Measuring intraocular pressure with OCT: the first approach.

The variability of corneal OCT speckle statistics is indirectly related to changes in corneal microstructure, which may be induced by intraocular pressure (IOP). A new approach is considered, which attempts to estimate IOP based on corneal speckle statistics in OCT images. An area (A) under trajectories of contrast ratio with respect to stromal depth was calculated. The proposed method was evaluated on OCT images from the ex-vivo study on porcine eyeballs and in-vivo study on human corneas. A statistically significant multivariate linear regression model was obtained from the ex-vivo study: IOP = 0.70 · A - 6.11, in which IOP was precisely controlled in the anterior chamber. The ex-vivo study showed good correlation between A and IOP (R = 0.628, at the least) whereas the in-vivo study showed poor correlation between A and clinical air-puff tonometry based estimates of IOP (R = 0.351, at the most), indicating substantial differences between the two studies. The results of the ex-vivo study show the potential for OCT speckle statistics to be utilized for measuring IOP using static corneal imaging that does not require corneal deformation. Nevertheless, further work is needed to validate this approach in living human corneas.

Indirectly assessing changes in corneal properties with OCT speckle after crosslinking in porcine eyes.

The aim of this study was to assess the effect of the standard crosslinking (CXL) procedure on corneal properties and subsequent changes in collagen bonds formation using optical coherence tomography (OCT) corneal speckle statistics and vibrational spectroscopy. Porcine eyes with intact corneal epithelium were randomly selected to one of the four study groups: (1) untreated eyes moistened with phosphate-buffered saline (PBS); (2) eyes after the epithelial debridement and riboflavin application; (3) eyes after CXL procedure according to the Dresden protocol; and (4) eyes after corneal epithelial debridement, regularly moistened with PBS. Before and after this selection, each eyeball was subjected to the constant intraocular pressure of 20 mmHg. Then, ocular biometry was performed and the central cornea was imaged using spectral-domain OCT. Following this, a nonparametric approach to speckle modeling (the Contrast Ratio (CR)) was utilized within the region of interest for each B-scan covering the central corneal stroma. To verify whether the CXL performed ex-vivo results in formation of new bonds in the cornea, Fourier Transform Infrared Attenuated Total Reflectance (ATR-FTIR) spectra of dried corneas, dissected from examined eyeballs, were collected and analyzed. Corneal epithelium removal alone or with the riboflavin application leads to a statistically significant decrease in the CR median value (the Wilcoxon signed-rank test, p < 0.05). However, the most pronounced change in CR median value, which decreases with the increased number of scatterers, was shown after the complete CXL procedure including riboflavin soaking and UVA irradiation (the Wilcoxon signed-rank test, p = 0.004). Analysis of ATR-FTIR spectra revealed influence of UVA irradiation on collagen matrix. The study has shown the increased dehydration accompanied by almost no alteration of collagen native triple-helical structure. Significant changes have been observed for bands related to collagen crosslinks. Specifically, the predominant changes occurred in the sugar region from 1150 to 975 cm-1, as well as in the absorbance of carbonyl groups. Furthermore, the ratio of two Amide I components at approximately 1660 cm-1 and 1675 cm-1 decreased after UVA irradiation. Together, these results provided the evidence for the creation of new corneal crosslinks. In conclusion, this study clearly indicates that the UVA exposure causes the substantial difference in optical scattering occurring in corneal stroma as a result of the induced biochemical changes at the molecular level in this tissue assessed with ATR-FTIR. The proposed speckle-based methodology brings a new insight into the development of OCT technology useful in an indirect assessment of some collagen changes.

Statistical analysis of corneal OCT speckle: a non-parametric approach.

In biomedical optics, it is often of interest to statistically model the amplitude of the speckle using some distributional approximations with their parameters acting as biomarkers. In this paper, a paradigm shift is being advocated in which non-parametric approaches are used. Specifically, a range of distances, evaluated in different domains, between an empirical non-parametric distribution of the normalized speckle amplitude sample and the benchmark Rayleigh distribution, is considered. Using OCT images from phantoms, two ex-vivo experiments with porcine corneas and an in-vivo experiment with human corneas, an evidence is provided that the non-parametric approach, despite its simplicity, could lead to equivalent or better results than the parametric approaches with distributional approximations. Concluding, in practice, the non-parametric approach should be considered as the first choice to speckle modeling before a particular distributional approximation is utilized.

The effect of intraocular pressure elevation and related ocular biometry changes on corneal OCT speckle distribution in porcine eyes.

The aim of this study was to evaluate the influence of increase in intraocular pressure (IOP) and cooccurring changes in ocular biometry parameters on the corneal optical coherence tomography (OCT) speckle distribution in ex-vivo experiments on porcine intact eyes. Twenty-three eyeballs were used in the inflation test where IOP in the anterior chamber was precisely set from 10 mmHg to 40 mmHg in steps of 5 mmHg and where eye biometry was utilized (IOL Master 700). To assess the influence of the duration of the experiment on the OCT speckle statistics, the second experiment was performed with 10 eyeballs at the constant IOP of 15 mmHg. Based on the OCT scans of central cornea (Copernicus REVO), spatial maps of the scale parameter (a) and the shape parameter (v) of the gamma distribution speckle model were estimated. The means of both parameters for each spatial map were computed within the 2 mm of the central stroma. Both distributional parameters statistically significantly varied with IOP and time (one way repeated measures ANOVA, all p-values < 0.001). The a parameter revealed a faster statistically significant increase in IOP up to 25 mmHg, regardless of time. Central corneal thickness (CCT), the anterior chamber depth, and the mean equivalent spherical power varied significantly with IOP, whereas CCT and axial length changed statistically significantly with time. Statistically significant correlation was found between CCT and the a parameter, after removing IOP as a confounding factor (r = -0.576, p < 0.001). The parameters of the gamma distribution can be used not only for identifying IOP induced changes in the optical scattering within the corneal stroma, but also in corneal geometry. The approach of corneal speckle analysis could be potentially utilized for an indirect and noninvasive assessment of some properties of corneal stroma.