Abstract:
An eye tracking and positioning system for use with a refractive laser system includes a camera interface, a computer, and a system for moving the patient relative to the laser beam. The computer includes a video frame grabber which extracts images of the eye from the camera, and is programmed to perform an eye tracking algorithm. The eye tracking algorithm calculates the exact center of the eye pupil, and compares the center with the desired location of the laser beam, as determined by a surgeon, with an image processing algorithm. If the relative location of the eye center and the laser beam fall outside a predetermined value, the patient chair, and thereby the patient, is repositioned relative to the laser beam, as opposed to the laser beam being repositioned relative to the patient. The repositioning counters the movement of the eye.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates broadly to eye surgery. More particularly, this invention relates to refractive laser systems for eye surgery. 
     2. State of the Art 
     The excimer laser refractive surgery field has exploded over the past few years with many new lasers systems to correct human vision. These systems use an ultraviolet excimer laser to change the shape of the cornea in a calculated pattern which makes it possible for the eye to focus properly. For example, in the treatment of myopia, the laser is used to remove tissue from the cornea in order to flatten its shape. The correction of hyperopia is produced by steepening the cornea. The correction of astigmatism requires the laser to remove tissue in a more complex pattern. All of these procedures require precise shaping of the cornea which depends on accurate placement of the laser beam. Therefore, any eye movement can affect the placement of the laser beam. 
     Most currently approved broadbeam refractive laser systems and scanning spot systems do not incorporate eye tracking. Therefore, the patient is required to minimize eye movement during surgery by voluntarily fixating their eyes on a small light located just above the patient. When the average person fixates on something, he or she has about five saccadic eye movements per second. Saccadic eye movements are rapid, involuntary movements that are random in amplitude and direction. These movements can cause eccentricity of the laser beam, resulting in degraded laser vision correction predictability and visual quality. 
     Newer generation refractive laser systems that use small scanning spots, usually less than 1 mm in diameter, have implemented eye-tracking techniques that move the laser beam to adjust for eye movement. Early indications suggest that this approach provides a higher accuracy ablation by virtually eliminating shaping error caused by eye movement. 
     Nevertheless, there already exists a large number of broadbeam and scanning spot systems which are not provided with eye tracking capability. There is currently no available means by which to provide existing systems with eye tracking capability. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an eye tracking and positioning system for a refractive laser system which compensates for eye movement in a manner different than prior art eye tracking devices. 
     It is another object of the invention to provide an eye tracking and positioning system which is capable of being retrofit to existing broadband and scanning spot refractive laser systems without modifying any hardware in the laser system. 
     It is another object of the invention to provide a refractive laser system which is adapted to move the patient, and thus the eye of the patient, relative to the laser beam. 
     In accord with these objects, which will be discussed in detail below, an eye tracking and positioning system is provided for use with a refractive laser system which produces a laser for surgically reshaping the eye. The eye tracking and positioning system includes a means for capturing images of the eye, a computer, and a means for moving the patient relative to the laser beam. The computer preferably includes a video frame grabber which captures images from a camera of the laser system, and is programmed to perform an eye tracking algorithm with respect to the images. The eye tracking algorithm calculates the exact center of the eye pupil in the image, and compares the center with the desired location of the laser beam, as determined by a surgeon, with an image processing algorithm. If the relative location of the eye center and the laser beam fall outside a predetermined value, the means for moving the patient relative to the laser beam are activated. The means for moving the patient is preferably a surgical bed, surgical chair, or headrest which is motorized to move the patient relative to the laser beam (as opposed to moving the laser beam relative to the patient) to make the necessary adjustment to the current position of the eye and thereby counter the movement of the eye. 
     The eye tracking and positioning system of the invention may be retrofit to the existing broadbeam and scanning spot systems which do not already include eye tracking capability. Additionally, the eye tracking and positioning system may be provided as an integral part of new refractive laser surgery systems. 
     Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a refractive laser system provided with the eye tracking and positioning system of the invention; 
     FIG. 2 is a flow chart of the process of the invention; and 
     FIG. 3 is a schematic view of the patient positioning interface of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to FIG. 1, a refractive laser system  10  is coupled to an eye tracking computer system  12  of the invention. The refractive laser system  10  includes a laser  14  coupled to a system processor  16 , a joystick  18  coupled to the system processor  16  for positioning a surgical chair  42 , and a surgical microscope  20  for viewing an eye of a patient seated in the surgical chair  42 . The refractive laser system  10  also includes a video-out camera port  22  coupled to the microscope, and a footswitch port  24  and a surgical chair port  26 . 
     The eye tracking computer system  12  generally includes a video camera interface  30  which is coupled to the video-out port  22 , a frame grabber  32 , and an image tracking processor  34 , preferably implementing a software algorithm described below. The computer system  12  also includes a footswitch interface  36  and a patient positioning interface  40 . The footswitch interface  36  is coupled between the footswitch port  24  and a footswitch  38 . When the footswitch  38  is activated, the laser  14  is activated to emit a laser beam, as described in more detail below. The computer  12  also includes a patient positioning interface  40  which is coupled between the surgical chair port  26  and the surgical chair (or bed or headrest)  42 . The surgical chair  42  is provided with motors capable of relatively rapidly repositioning the chair such that an eye of a patient in the chair is moved relative to a laser beam emitted by the laser  14 . 
     More particularly, the video-out camera port  22  to which the video camera interface  30  is coupled is typically a microscope beam splitter optical port which permits users to attach cameras thereto for recording the surgery and audience viewing of the surgery. The eye tracking system  12  takes advantage of one of these microscope beam splitter optical ports in order to monitor the eye via a provided video camera. For example, in the VISX™ laser, an electronic output signal port connector is provided which is attached to an internal CCD camera. On other systems an electronic signal splitter can be attached at the output of the camera so the signal may be captured by the video camera interface  30 . Alternatively, a separate camera (not shown) may be provided with the eye tracking system of the invention and added to the microscope beam splitter optical port in order to capture the images. That is, any number of methods and systems may be utilized to capture the image of the eye from the surgical microscope  20  used in performing the refractive laser surgery. 
     The frame grabber  32 , e.g., a National Instruments PCI 1408, takes the signal from the video camera interface  30  and converts it to a digital signal, preferably in real-time. The frame grabber  32  can be configured to capture color, monochrome or infrared images at varying speeds. The digitized signal is then converted to a digital image matrix for processing. This conversion occurs at the rate of the camera (from a typical 30 Hz for color cameras to as fast as 800-Hz for monochrome cameras). 
     The image tracking processor  34  receives the digitized image from the frame grabber  32  and generally (1) processes the digitized image for laser beam center, (2) processes the digitized image for pupil recognition, (3) determines the center of the pupil, and (4) calculates the offset of the current eye pupil center from a surgeon-desired offset and drives the appropriate surgical chair motors. Each of the functions of the image tracking processor  34  are preferably performed by an algorithm carried out by the image tracking processor  34 , and will now be described in more detail. 
     Referring to FIGS. 1 and 2, when the surgeon presses the footswitch  38 , the current image in the camera is digitally captured at  50  by the frame grabber  32  for processing. The digitized image is processed at  52  to determine a laser beam center (reference location) preferably by one of two methods. Both methods begin by monitoring the footswitch  38  during continuous acquisition of the eye image signal entering from the video camera interface  30 . 
     According to a first method  54 , the center of the pupil is used as the laser beam center. That is, the surgeon positions the eye with respect to an alignment marker (e.g., a reticle appearing in the microscope  20  or displayed on a video monitor). Positioning is performed by adjustment of the surgical chair  42  via movement of the joystick  18 . Once the eye is properly positioned, the surgeon presses the footswitch  38  and the image tracking processor  34  uses the current eye pupil center as the laser beam center. The image tracking processor  34  then continues to the next step of actively tracking the eye pupil center and directing the surgical chair motors so that the laser beam and eye are properly relatively positioned. 
     According to a second method at  56 , the surgeon or a surgical technician uses a laser system alignment marker (e.g., a reticle or a visible laser diode spot) to designate the laser beam center. This second method permits the laser beam to be centered away from the pupil center. When the surgeon has the eye at a desired location, the surgeon presses the footswitch  38  and the image tracking processor  34  captures the current image. For surgeon positioning, this image is preferably then processed to highlight the alignment marker, which appears brighter than the pupil in a monochrome image, or appears as a different color than the pupil in a color image. The alignment marker is determined and its center is found and recorded as the desired laser beam center. For surgical technician positioning, preferably a PC mouse cursor is used by the technician to mark the position of the reticle or the alignment spot. The coordinates of the mouse cursor are recorded as the desired laser beam center. The image tracking processor  34  then continues to the next step of actively tracking the eye pupil center and directing the surgical chair motors. 
     Regardless of whether the first or second methods are used for determining the laser beam center, the image tracking processor next determines at  58  the eye pupil center. That is, once the desired laser beam reference location is determined at  54  or  56 , the image tracking processor  34  begins to track the eye pupil center. There are several image processing methods that can be used to find the pupil and pupil center. One preferred method implemented by the image tracking processor  34  uses the contrast between the pupil and the iris to determine the pupil and then the center of the pupil by the following seven steps. 
     First, the alignment marker is removed at  60  from the image. Although the alignment marker is useful to help determine the desired laser center beam, it does interfere with detection of the pupil. As the alignment marker appears white against the darker pupil, alignment marker removal from a monochrome image is preferably performed with a high order low pass filter. In a color image, the marker is removed by ignoring the color plane that corresponds to its color (usually red). 
     Second, at  62 , the contrast between the pupil and the iris is enhanced. This is preferably done by applying a transfer function to the intensity values in the image to produce a bimodal histogram of intensity values. The transfer function increases the brightness and contrast in dark regions (the pupil) and decreases the contrast in brighter regions (iris). The intensity values are preferably then reversed to produce a photometric negative of the image. Preferably, the photometric image is used in the image processing steps which follow. 
     Third, a threshold function is applied to the photometric image at  64  to create a binary representation of the image which permits faster image processing. The threshold function replaces the image intensity values below some threshold value to black (a value of zero) while placing the intensity values above the threshold value to all white (a value of 256 in an 8-bit image representation); i.e., a binary representation of the image is created. At this step the image of the pupil is now totally white against a black background. 
     Fourth, the binary representation is preferably further processed at  66  by a technique known as dilation. Dilation ensures that the pupil is a solid circular object. This step is preferred as sometimes the alignment marker, especially the reticle, will leave black holes or lines in the pupil area. 
     Fifth, at  68 , the binary image undergoes a characterization process to determine a set of parametric values from the image. Since all pupils are nearly the same diameter (for recognition purposes), the search of binary objects can be limited to a range defined by pupil diameters. This range is typically 2-3 millimeters. A search is then performed on the binary image for objects matching the criterion. Those objects found in this range are returned with several pieces of information. By limiting the field-of-view to the eye image, only one object, the pupil, will be detected. The information returned from this function include object area, width and height, and object center. The object center is used for the pupil center. The object width and height are used to apply a bounding circle around the displayed pupil during the tracking procedure. 
     Sixth, an offset calculation is made at  70  in which the object (pupil) center is compared against the previously defined laser beam center and differences are recorded. If, at  72 , the difference is less than or equal to a surgeon preset value, i.e., a predetermined tracking zone, the surgical chair is not adjusted, the laser is permitted to operate, and the image tracking system  34  continues to receive, at  50 , and process, at  52 ,  54  (or  56 ),  58 ,  60 ,  62 ,  64 ,  66 ,  68  and  70 , images of the eye. 
     Seventh, if the eye center value falls outside the surgeon preset value, at  72 , the patient, and consequently the eye, is repositioned at  76  by adjusting the surgical chair  42  in the proper direction, as described below. 
     Furthermore, if the eye center falls outside the surgeon preset value at  72  and also a predetermined safety zone (a second predetermined value greater than the predetermined tracking zone) at  74 , the image tracking processor  34  sends a command to interrupt one stage (FS 1  in FIG. 3) of the footswitch  38 , thus pausing the laser surgery procedure, and also repositions the patient at  76  by adjusting the surgical chair  42  in the proper direction, as described below. Then, once repositioned, the laser is again permitted to operate (i.e., the footswitch is reset), and, at  50 , the image tracking system  34  continues to receives and process images of the eye. 
     Referring to FIGS. 1 and 3, if a command is sent to adjust the surgical chair at  76 , the patient positioning interface  40  performs two functions: (1) adjustment of the chair  42  and patient to counter the eye motion and, if the eye center is outside the safety zone at  74 , (2) interruption at  78  of the signal from the footswitch  38 , as a safety measure. Each of these functions will now be described in more detail. 
     First, turning to FIG. 3, an adjustment signal determined by the image tracking processor  34  is sent at  76  to the patient positioning interface  40  for adjustment of the surgical chair  42 . The patient positioning interface  40  provides the appropriate signal to direct the chair motors. For example, one model of a Dexta™ surgical chair, well-known in the field of eye refractive surgery, utilizes relays to provide fixed voltages to positioning motors in the chair. In the Dexta™ surgical chair, four relays provide +X, −X, +Y, and −Y directional movement. Z directional control voltages may also be used. According to a preferred patient positioning interface  40 , the relays are turned OFF or ON depending on the tracking adjustment required. Additionally or alternatively, variable control voltages may be supplied to the chair controller. These voltages then control silicon controlled rectifier (SCR) drives to control the motors moving the chair. This allows for finer motion control of the patient. The patient positioning interface  40  provides these variable voltages to the appropriate motors to adjust the eye position of the patient. Other beds or chairs used in this field have similar controls and may likewise be coupled to the patient positioning interface  40 . In addition, the invention may be used with a headrest which includes motor control to appropriately adjust the eye position of the patient. 
     Second, interruption at  78  of a stage of the footswitch  38  (i.e., interruption of the footswitch signal FS 1  in FIG. 3) pauses the laser procedure when the eye drifts outside the safety zone at  74 . That FS 1  stage of the footswitch is re-enabled once the eye is returned to the safety zone by adjustment of the surgical chair. 
     There have been described and illustrated herein several embodiments of an eye tracking and positioning system for a refractive laser system and a method of tracking an eye and repositioning the eye relative to a laser beam. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while particular functional systems have been disclosed, it will be appreciated that other functional systems may be used as well. That is, the image tracking processor and patient positioning interface may be combined in a single system or further divided to perform the required tasks of the invention. Furthermore, while a particular preferred method has been disclosed for tracking the pupil of the eye, it will be appreciated that other algorithms may be used. For example, the eye center can be determined from a marker in a manner other than described above. In one implementation, an active infrared diode system places one or more small, collimated spots on the cornea, and an infrared detector system is then used to monitor the one or more spots. For example, three spots can be monitored to triangulate the center of the eye during eye movement. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as so claimed.