Patent Publication Number: US-2005137586-A1

Title: Hybrid eye tracking system and associated methods

Description:
FIELD OF THE INVENTION  
      The present invention relates to systems and methods for performing eye tracking, such as during laser surgery, and, more particularly, to such systems and methods for tracking both rapid and slow eye movements.  
     DESCRIPTION OF RELATED ART  
      The measurement of wavefront aberrations emanating from an eye is known in the art to provide data for driving laser-surgical corrective systems. Various embodiments of a method and system for objectively measuring and surgically correcting aberrations of optical systems by wavefront analysis have been disclosed in commonly owned application Ser. No. 09/566,668, “Apparatus and Method for Objective Measurement and Correction of Optical Systems Using Wavefront Analysis,” filed May 8, 2000, and in commonly owned issued U.S. Pat. Nos. 5,632,742 and 5,980,513, all of which are hereby incorporated by reference herein.  
      Corrective laser surgery may be performed, for example, by laser ablation of portions of the corneal surface to achieve a calculated shape for improving visual acuity. In this case it is desirable to account for eye movement during surgery while delivering laser shots to the cornea. Rapid, involuntary (“saccadic”) eye movement comprises motion of very short duration, in the 10-20 msec range, and up to 1□ of rotation. This movement makes it difficult to determine a visual axis from which to calculate a movement of a treatment laser. The &#39;513 patent addresses this problem with an x,y eye tracking system such as will be described more fully in the following.  
      Two other types of eye movement, which are slower than saccadic eye movement, comprise cyclo-torsion and translation movements. Translation movement is at present eliminated by paralyzing the pupil so that asymmetric pupil dilation and constriction do not shift the tracking point, resulting in decentration. Manual observation of indicia such as ink marks and/or reticle adjustment are typically used to account for cyclo-torsion during the procedure, which can result from postural changes between the patient&#39;s measurement and surgical positions.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the present invention to provide a system and method for tracking eye movement during laser surgery to correct ocular aberrations.  
      It is a further object to provide a system and method for tracking eye movement that obviates the need for eye dilation during laser surgery.  
      It is another object to provide a system and method for tracking eye movement that obviates the need for paralyzing the patient&#39;s pupil during laser surgery.  
      It is also an object to provide a system and method for tracking both rapid and slow eye movement.  
      It is an additional object to provide a system and method for automating adjustments to beam delivery parameters during laser surgery.  
      These and other objects are achieved by the present invention, a hybrid eye tracking system and associated methods. The method is for tracking ocular changes during a surgical procedure such as laser vision corrective surgery, and comprises the step of directing an eye-safe optical beam toward an eye. The eye can be substantially untreated (minimally treated by drugs or otherwise) to achieve dilation and paralysis. Next a reflected optical beam from the eye is detected, and a plurality of measurements are performed based upon data contained in the reflected optical beam. The measurements are made of at least one geometric parameter of the eye at a predetermined frequency, and from them is calculated a change in the at least one geometric parameter.  
      Another aspect of the invention is directed to a method for performing a corrective procedure upon an eye. This method comprises the steps of directing a plurality of ablating laser beam shots at a cornea of an eye in a predetermined pattern. Again, the eye can be substantially untreated to achieve dilation and paralysis. Ocular changes are tracked as above. Ocular changes are compensated for by dynamically adjusting the directing of the laser beam shots based upon the calculated change.  
      Systems are also provided as part of the present invention for performing the above-described methods.  
      The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the following description, taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein:  
       FIG. 1  is a schematic diagram of the hybrid eye tracking system of the present invention;  
       FIG. 2  is a flow diagram of the slow movement tracker control system of the present invention; and  
       FIG. 3  is a schematic diagram of an eye, illustrating the parameters monitored by the system.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A description of the preferred embodiments of the present invention will now be presented with reference to  FIGS. 1-3 .  
      A system  10  ( FIG. 1 ) and method  100  ( FIG. 2 ) for performing a corrective procedure upon an undilated, unparalyzed eye are provided. The system  10  comprises a surgical component  11  and a tracking component, which in turn comprises a rapid eye movement (saccadic) tracker  12 , such as disclosed in the &#39;513 patent, and a slow movement tracker  13 . The tracking components  12 , 13  are for tracking ocular changes during the surgical procedure.  
      The surgical component  11  comprises an ablating laser  14  and associated optics  15  adapted for emitting laser beam shots at a cornea  91  of an eye  90  in a predetermined pattern (block  101 ) based upon, for example, a wavefront measurement, although this is not intended as a limitation. The surgical component  11  is under control of a beam translation component  16  that is in turn under control of a processor  17  having software  18  resident thereon. The software  18  comprises means for performing calculations on data received from the tracking components  12 , 13 .  
      The slow tracker  13  comprises a video system  19  comprising an illumination means  20  adapted to direct an eye-safe optical beam  21  toward the eye  90  (block  102 ). The video system  19  further comprises a detector  22  for detecting a reflected optical beam  23  from the eye  90  (block  103 ), tracking features such as, for example, scleral features such as blood vessels, limbus shape and ellipticity, iris features, and artificial eye marks. Data from the detector  22  are routed to the processor  17  (block  104 ), from which a plurality of measurements are made of at least one geometric parameter of the eye  90  at a predetermined frequency (block  105 ), in one embodiment preferably less than 250 Hz. From these measurements is calculated a change in the at least one geometric parameter (block  106 ), including a translational and/or a cyclo-torsional change.  
      The geometric parameter (see  FIG. 3 ) may comprise (block  107 ), for example, at least one of a diameter  92  of a pupil  93  (block  108 ), a pupil (centroid) position  94  relative to the limbus  95  (block  109 ), and a position of the iris  96  in x,y coordinates  97  (block  110 ) and as a cyclo-torsion angle  98  (block  111 ).  
      If the pupil diameter  92  changes, either (block  112 ) the zoom motor  24  adjusts (block  113 ) or the ambient illumination level in the room is adjusted to maintain a substantially constant pupil diameter  92  (block  114 ). Preferably this measurement is updated at a rate approximately  10  times the rate of change of the pupil diameter  92 .  
      If the pupil position  94  changes relative to the limbus  95 , an x,y offset is calculated from the limbus  95 , and the centroids of the pupil  93  and the limbus  95  are tracked, along with their relative locations (block  115 ). Preferably this measurement is updated at a rate approximately 10 times the rate of change of the pupil diameter  92 . Pupil centroid offset or “pupil drift” depends on the state of the iris and may be larger or smaller from patient to patient and may increase or decrease slightly during surgery. This displacement may not be noticed by the fast iris/pupil boundary tracker and may need to be added vectorally to every shot position with the excimer scanner mirrors so that the shots end up on the desired locations on the cornea independent of pupil centroid drift.  
      A preferred way to account for pupil center drift due to pupil size changes is to characterize the pupil center position relative to the center of the limbus at the pre-operative visit using the video images of the pupil under different lighting conditions. An equation for how pupil center shifts with respect to size changes of the pupil can be developed from such a pre-operative evaluation. It is assumed that during surgery, if the video tracker observed pupil size changes, the same drift offset will be present. This means that an equal and opposite offset in the location of where the excimer scanners will place these shots must be applied to “null out” the effect that the pupil drift would have on the high speed tracked position, which is only closed around the real-time pupil center and will not account for changes in relative position of the pupil center with respect to the limbus or cornea.  
      If the iris position in x,y coordinates  97  changes relative to the excimer laser center, due to, for example, the normal head shifts, chin rolling or other movements that cause the entire eye to move, the eye will be tracked-in using the high-speed four IR spot closed-loop LADAR tracker, which uses the tracking galvonometer mirrors to constantly adjust the position of the tracked eye to maintain a space stabilized image  
      If the rotational position  98  of the iris  96  changes, the new position is used to calculate and perform a rotation of the surgical pattern generated by the excimer scanning mirrors to compensate for cyclo-torsion (block  117 ). Video tracking may be performed using iris feature recognition, scleral blood vessel detection  96  or on features marked on the eye  90 . Preferably this measurement is updated at a rate 10 times the rate of change of the ocular rotation.  
      Ocular changes noted on the video images that cause the locations on the cornea of the desired laser ablation pattern to be shifted relative the location of the excimer beam scanning positions are compensated for by dynamically adjusting the directing of the laser beam shots via the excimer laser scanning mirrors based upon the calculated change.  
      It will be seen by one of skill in the art that other embodiments and uses may be contemplated for the present invention. For example, the eye tracker monitoring system and method may be used in settings other than surgical sites, including such sites as for psychological and physiological testing applications.  
      In the foregoing description, certain terms have been used for brevity, clarity, and understanding, but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such words are used for description purposes herein and are intended to be broadly construed. Moreover, the embodiments of the apparatus illustrated and described herein are by way of example, and the scope of the invention is not limited to the exact details of construction.  
      Having now described the invention, the construction, the operation and use of preferred embodiment thereof, and the advantageous new and useful results obtained thereby, the new and useful constructions, and reasonable mechanical equivalents thereof obvious to those skilled in the art, are set forth in the appended claims.