Objective and quantitative analysis of corneal transparency with clinical spectral-domain optical coherence tomography

Abstract

Purpose : To demonstrate the feasibility of deriving quantitative measures of corneal transparency from objective analysis of depth-resolved clinical images.

Methods : 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 “full-field” optical coherence tomography (OCT). Here, we apply our algorithm developed in Matlab (Mathworks, Inc., USA) in conjunction with R (R Core Team, 2014), to clinical, depth-resolved images of in-vivo corneas. That is, after automatic segmentation, flattening, and normalization based on signal intensity at the epithelial surface, the data is analyzed with respect to the propagation properties of the coherent mean of backscattered light in depth (z) of the stroma. Computed stromal intensity depth profiles were fitted using a Bayesian method to an exponential function I(z) ~ exp(-z/ls). As a proof-of-concept demonstration, this approach is illustrated by means of spectral-domain OCT images of one normal, transparent cornea and one pathological cornea with compromised transparency (Fig. 1), as per “gold-standard” subjective and qualitative image inspection.

Results : A graphical representation of the results is shown in Fig. 2. Intensity depth profiles are depicted, representing the average grey value as a function of stromal depth, and illustrating the exponential fitting procedure and ls extraction: <ls> = 263 ± 7 microns and <ls> = 70.6 ± 3 microns for the normal and pathological cornea respectively.

Conclusions : We demonstrated the feasibility of 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.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.