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Guide ocular pursuits - eye tracker system

How to ensure the alignment of the eye and the laser during the operation?

 

Centering of the excimer laser correction is a fundamental parameter to ensure a precise and effective correction. It is important regardless of the type of Visual defect corrected. The experience of many practitioners who perform refractive surgery, a good balance of treatment is particularly crucial for the correction of hyperopia and some astigmatismes.

Eye tracking (eye tracking) system

Modern lasers are equipped with sophisticated systems grouped are the term 'Eye tracking system' or 'Eye tracker', which allow the licensing system (the system to deliver the laser impacts) to follow the eye, although it travels small, jerks, during surgery. This technology is comparable in its principles to that team some military equipment (guidance of shooting lasers to destroy missiles in flight), where also the problem of a shot on a moving target.

During surgical correction laser, the surgeon tells systematically the patient a fixation point, which is a focus of green or red (most often). The patient then directs his eye in such a way that it forms a retinal image Foveal of this benchmark. It is important to do this step correctly, because the attachment is not automatic and requires the participation of the patient.

The eye naturally performs a small micro-saccades, even when he fixed a stationary marker.  Eye-tracking systems that equip the excimers lasers are equipped with a video camera high frequency, able to acquire very many images of the eye per second: more than the eye itself, these infra red cameras are able to identify the IRIS Pupil (illuminated by infrared lamps that illuminate the ocular surface without dazzle the patient).

eye tracker camera

Camera eye (laser Wavelight EX500) prosecution system

 

 

For these cameras, the pupil appears as a dark disc, surrounded by the iris whose details can be used for procedures with Iris recognition. In this type of procedure, surgery is preceded by a black and white of the iris image acquisition (ex: instrument Topolyzer Vario) (see the) page dedicated to the centering of a laser treatment procedure by recognition with with laser EX500)

If the laser treatment is designed to be centered on this disc (Center of the pupil) or on a supposed Visual axis (usually slightly in nasal of the center of the pupil), but as the eye moves during treatment, or that it "turns" on its axis (rotation cyclo) laser rectifies in real time the position of the mirrors which are responsible for guided the impacts to the cornea. This is done while the latest lasers have a frequency equal or greater than 500 Hertz (or 500 shots per second).

These sophisticated eye-tracking and guidance systems can be completed by passive mechanisms to stabilize the eye. A system called "Neurotrack" prevents the occurrence of a cyclorotation, i.e. a mismatch between management assumed horizontal, and orientation of the eye of the surgery patient. With the laser Wavelight EX500, 4 Orange LEDs form a rectangle large horizontal axis at the centre of which is located the target set. The eye of the patient tends to align "instinctively" according to the centerline of the rectangle formed by the diodes.

These technologies complexent requires a particular system of illumination of the operating field. A patient who goes for the first time under an excimer laser discovers a complex Assembly of lights, flashing lights, LEDs...

The following figure shows the system allowing the EX500 laser to acquire images of the eye during treatment, to illuminate the operative field, provide a cue to fix during treatment eye.

eye-tracking laser excimer EX500

Representation of the system for enlightenment and the video capture of the field operation, useful to the servo to the eye prosecution system (eye-tracking system) - EX500 Wavelight excimer laser

 

At the end of each issued treatment, the EX500 excimer laser provides a specific statement that has a Cartesian coordinate chart to represent the possible displacement of the center of the pupil with respect to its original position.

eye tracker EX500

The report provided at the end of a correction of hyperopia laser EX500 (Wavelight). Eventual displacement of the eye for the fixing of the alignement(un point vert clignotant) indicator is offset by the system of eye-tracking, which identifies the shift of pupillary Center and shifts firing of the laser as a result. On the right, the graph represents, relative to the center of the tracking camera, the apparent movement (in Cartesian coordinates) of the pupil of the treated eye. 

The following video demonstrates the effectiveness of an eye tracker modern. Despite movements random printed to a target, the laser is able to deliver impact to high frequency (500 Hz or 500 shots per second) thanks to the video loop and the information transmitted to the laser mirrors, which allow to correct the path of laser spots.

 

 

A video "subjective camera" allows to view the appearance of the licensing system, from the patient's side during the surgery.

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