Noninvasive Assessment of Corneal Crosslinking With Phase-Decorrelation Optical Coherence Tomography

Abstract

Purpose: There is strong evidence that abnormalities in corneal biomechanical play a causal role in corneal ectasias, such as keratoconus. Additionally, corneal crosslinking (CXL) treatment, which halts progression of keratoconus, directly appeals to corneal biomechanics. However, existing methods of corneal biomechanical assessment have various drawbacks: dependence on IOP, long acquisition times, or limited resolution. Here, we present a method that may avoid these limitations by using optical coherence tomography (OCT) to detect the endogenous random motion within the cornea, which can be associated with stromal crosslinking.

Methods: Phase-decorrelation OCT (PhD-OCT), based in the theory of dynamic light scattering, is a method to spatially resolve endogenous random motion by calculating the decorrelation rate, Γ, of the temporally evolving complex-valued OCT signal. PhD-OCT images of ex vivo porcine globes were recorded during CXL and control protocols. In addition, human patients were imaged with PhD-OCT using a clinical OCT system.

Results: In both the porcine cornea and the human cornea, crosslinking results in a reduction of Γ (P < 0.0001), indicating more crosslinks. This effect was repeatable in ex vivo porcine corneas (change in average Γ = -41.55 ± 9.64%, n = 5) and not seen after sham treatments (change in average Γ = 2.83 ± 12.56%, n = 5). No dependence of PhD-OCT on IOP was found, and correctable effects were caused by variations in signal-to-noise ratio, hydration, and motion.

Conclusions: PhD-OCT may be a useful and readily translatable tool for investigating biomechanical properties of the cornea and for enhancing the diagnosis and treatment of patients.

Figure 1

Phase decorrelation analysis. A and B show the g¹(t) decorrelation (Equation 1) taken from a 3-mL sample of 25% glycerol (w/v) solution with 0.025% v/v 0.5-μm-diameter polystyrene beads (Magsphere, Inc., Pasadena, CA, USA) The data were acquired in an M-scan of 5000 A-lines. To calculate a single g¹(t) curve, the correlation of six adjacent pixels within one A-line was computed as a function of time across successive A-lines. The nth point in the curve is the median correlation value between all pairs of A-lines separated in time by n/(A-line rate) seconds. To produce A and B, 20 g¹(t) curves from the same sample were calculated, and the mean and SD of all 20 curves was calculated. (A) g¹(t) is shown from a relative time lag of 0 (1 A-line) to 53 ms (2499 A-lines). The data are fit to an exponential function. (B) The same data are shown from a relative time lag of 0 (1 A-line) to 0.3 ms (15 A-lines). The data are fit to a linear function in the form y = −Γx + B.

Figure 2

Photograph of the Bioptigen Envisu (C-class; Leica, Wetzlar, Germany) OCT scanner using the 12-mm telecentric objective lens for anterior imaging configured to acquire clinical data for the PhD-OCT analysis.

Figure 3

Glycerol phantoms. Controlled phantoms were used to preliminarily characterize the phase decorrelation computation. (A) Measured absolute viscosity of various glycerol-water mixtures with 0.025% polystyrene plotted with expected values (dashed line). Values are plotted as mean ± SD (N = 5). (B) Gelatin phantoms. Varying concentrations of gelatin phantoms were imaged. The mean and SD Γ values are plotted with a linear fit (r² = 0.971).

Figure 4

Apparent diffusion coefficient as a function of scatterer concentration. The concentration of beads in a 25% glycerol solution was varied. The observed diffusion coefficient is shown to trend toward an asymptote as the bead concentration increases. (Dotted line is exponential trend line, r² = 0.82.) On the right axis (red), the scattering coefficient is shown to proportionally increase with logscale bead concentration (dotted line is linear trend line, r² = 0.99). Representative B-scan sections of varying bead concentrations are inset to show speckle development.

Figure 5

Characterization of confounding factors in cornea samples. (A) As the incident light intensity on the sample is reduced, decreases due to the decrease in SNR. A quadratic trend line was fit to the data (n = 3; r² = 0.94). (B) As hydration is varied, with change in central corneal thickness taken as a surrogate measure, Γ tends to decrease (r² = 0.512). (C) As IOP of the porcine globe is varying across a physiologic range, the effect (r² = 0.004) on Γ is observed to be small compared with other variations in the measurement.

Figure 6

Effect of motion on Γ. (A) Increased lateral motion from a translation stage increases the induced motion artifact observed in porcine cornea (N = 3), as does increased maximum lag time. The mean and SD of the average Γ within the cornea are shown. Points are staggered for visualization. (B) Purely axial motion was obtained by continuously translating the reference mirror while imaging a porcine cornea (N = 1). This motion, along with lag time, corresponds to increased measured D. The mean and SD of the average Γ within the cornea are shown. Points are staggered for visualization.

Figure 7

Corneal crosslinking of ex vivo porcine cornea. Examples of OCT reflectance images (upper images) and PhD-OCT images (lower images) of ex vivo porcine corneas before and after (A) a crosslinking procedure and (B) a sham procedure. The black dotted line on the PhD-OCT images indicates the segmentation of the anterior third of the cornea. Field of view is 4.5 mm lateral by 1.2 mm axial. (C) The average Γ within the anterior third of the corneas was computed for samples before and after CXL (n = 5) and sham (n = 5) procedures. The relative postprocedure change in Γ was calculated for each sample and plotted. The two groups are significantly different (P < 0.001). (D) For three additional ex vivo porcine globes, corneal imaging was conducted during the crosslinking procedure, at the start of UV irradiation, and every 10 minutes thereafter. The average Γ of the anterior third is plotted. Note that in the post-CXL example, in the OCT reflectance image, the corneal demarcation line is highlighted with white arrows. This region corresponds to the region of decreased mobility in the PhD-OCT image.

Figure 8

Images acquired with a clinical OCT system. Field of view is 3.7 mm lateral by 1 mm axial. (A) To generate interpretable images, corrections were applied to account for SNR and patient motion. An example of a clinically obtained image before and after corrections have been applied is shown. (B) Examples of four unrelated scans are shown. Two pre-LASIK patients were imaged and found to have relatively homogenous corneas. However, two patients at 3 months after CXL exhibit patterns of decreased Γ in the anterior third, very similar to the ex vivo porcine eyes post-CXL.