Method and apparatus for providing precise location of points on the eye

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to improvements to optical aiming systems used in conjunction with excimer laser eye surgery systems, and more specifically, to a method and apparatus for improving the effectiveness of such an optical aiming system in locating points on the eye.

2. Description of the Related Art

The advent of the excimer laser opened a whole new realm of possibilities for eye surgery, providing a non-invasive technique for resculpting the surface of the eye itself to match a desired curvature. Such systems are well known in the art, and are further described, for example, in the inventor's PCT Application PCT/EP93/02667 which is hereby incorporated by reference, as well as in various patents to L'Esperance, such as U.S. Pat. No. 4,665,913.

To improve the accuracy of these excimer laser surgical devices, it is preferable to precisely place each shot from the excimer laser at the desired location. A number of techniques and devices have been developed to achieve this end. For example, a simple fixation light has often been used. Patients fixate their gaze upon that light, generally lessening slow eye movement. This technique does not, however, prevent rapid movements of the eye. Further, a momentary lapse in fixation could result in an ablation shot far from the intended shot location. As an alternative, physical fixation devices have been used which immobilize the eye by physically connecting to the eye, thereby holding it steady.

A more recent technique involves the use of computer aided eye tracking devices. These are optical or topographic location systems that typically use a video camera to either optically or topographically locate and track the center of the eye. Each shot can then be placed at any desired location on the eye relative to that center. Examples of such systems can be found in U.S. Pat. Nos. 5,098,426 to Sklar et al., 5,162,641 to Fountain, and 4,848,340 to Bille. These systems use various techniques to track the center of the eye, such as a computer mapped digital image from a video camera. For example, U.S. Pat. No. 5,098,426, to Sklar, et al., hereby incorporated by reference, describes an eye tracking system that generates a three dimensional profile of the eye and tracks movement by noting changes in that profile. The Sklar patent shows an eye tracker using a slow control loop and a fast control loop. The slow control loop relies on a video camera to provide topographical information that the eye tracker then uses to aim the system optics.

An alternative eye tracking system is shown in U.S. Pat. No. 4,848,340 to Bille, also incorporated by reference. The Bille patent shows a strictly optical, rather than topographical, based system that tracks a reference grid which has been ablated into the eye.

Another eye tracking system using infrared light to illuminate the pupil of the eye has been announced by ISCAN, Inc. This system is described as using infrared light to illuminate the eye, with the system then returning positioning information to a variety of applications, such as computer control through eye movement and assistance to the disabled.

Any of a various number of techniques for locating objects can be readily adapted to locate the center of the eye. It would be desirable, however, to improve the effectiveness and accuracy of such systems. That is, given an object location system used in conjunction with an excimer laser system, it would be desirable to provide other improvements that enhance the ability of those systems to accurately locate the center of the eye and provide for accurate aiming of the pulsed excimer beam onto the eye.

These eye tracking systems are not without problems, however. First, misalignment of the optics can result in offsets of where each excimer laser shot actually falls relative to where it should fall. For example, servomotors can be slightly miscalibrated, resulting in these offsets. It would thus be desirable to provide a method and apparatus for eliminating the effects of such miscalibrations.

Second, these systems tend to be either invasive or complicated, in the sense that they require actual physical markings to be made on the eye, as shown in the Bille patent, or require highly complex topographical location systems and multiple feedback loops for locating the center of the eye, as shown in Sklar patent. Thus, simpler methods of providing a reference to the center of the eye would be desirable.

SUMMARY OF THE INVENTION

In accordance with the invention, an aiming fixture is provided for an eye tracking system so that the eye tracking system can achieve more accurate registration of the location of the eye. In variations according to the invention, the aiming fixture is provided as a triangle, hexagon, or other regular object. Further, the fixation ring is preferably constructed to provide registration at one point to determine rotation of the fixation ring.

Further according to the invention, a registration laser is provided along the optical path of the pulsed excimer beam, allowing an eye tracking system to accurately determine where the next excimer laser shot will fall on the surface of the eye.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which:

FIG. 1 is a diagram illustrating a typical excimer laser eye surgery system in which can be implemented the apparatus and method according to the invention;

FIG. 2 is a diagram illustrating how a registration beaming from an aiming laser spot according to the invention can be used to precisely position the excimer laser system for the next pulsed excimer shot;

FIG. 3 is a flowchart illustrating how software can be used in conjunction with an eye tracking system and an ablation profile system to more precisely aim the excimer laser using the registration spot illustrated in FIG. 2;

FIGS. 4A and 4B are top and side views of an aiming fixture according to the invention to improve performance of an eye tracking system;

FIGS. 5A and 5B are top and side views of an alternative embodiment of the aiming fixture of FIGS. 4A and 4B; and

FIG. 6 is a top view of another alternative embodiment of the aiming fixture of FIGS. 4A and 4B.

FIG. 7 illustrates a cross-sectional view of FIGS. 5A and 5B; and

FIGS. 8A-8D illustrate alternative profiles to the profiles shown in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, FIG. 1 shows the typical eye surgery system 10 in and with which the method and apparatus according to the invention would be implemented. An excimer laser 20 provides a pulsed beam 22 to a beam homogenizer 24 after reflection from optics 26. A shutter 28 is also provided to block transmission of the pulsed beam 22 to the beam homogenizer 24. The excimer laser 20 is a typical excimer laser as is well known in the art. It preferably provides a 193 nm wavelength beam with a maximum pulse energy of 400 mJ/pulse. The excimer laser 20 preferably provides maximum average power at the treatment site of 1 W, with a pulse frequency of 10 Hz and a pulse length of 18 ns. Of course a variety of other excimer lasers could be used, and the apparatus and method according to the invention further have application where a laser other than an excimer laser is used. By way of example, the wavelength of the light from the laser is preferably less than 400 nm, as that provides the desired ablating action with reduced thermal heating. Further, other pulse energies can be provided, such as all the way down to 200 mJ/pulse, with typical repetition rates of 60 to 100 pulses per second with a typical pulse length of 10 to 30 ns. Again, all of these are merely typical values, and deviation from them can be made without changing the spirit of the apparatus and method according to the invention. Further examples of such laser systems can be found in U.S. Pat. Nos. 4,665,913, entitled "Method for Ophthalmological Surgery," issued May 19, 1987, and 4,729,372, entitled "Apparatus for Performing Ophthalmic Laser Surgery," issued Mar. 8, 1988.

The beam homogenizer 24 preferably includes standard homogenization and focusing hardware, which can be based both on optical mixing of the beam and on rotation of the beam. For an example of typical beam homogenization hardware, see U.S. Pat. No. 4,911,711 entitled, "Sculpture Apparatus For Correcting Curvature Of The Cornea," issued Mar. 27, 1990. From the beam homogenizer 24, the pulsed beam 22 is then reflected off of optics 30, which also passes an aiming beam from an aiming laser 32. This aiming laser 32 is preferably a red 633 nm helium neon laser of less than 1 mW/cm.sup.2 of power. The aiming beam from the aiming laser 32 can also be blocked by a shutter 33. The aiming laser 32 is aligned so that its optical pathway coincides with the pulsed beam 22. The aiming laser 32 provides an aiming beam spot that coincides with the central axis of the laser shot of the pulsed beam 22.

A registration laser 35 also provides a registration beam reflected by optics 34. The registration laser 35 preferably is of a wavelength of approximately 950 nm, or near infrared, and preferably is low power, less than 1 mW/cm.sup.2.

The size of the registration beam from the registration laser 35 is preferably small, less than 0.5 mm in diameter. This registration beam provides for precise aiming of the pulsed beam 22, as is discussed below in conjunction with the discussion of FIGS. 2 and 3. Although the separate aiming laser 32 and registration laser 35 are disclosed, these could be combined to provide a single aiming/registration beam depending on subsequent optics in the system. The aiming beam from the aiming laser 32 and the registration beam from the registration laser 35 are preferably both coaxially aligned with the pulse beam 22.

In the disclosed embodiment, the registration laser 35 and the aiming laser 32 are separate because, as discussed below, while the aiming laser 32 provides a spot of visible light to the surgeon, that light is filtered out by the imaging system. Surgeons need to directly observe the eye 44 during manual surgery, for example, such as when they manually perform theraputic surgery or when they manually designate the location of the center of the eye. Then, the visible light is necessary. With an alternative embodiment in which the imaging system does not remove the visible light, for example, the aiming laser 32 and the registration laser 35 could be one and the same.

From the optics 30, the pulsed beam 22 (now also co-aligned with the aiming beam from the aiming laser 32 and the registration beam from the registration laser 35) then passes through an adjustable diaphragm 36, which allows the beam size of the pulsed beam 22 to be adjusted before it enters the final optics.

Following the adjustable diaphragm 36, a focusing lens 40 directs the pulsed beam 22 onto a scanning mirror 42, which then reflects the beam 22 onto a patient's eye 44. The scanning mirror is preferably capable of moving a beam at 5000 mm/sec at the surface of the eye 44. The focusing lens 40 focuses light such that when the eye 44 is at the optimal distance, the pulsed beam 22 is properly focused onto the eye 44.

These various lenses and mirrors thus combine to form an optical system providing an excimer beam to the cornea. The optical system creates a laser spot on the cornea, and the spot size is adjustable, along with its location. It will be readily appreciated that a wide variety of different systems could be used to optically provide such a beam. For example, a lens could be used to adjust the spot size rather than an aperture, and instead of a scanning mirror, the patient or the patient's eye 44 could be physically moved to provide for shots at different locations on the eye 44.

Also provided in the system according to the invention is a focusing laser 46, whose beam can also be blocked by a shutter 48. The focusing laser 46 is preferably a green helium neon laser providing a beam of a wavelength of 535 nm and less than 1 mW of power. The beam from the focusing laser 46 travels through optics 50 and impinges on the eye 44 at an angle. The distance of the eye 44 from the eye surgery system 10 is adjusted such that both the beam from the aiming laser 32 and the beam from the focusing laser 46 impinge on the surface of the eye 44 at the same point.

A clean gas purge unit 54 ensures that the optics and the beams in the system are free from any floating debris.

A microscope 56 is provided for the physician to observe progress during ablation of the surface of the eye 44. The microscope 56 is preferably a ZEISS OPMI "PLUS" part No. 3033119910, with magnifications of 3.4, 5.6 and 9.0 times. Field illumination is provided by a cold light source not shown, which is preferably the Schott KL1500 Electronic, ZEISS part number 417075. This microscope 56 focuses through the scanning mirror 42 and also focuses through a splitting mirror 58. The splitting mirror further provides a view of the eye 44 to a video camera 60. The video camera 60 is preferably sensitive to both visible and infrared light, and is preferably a high resolution S-VHS camera with 400,000 pixels, and generating 50 frames per second, although it could be any of a variety of other cameras or detection grids, including an NTSC camera with a 60 frame per second rate. The video camera 60 preferably provides an image output to a capturing video screen 62 and to a control unit 64. The video camera 60 is preferably capable of producing digitized output to provide to the control unit 64.

Preferably filtering light into the video camera 60 is an infrared filter 66, which only permits infrared light to pass through. This would permit for example, a spot created by the registration beam from the registration laser 35 to be perceived by the video camera 60. Thus, the video camera 60 and infrared filter 66 combine to form an infrared sensitive video unit.

In addition to visible light, the eye 44 is also illuminated by an infrared light source 68. The infrared light source 68 is preferably a 880 nm diffuse light source from a 10 LED array, but could be any of a number of other known sources, such as a halogen lamp with an appropriate filter. The infrared light source 68 is preferably of lower intensity than the registration beam from the registration laser 35, and in any case less than 1 mW/cm.sup.2. The infrared light source 68 is preferably controlled by the control unit 64 and provides either a fixed or adjustable degree of illumination of infrared light onto the eye 44. It will be appreciated that the image of the eye 44 illuminated by the infrared light source 68 is also perceptible by the video camera 60 through the infrared filter 66.

It has been found that use of the infrared light source 68 in conjunction with the infrared filter 66 improves the contrast of the features of the eye 44, no matter what intensity of visible light is cast on the eye 44. The control unit 64 can then control the infrared light source 68 to provide the desired contrast at the video camera 60. In this way, the aiming function of the control unit 64 and the movable mirror 42 are unaffected by changes in visible light onto the eye 44. For example, if the surgeon needs more illumination, he can adjust an visible light source, while the infrared light source 68 remains being controlled by the control unit 64. By providing the infrared light source 68, contrast is thus improved, and the performance of an eye tracker used in conjunction with the eye surgery system 10 is thus improved. This improved contrast is even apparent when the infrared filter 66 is omitted.

The control unit 64, which is typically a high performance computer compatible with an IBM PC by International Business Machines Corp., preferably controls all components of the eye surgery system 10, including the shutters 28, 34, and 48, the diaphragm 36, the spot mode lens 38, and the scanning mirror 42, and the infrared light source 68. Ablation profiles software runs on the control unit 64, such as the ablation software described in the inventor's PCT application PCT/EP93/02667. The various types of ablation software known to the art would all benefit from the improved aiming and registration provided by the invention.

The control unit also preferably contains an eye tracking system 70. In one embodiment, the eye tracking system 70 may be a proprietary software system developed for Chiron Vision/Technolas, which runs on one Transputer.TM. manufactured by INMOS Limited used in conjunction with a Transputer Frame Grabber.TM. manufactured by Parsytech, GmbH. The eye tracking system 70 preferably receives the digitized output from the video camera 60 and then provides coordinates of the center of the eye on that video image relative to a preset origin. Further, the eye tracking system 70 should provide the coordinates of an infrared spot on the eye 44 created by the registration laser 35. These coordinates are then used by the ablation profile software in the control unit 64 to aim the scanning mirror 42 for the next shot from the excimer laser 20.

Other eye tracking systems which can be utilized with similar effect in alternate embodiments are known to the art and include those described in the Sklar and Bille patents discussed above, as well as the devices marketed by ISCAN.

FIG. 2 is a diagram illustrating how th