed a method for corneal transparency assessment based on a new optical data analysis-based approach. Our method allows the objective extraction of quantitative parameters (including the scattering mean-free path, ls, a major indicator of scattering extent and thus of transparency of a medium) based on a physical model of corneal transparency and has been validated by laboratory experiments, using high-resolution, ex-vivo “fullfield” optical coherence tomography (FF-OCT). Here, we apply our algorithm to depth-resolved spectral domain OCT (SD-OCT) images of in-vivo corneas and demonstrate the feasibility of our approach by means of four representative clinical cases. Specifically, we illustrate its potential in discriminating between the four clinical cases and, if applicable, deriving the scattering mean-free path as a quantitative measure of corneal transparency from objective analysis of stromal light backscattering (attenuation of the coherent mean) with SD-OCT. This measure may be related to, or expressed as, Strehl ratio reduction and thus retinal PSF broadening. As such, our approach not only has the potential to supply the demand for an objective means to quantify corneal transparency in the clinical setting, but also to create an association with visual function.
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