Assessing collagen microstructural changes in an ex vivo keratoconic eye model with Stokes polarization-sensitive optical coherence tomography

Abstract

Purpose : Collagen structure is disrupted in keratoconus (KC), a progressive disease currently diagnosed by advanced morphological features. Because biomechanical (BM) changes are known to occur before morphology is affected, there is a pull to find effective BM biomarkers for early diagnosis, which currently rely on complex finite-element modeling (FEM) of the eye globe. An alternative to current BM diagnoses avoiding FEM would be highly attractive. We hypothesize that Stokes processing of polarization-sensitive optical coherence tomography (PS-OCT) can directly provide information on the collagen microstructure; in particular the degree of polarization (DOP), known to be sensitive to disorder of anisotropic, sub-resolution structures.

Methods : We treated 3 freshly enucleated (<48h) and de-epithelized bovine eyes with a solution of 10 mg/mL collagenase type II (CLG) with 15% dextran in a peripheral 7 mm-diameter region.

We built a PS-OCT system based on a 1.3 μm VCSEL source with polarization diverse detection and modulation of the sample arm polarization state. Based on Beer et al. (BOE, 2017), we designed the system to match the chief rays’ angles to the corneal normal surface, effectively suppressing Fresnel diattenuation at the interface. Spectral binning Stokes PS processing was used to obtain high-quality local retardation, optic axis and DOP maps (Villiger et al., Opt. Express, 2013).

We performed PS-OCT measurements under controlled intraocular pressure (15 mmHg) at baseline and after CLG treatment at 30 and 60 min. We also used a second harmonic generation (SHG) microscope in a backward scattering configuration to record images of the treated and untreated corneal collagen.

Results : After 30 min CLG, DOP ([0, 1] range) exhibits strong contrast between untreated (0.56±0.10 DOP) and treated (0.94±0.01 average DOP) CLG regions, while structural OCT shows little unspecific contrast. The uniformly high DOP after CLG treatment suggests that collagen breaks down into small, isotropic structures. In agreement, mean SHG signal dropped 25%.

Conclusions : Stokes PS-OCT and depolarization analysis provides high sensitivity to the state of collagen fibers in the cornea and enable new diagnostic and surveillance techniques, more directly sensitive to the corneal collagen microstructure.

This is a 2020 ARVO Annual Meeting abstract.

 

(a) OCT intensity (b) DOP en face views at 100 μm from corneal surface