Intraoperative corneal topography for image registration

PURPOSE: To present a method to measure the three-dimensional shape of the cornea and to use the data for registration purposes in order to optimize ablation pattern alignment during corneal laser surgery. METHODS: The three dimensional shape of the cornea can be measured with a modified fringe projection technique using UV laser pulses. A method to register these shape images is presented. The registration is done via established algorithms that use peripheral elevation data, which is not affected during the laser treatment. The method also provides a means to control the absolute amount of tissue removal. The three-dimensional registration method is compared with conventional two-dimensional eye tracking. RESULTS: Due to the parallax of the cornea with respect to the pupil center, considerable decentration of laser ablation patterns can occur when tracking just the pupil center. Registration using three-dimensional shape measurements provides a more accurate means to control ablation pattern application. CONCLUSIONS: A new method to register corneal shapes is discussed. It should allow monitoring the real ablation rate online during the treatment and might eventually serve as an online feedback system to control the laser ablation-induced corneal shape changes.