Analysis of keratoscopic images for detecting fixational eye movements and ocular surface deformation

Abstract

A sequence of videokeratoscopic images was registered using commercially available instrument E300 at a rate of 50 fps. During the 20 seconds measurement, subject's head was fixed strongly. Acquired images were analyzed for detecting fixational eye movements and corneal surface deformation. For this purpose two rings were extracted from each frame and the ellipses were fitted to them, using least square method. The time series of the ellipses geometrical parameters were considered: minor and major axes length as well as the ellipses center and the orientation. The frequency spectra of mentioned parameters were obtained by application of the Fast Fourier Transform. The longitudinal position of the corneal apex was controlled, thanks to the cone side viewer installed inside the videokeratoscope. The average amplitude of the variation of the ellipse's axes length is around 20μm and of the orientation of the ellipse around 0,1 rad. In the signals frequency characteristics, appear the peak corresponding to the heart rate. No clear relationship was found between the variations of the fitted ellipse parameters and the longitudinal position of the corneal apex. The fixational eye movements were examined using two different methods. One of them consists of calculating the correlation function between the first and successive frame of the sequence and searching its maximum. The other is based on tracking the center of the ellipse fitted to particular ring of the videokeratoscopic image. The accuracy of the second method found to be higher. Simple methods proposed in this work can extend the application of videokeratoscopic measurements.

Ultrasonic In Vivo Measurement of Ocular Surface Expansion

“…However, examination of corneal curvature pulsatile changes using commercial or custom designed videokeratoscopy did not provide any clear evidence of existence of such deformations [20], [21], [28].…”