Tracking irregular peripheral image rings in videokeratography

摘要

Videokeratography is a common method used by clinicians and researchers to estimate the surface topography of the human cornea. It is based on the object-to-image relationship of concentric rings reflected off the surface of the cornea. This technique works reliably in most cases for central cornea. However, the accuracy of corneal topography is reduced for peripheral cornea because of shadows caused by brows and nose and occlusions caused by eyelids. To achieve a broader coverage of the peripheral cornea, images of off- centered gaze in four directions could be combined. One of the difficulties associated with this approach is that the shape of image rings in the peripheral cornea become very irregular, z-shaped, due to abrupt change sin surface topography near the limbus. These irregularities cause complications for current algorithms for estimating the location of edges along each image ring. Many current algorithms make assumptions about he shape and relative positions of image rings to distinguish between different rings. These assumptions no longer hold with off-centered images since the image rings can deviate dramatically from an ellipsoid. Our algorithm overcomes this problem by using fewer assumptions combined with a robust segmentation algorithm to distinguish between image rings.