The MEL 80 Excimer Laser is the sixth-generation laser from Carl Zeiss Meditec AG (Jena, Germany); it possesses the Oculign, a highly sophisticated eye tracking and registration system designed for high volume practices that demand efficient patient flow. The MEL 80's advanced eye tracking system balances very high reliability with ease of use.

Eye tracking comprises a set of events intended to place an excimer laser spot in a specific predetermined location on the cornea relative to the visual axis. This closed loop of events includes imaging of the eye; transferring data; interpreting landmarks; computing the position of the eye in x-, y-, and z-space; adjusting aiming beam mirrors and delivering the excimer laser pulse to a specific location. Each step adds delay, and the total delay (ie, latency) for the system is what determines the efficacy of eye tracking.

As a rule of thumb, it is necessary to accurately position a laser spot within at least one-third of the diameter of the spot. Therefore, the complete eye-tracking loop must be fast enough to enable a shot to be fired before the eye can move more than one-third of the spot diameter away from the detected position. This is a similar situation to baseball batsmen predicting the flight of a pitch. A batsman must judge the flight of the ball; they swing the bat based on this judgment. The batsman cannot—realistically—change the course of the swing once committed, so a curve ball may lead to a strike. The time between deciding where the ball is going (ie, position detection), activating the right muscle groups (ie, mirror alignment), and swinging (ie, laser shot) is the latency. The actual latency of the Oculign MEL 80 tracking system is 2 milliseconds to 8 milliseconds, depending on the amount of eye movement and the distance that must be corrected for between laser pulses. The mean total latency during a treatment is approximately 6 milliseconds.

Because the Oculign system tracks multiple anatomical elements within the eye, tracking is still possible under difficult conditions, allowing both eye position and rotation to be accounted for during ablation. Undilated pupil margin, iris features, limbus, and conjunctival vessels are all employed to make the Oculign an extremely stable system.

By considering every component of the eye-tracking cycle, latency is minimized. The Oculign system uses a high-resolution full field 250-Hz infrared camera that matches the 250 Hz firing rate of the MEL 80's 0.7-mm Gaussian spot. One limitation of previous infrared eye trackers was a limited infrared dose, as excess exposure could lead to tissue heating or damage. This restriction limited the detection contrast intraoperatively through a roughened ablated surface. The Oculign system overcomes this problem by using a unique 250-Hz pulse illumination for the infrared light source. This is designed to allow higher infrared intensity for image acquisition, while minimizing ocular exposure when images are not being captured. It is based on the principle of flash photography. Imagine trying to keep your eyes open if the camera's flash unit was continuous, rather than a short flash when the image is captured. The increased infrared intensity of the Oculign system vastly improves the detection contrast, and so the Oculign system can use contrast thresholding to automatically lock onto the eye during treatment. Thus, there is no need for the surgeon to adjust knobs to lock the tracker onto the eye, which greatly improves clinical flow efficiency and ease of use.

STABILITY, IMAGE QUALITY
The Oculign system optimizes the image processing time by weighting the computation toward pupil tracking as it is the least computationally intensive component. Concurrent limbus tracking provides increased stability during treatment, where the image quality can vary due to the ablated surface.

Internal algorithms account for parallax errors due to eye-roll. Additionally, passive tracking exists, so that the eye moves outside of a specific hot-zone—firing stops until return of the eye to within the hot zone.

The number of eye position detection instances with the Oculign system is equivalent to the number of laser shots fired, which is important to ensure that spots are not fired into stray positions. Trackers that detect eye position slower than the shot frequency are known to produce rougher ablation surfaces due to small errors. This is particularly true for smaller spot scanning lasers.

EYE REGISTRATION
As mentioned earlier, the Oculign system uses more than just iris features to register the eye; it uses a unique combination of the pupil edge, iris features, limbus, and conjunctival blood vessels to optimize ocular registration. Using these features, the Oculign can be used to offset treatment in x and y directions. It also measures and compensates for cyclotorsion (Figure 1).

One of the most interesting features of the Oculign algorithm is in the pupil centroid shift compensation. The WASCA high-resolution wavefront sensor is used to acquire images of the eye under both scotopic and photopic conditions (Figure 2). This enables the optimization of custom treatment registration to the pupil center, accounting for possible pupil center shifts due to variable lighting conditions between wavefront or topography data acquisition and the operating microscope. Online compensation for pupil centroid shift probably contributes to the superior wavefront-guided outcomes of the MEL 80.

ERGONOMICS
From the user's standpoint, several features make the Oculign system ergonomic and easy to use. Particularly from a surgeon's standpoint, the automatic contrast thresholding (ie, manual adjustment is not required for the tracker to lock onto the eye) means that setting the tracker is a matter of switching it on. Another user-friendly feature is the logical and simple arrangement of buttons, allowing the center of treatment to be quickly and easily adjusted to compensate for angle-kappa fixation shifts in individual eyes. Oculign works like a point-and-shoot system, despite its very high degree of complexity. With the patient on the table, the action of swinging-in the cone for controlled atmosphere activates the cascade of computation within seconds, enabling the surgeon to start the ablation without delay.

Dan Z. Reinstein, MD, MA(Cantab), FRCSC, DABO, is medical director of the London Vision Clinic and Consultant Ophthalmologist at St. Thomas' Hospital–Kings College, in London. He states that he is a consultant to Carl Zeiss Meditec AG. Dr. Reinstein may be reached at dzr@londonvisionclinic.com or +44 20 7224 1005.