Digital tracking and control of retinal images

Laser induced retinal lesions are used to treat a variety of eye diseases such as diabetic retinopathy and retinal detachment. Both the location and the size of the retinal lesions are critical for effective treatment and minimal complications. Currently, once an irradiation is begun, no attempt is made to alter the laser beam location on the retina. However, adjustments are desirable to correct for patient eye movements and tissue inhomogeneities. Lesions form in much less than one second and typical treatment for a disease such as diabetic retinopathy requires as many as 2,000 lesions per eye. This type of tedious task is ideally suited for computer implementation. An instrumentation system has been developed to track a specific lesion coordinate on the retinal surface and provide corrective signals to maintain laser position on the coordinate. High resolution retinal images are acquired via a CCD camera coupled to a fundus camera and video frame grabber. Optical filtering and histogram modification are used to enhance the retinal vessel network against the lighter retinal background. Six distinct retinal landmarks are tracked on the high contrast image obtained from the frame grabber using two-dimensional blood vessel templates. The frame grabber is hosted on a 486 PC. The PC performs correction signal calculations using an exhaustive search on selected image portions. An X and Y laser correction signal is derived from the landmark tracking information and provided to a pair of galvanometer steered mirrors via a data acquisition and control subsystem. This subsystem also responds to patient inputs and the system monitoring lesion growth. To confine the laser position within a 100 micron radius circle at a retinal velocity of 50 degrees per second requires a position update 150 times per second. The development system is currently implemented on a 486 personal computer hosting a video frame grabber and data acquisition and control hardware. With a 33 MHz processor the current implementation provides 100 micron target radius at retinal velocities less than two degrees per second. This paper begins with an overview of the Robotic Laser System design followed by implementation and testing of a development system for proof of concept. The paper concludes with specifications for a real time system.