Birefringence properties of the human cornea measured with polarization sensitive optical coherence tomography

We map the three-dimensional distribution of birefringence of the normal human cornea and provide insight into structures and mechanisms causing corneal birefringence, establishing standard patterns of 3D birefringence distribution. A polarization sensitive optical coherence tomography (PS-OCT) system was developed that allows measurement and imaging of three tissue parameters simultaneously: reflectivity, retardation, and slow optic axis orientation. This instrument was used to obtain 3D PS-OCT data sets of normal human corneas in vitro. From the 3D data sets, conventional cross sectional, as well as en face images of reflectivity, retardation, and optic axis orientation were derived. Preliminary results from a healthy cornea in vivo and a keratoconus cornea in vitro are also presented. In transversal direction the retardation distribution of the normal cornea has a radially symmetric shape; retardation is lowest at the center of the cornea and increases towards the periphery. At peripheral regions, retardation also increases with depth. The distribution of the optic axis is not constant with the parallel illumination scheme used. Optic axis orientation is an approximately linear function of azimuth angle, however, if averaged over the entire cornea, a preferential optic axis orientation is observed. In a keratoconus cornea, the normal birefringence pattern is heavily distorted. The results provide additional insight into corneal birefringence as compared to published work where corneal birefringence is usually averaged over a larger area. The results can be explained by a birefringence model based on stacked collagen fibril lamellae of different orientations. The observed birefringence patterns in normal corneas might be used as standard patterns for comparisons with pathologic changes.