The eye tracker is an important component of any excimer laser system, and there is not one procedure in which I do not employ its use. One indication of a good tracking system is when the camera frequency is larger than the laser spot frequency. The Schwind eye tracker (Schwind eye-tech-solutions, Kleinostheim, Germany), my system of choice, has this capability.

The reason that the camera frequency should be larger than the laser frequency is to avoid having more laser spots with less information from the eye tracker. I have used the ESIRIS Schwind Technology laser (Figure 1) for more than 4 years, and I have never had any problem.

Since the establishment of a passive eye tracker in 1994, the company has improved the device tremendously. One improvement is that the Complementarty Metal Oxice Semiconductor camera now works at a 330 Hz frequency, and the algorithm is more robust and sophisticated, allowing for better pupil recognition throughout the surgical procedure. Now, even if the pupil is partially covered by a surgical instrument, the tracking is capable to recognize the proper public center. The frequency of the laser source in my ESIRIS is 200 Hz.

The laser beam automatically follows the movements of the patient's eye and supports the eye tracking, so that an optimum treatment result is achieved, even when the patient does not fixate properly. For safety reasons, the active tracking is reduced to a limited area of ±1.5 mm. If the patient's eye is found outside of this area, the treatment will be interrupted. In this case, the function is equivalent to a passive eye tracker.

Another feature of the Schwind eye tracker is the arbitrary moving of the ablation center and shifting away from the pupil center. The eye tracker recognizes the pupil, but as a surgeon I can decide where to go with my centration of the ablation. On demand, the ablation can be precisely shifted in accordance to the diagnostic information. Because of this, it might be shifted in respect of the Purkinje image or in respect of the corneal apex.

The tracker is based on a straightforward design. Nothing has to be moved to a special position to make the tracker function. I simply prepare my patient, and in the moment the foot pedal is pressed the treatment will start. Furthermore, the eye tracker is independent from the environmental light conditions and from the pupil size and iris color.

COMPENSATES FOR MOVEMENT
Safety is a major concern during any ophthalmic procedure. The Schwind eye tracker ensures a safer procedure by compensating for any patient movement, the slow-drifting motions, and the quick saccadic eye movements.

If a short interruption occurs (eg, the patient twitches); the pupil is out of the active tracking area; or you have to stop the laser with your foot for any other reason, the laser delays for a few milliseconds before the treatment is continued. The eye tracker automatically calculates the next spot location, and the surgery resumes.

A highly specialized electronic processor extracts the center of the pupil by using a very safe tolerant algorithm. It is tolerant in the way of there being less interference through other circumstances (ie, a partly visible pupil, if the pupil is covered by a sponge or surgical instruments.)

The tricky part is that the tracker does not use a standard algorithm to find the center of the pupil by gravity; it uses a mathematical calculation of putting tangential lines to the pupil's edge. Each tangential line has, in the intersection point, another perpendicular line. These perpendicular lines will all meet in one point, which represents the center of the pupil. The algorithm itself looks for this accumulation of intersections.

As an indication of the reliability, the software calculates a quality in values between zero and 255. All quality values between 120 to 255 indicate a reliable recognition of the pupil, resulting in a green highlighted bar on the computer screen and all values below will not be accepted. These values will result in a warning message in a red highlighted bar.

The treatment is done by turning/diversion of the scanning-mirrors (which work at a frequency less than or equal to 700 Hz) according to the movements of the pupil from the taught-in position and the algorithm for the chosen treatment. If the diversion gets larger than the limit, the treatment is interrupted, and the patient is asked to fixate again. If the foot pedal is still pressed, the treatment continues automatically at the correct point, as soon as the eye is repositioned and the patient fixates the target again.

The summary of an eye tracker's reaction time is based on the speed of the camera, plus the time to process this data and the time for the scanner to react and to position. It is also dependent on the time needed at the excimer laser to provide the pulse. With these factors, an overall time of more than 6 milliseconds is needed.

COMPARE SYSTEMS
When deciding on what eye tracker is best for you, first compare multiple eye-tracker systems. Look at each system. Does it look like a system that will be easy to use? Do you think it will improve the number of spots that are within your intended ablation position? What is the time of response? Remember the accumulation of times; we have a chain of parts responsible for this response, and not only the camera. Also, make sure that you can shift the treatment from the pupil.

In the future, I think that eye-tracking systems will become even faster than they are now. If there is an advantage to working with a 500-Hz frequency, then I would like to see Schwind's system move to this speed. With that being said, I must admit that I have never had problems with my eye tracker. Although I cannot compare the Schwind eye tracker with other systems, I do believe it is extremely important to use any eye-tracking device during every surgery. I cannot imagine obtaining such exacted and safe results without one.

D. de Ortueta, MD, FEBO, is medical director at the Eye Centre Recklinghausen, in Recklinghausen, Germany. Dr. de Ortueta states that he is a consultant for Schwind eye-tech solution. He may be reached at ortueta@gmx.de.