Abstract:
A closed-loop control system for the intrastromal photoablation of tissue includes an active mirror for individually directing the component beams of a diagnostic laser beam to a focal point on the retina of an eye. The reflected beam is analyzed to identify a distorted wavefront indicative of required corneal corrections, and an induced wavefront indicative of optical aberrations introduced by bubbles formed during tissue ablation. A comparator alters the induced wavefront with a desired wavefront to create a rectified wavefront, and a comparator compares the rectified wavefront with the distorted wavefront to create error signals. The error signals are then used to operate the active mirror and to control an ablation laser until the absence of error signals indicate the required stromal tissue has been photoablated.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to laser systems that photoablate corneal tissue to improve the visual acuity of an eye. More particularly, the present invention pertains to laser systems and methods that perform intrastromal photoablation of tissue during corrective optical surgery. The present invention is particularly, but not exclusively, useful as a closed-loop control system to accurately and precisely maintain control of an ablation laser beam when bubbles form in the stroma, and introduce induced optical aberrations during a laser surgery. 
     BACKGROUND OF THE INVENTION 
     It is well known that the optical characteristics of an eye can be altered through laser surgery. For example, U.S. Pat. No. 6,050,687 which issued to Bille et al. for an invention entitled “Method and Apparatus for Measuring the Refractive Properties of the Human Eye,” and which is assigned to the same assignee as the present invention, discloses a laser system that can be used for such purposes. In any event, a consequence of photoablation, is that individual cells in the tissue are vaporized. Gas is, therefore, a product of photoablation. When a surgical laser procedure involves the superficial photoablation of tissue, the fact that such gases are created does not cause much of a problem. This is not the case, however, when internal tissue is photoablated. 
     For specific surgical procedures that involve the intrastromal photoablation of corneal tissue, it is known that such photoablation results in the formation of tiny bubbles in the stroma. Further, it is known that the formation of these bubbles introduces induced aberrations that change the optical characteristics of the cornea. The reason for this change is essentially two-fold. First, the gas bubbles have a different refractive index than that of the surrounding stromal tissue. Second, and perhaps more important, the gas bubbles tend to deform the stroma and, thus, they alter its refractive effect on light passing through the cornea. In a controlled surgical procedure these induced aberrations must be accounted for. 
     Wavefront analysis provides a useful and helpful conceptual too) for determining the effect a particular medium or material (e.g. the cornea of an eye) will have on a light beam, as the beam passes through the medium (material). For wavefront analysis, a light beam can be conveniently considered as being a so-called “bundle” of component light beams. These component light beams are all mutually parallel to each other, and when all of the component beams of a light beam are in phase with each other as they pass through a plane in space, it is said they define a plane wavefront. However, when a fight beam passes through a medium, the medium will most likely have a different refractive effect on each of the individual component beams of the light beam. The result is that the phases of the component light beams will differ from each other. When now considered collectively, these component light beams will define something other than a plane wavefront. In summary, a particular wavefront will define the refractive effect a medium, or several media, have had on a light beam. 
     Insofar as laser surgery is concerned, it is the objective of such surgery to remove unwanted aberrations from the light beams that a patient perceives visually. As implied above, wavefront analysis can be a helpful tool in evaluating and determining the extent to which refractive properties of a cornea may need to be altered or corrected. Indeed, such an analysis has been helpful for surgical procedures involving superficial photoablation. For example, U.S. Pat. No. 6,428,533B1 which issued to Bille for an invention entitled “Closed Loop Control for Refractive Laser Surgery (LASIK),” and which is assigned to the same assignee as the present invention, discloses such a system. 
     As recognized by the present invention, when intrastromal photoablation is to be performed, and the evaluations and determinations of a wavefront analysis are put into practice, it is desirable to establish control over each individual component beam defining a wavefront. With this control, induced aberrations such as mentioned above, can be accurately compensated for, and the overall control of the procedure is enhanced. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a closed-loop system is provided which will control the photoablation of stromal tissue in an eye during an intrastromal surgical procedure. More specifically, in addition to controlling the photoablation of the stromal tissue that is necessary for a corrective procedure, the control system of the present invention also compensates for optical aberrations that are induced by gas bubbles as they form in the stromal tissue that is being photoablated. 
     For purposes of wavefront analysis, a light beam is properly considered as including a plurality of individual component beams. Collectively, these constituent component light beams define a wavefront for the larger inclusive light beam. With this in mind, and in the context of the present invention, several definitions for light beam wavefronts are helpful. Specifically, these definitions pertain after a light beam has passed through the stroma of an eye. A “desired wavefront” results from the stroma of a corrected eye, and is the objective of a surgical procedure. A “distorted wavefront” results from the stroma of an uncorrected eye, and exhibits the actual real-time characteristics of the cornea, before correction. An “induced wavefront” results from the formation of bubbles in the stroma, and includes the “distorted wavefront.” A “rectified wavefront” results by incorporating an “induced wavefront” with a “desired wavefront.” 
     Structurally, the system of the present invention includes two distinct laser sources. One is for generating an ablation laser beam that will be used to photoablate stromal tissue. The other is for generating a diagnostic laser beam. Conceptually, as mentioned above, the diagnostic laser beam is properly considered as including a plurality of individual component beams. 
     Along with the two laser sources just mentioned, the system of the present invention also includes an active mirror and a detector. More specifically, the active mirror comprises a plurality of separate reflective elements for individually reflecting respective component beams of the diagnostic beam. Together, these elements of the active mirror are used, in concert, to direct the diagnostic laser beam to a focal spot on the retina of the eye. The detector is then used to receive the diagnostic beam after it has been reflected from the retina. 
     In the operation of the present invention, a compensator incorporates a desired wavefront (predetermined for the patient), with the induced wavefront as it is received by the detector. This incorporation creates a rectified wavefront. A comparator is then used to compare the rectified wavefront with the distorted wavefront (diagnostically predetermined) to create an error signal. Consistent with the wavefront analysis used to define light beams, the error signal is properly considered as having a plurality of error segments. These error segments are then used by the system of the present invention to individually activate a respective reflective element of the active mirror, and to thereby maintain the focal spot of the diagnostic beam on the retina. The ablative laser source can then be continuously operated to photoablate stromal tissue using a so-called “spot-by-spot” local ablation strategy until the error signal is substantially a nullity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1  is a schematic layout showing the interrelationships of components in a system for controlling the intrastromal photoablation of corneal tissue in an eye in accordance with the present invention; 
         FIG. 2  is a functional representation of the wavefront analysis techniques used in the operation of the system of the present invention; and 
         FIG. 3  is a schematic representation of the interrelationships between the component beams of a wavefront, corresponding reflective elements of the active mirror, and required laser pulses from the ablation laser source. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , a closed-loop system for intrastromal photoablation of corneal tissue in accordance with the present invention is shown and is generally designated  10 . In detail, the components of system  10  include a source  12  for generating an ablation laser beam  14 , and a source  16  for generating a diagnostic laser beam  18 . Further, the system  10  includes an active, multi-facet mirror  20 , a beam splitter  22  and a beam splitter  24 . More particularly, the active mirror  20  is preferably of a type disclosed in U.S. Pat. No. 6,220,707 which issued to Bille for an invention entitled “Method for Programming an Active Mirror to Mimic a Wavefront” and which is assigned to the same assignee as the present invention. As shown, the active mirror  20  and the beam splitters  22  and  24  direct the diagnostic laser beam  18  from diagnostic laser source  16  toward an eye  26 . Likewise, the beam splitters  22  and  24  are used to direct the ablation laser beam  14  from the ablation laser source  12  toward the eye  26 . 
       FIG. 1  also shows that the system  10  of the present invention includes a detector  28 , a comparator  30  and a compensator  32 . In particular, the detector  28  is preferably of a type commonly known as a Hartmann-Shack sensor. The comparator  30  and compensator  32  are electronic components known in the pertinent art that will perform the requisite functions for the system  10 . 
     Still referring to  FIG. 1 , it is to be appreciated and understood that during an intrastromal photoablation procedure, as performed by the system  10  of the present invention, the ablation laser beam  14  is focused (by optical components not shown) onto stromal tissue  34  in the cornea of the eye  26  for the purpose of accomplishing intrastromal photoablation. A consequence of this photoablation of the tissue  34  is the formation of gas bubbles  36  that introduce optical aberrations in the stromal tissue  34 . At the same time, the diagnostic laser beam  18  is focused (by optical components not shown) to a focal spot  38  on the retina  40  of the eye  26 . In this combination, control by the system  10  over the ablation laser beam  14  is actually accomplished using the reflected diagnostic laser beam  18 ′, as it is reflected through the stromal tissue  34  from the focal spot  38  on the retina  40  of eye  26 . 
       FIG. 1  shows that as the reflected diagnostic laser beam  18 ′ exits from the eye  26  through the stromal tissue  34 , the beam  18 ′ is directed by beam splitter  24  toward the detector  28 . Using wavefront analysis considerations, the reflected diagnostic beam  18 ′ can be conceptually considered as including a plurality of individual and separate laser beam components. Together, these components can be characterized as a distorted wavefront  42 . Further, this distorted wavefront  42  will result from two contributions. One contribution results from the uncorrected eye  26  and is an actual real-time consequence of light passing through the stromal tissue  34 . It is this contribution that is to be corrected. The other contribution results from the aberrations that are introduced by the presence of the gas bubbles  36  in the stromal tissue  34 . Again using wavefront analysis, the contribution introduced by the gas bubbles  36  can be conceptualized as a wavefront having a plurality of components that are collectively characterized as an induced wavefront  44 ,  FIG. 1  further shows a desired wavefront  46 . This desired wavefront  46  will most likely be either a plane wavefront, or a wavefront that is relatively similar to a plane wavefront. In any event, it is the desired wavefront  46  that is the objective of the procedure to be performed by the system  10 . 
     By cross referencing  FIG. 1  with  FIG. 2 , it will be appreciated that in the operation of the system  10 , the distorted wavefront  42  is first received by the detector  28 . Using predetermined diagnostic information about the corrections that are to be made to the eye  26  by system  10 , the detector  28  determines and generates the induced wavefront  44 . The compensator  32  then alters a predetermined, desired wavefront  46  with this induced wavefront  44 . This alteration creates a rectified wavefront  48 . The rectified wavefront  48  is then compared with the distorted wavefront  42  to generate an error signal  50 . In turn, this error signal  50  is used to manipulate the active mirror  20  for control of the diagnostic laser beam  18 . Importantly, the error signal  50  is also used to activate the ablation laser source  12  and, specifically, the error signal  50  causes the ablation laser source  12  to cease its operation when the error signal  50  is a null. 
     In response to the error signal  50 , the operation of the active mirror  20 , as well as the operation of ablation laser source  12  will, perhaps, be best appreciated with reference to FIG.  3 . Again, using a wavefront analysis, the error signal  50  can conceptually be considered as comprising a plurality of component error signals. For this analysis, the component error signals  50   a ,  50   b ,  50   c  and  50   d  shown in  FIG. 3  are only exemplary. In general, what is important here, is that each of the exemplary component error signals  50   a-d  result from the interaction of corresponding components of the wavefronts  42 ,  44 ,  46  and  48 . As disclosed above, these wavefronts  42 ,  44 ,  46  and  48  directly result from the refraction of corresponding beam components of the diagnostic laser beam  18 . Stated differently, each component beam of the diagnostic laser beam  18  is present in each of the wavefronts: namely, the distorted wavefront  42 , the induced wavefront  44 , the desired wavefront  46 , and the rectified wavefront  48 . Consequently, each component beam of the diagnostic laser beam  18  generates a corresponding error signal component  50   a-d . Depending on its refractive history as it passes through the system  10 , each error signal component  50   a-d  will have a respective magnitude  52 . 
       FIG. 3  also indicates that the active mirror  20  includes a plurality of reflective elements  54 , of which the reflective elements  54   a-d  are exemplary.  FIG. 3  also indicates that each reflective element  54  is at a respective distance  56  (i.e. distances  56   a-d ) from a datum  58 . For example, each; error signal component  50  (e.g. error signal component  50   a ) is used by the system  10  to establish a respective distance  56  for a corresponding reflective element  54  of the active mirror  20  (e.g. signal  50   a  and distance  56   a ). 
       FIG. 3  also shows that the ablation laser source  12  will generate a plurality of separate laser pulse trains  60  that correspond to each corresponding error signal component  50   a-d . For instance, the error signal component  50   a , will generate a laser pulse train  60   a . The laser pulse train  60   a  is then continued until the error signal component  50   a  is a null. Similarly, the pulse trains  60   b-d  react to corresponding error signal components  50   b-d . While this is happening to ablate the stromal tissue  34 , the; error signal components  50   a-d  also interact with the active mirror  20 . Specifically, as the error signal component  50   a  decreases in its magnitude  52 , the distance  56  of reflective element  54   a  from datum  58  also decreases. This, is done to maintain the focal spot  38  fixed on the retina  40  of eye  26  so that the distorted wavefront  42  is maintained as an accurate measure of the progress of the intrastromal photoablation procedure. The ablation laser source  12  is inactivated, when all of the error signal components  50   a-d  (i.e. error signal  50 ) are a nullity. 
     While the particular Closed Loop Control for Intrastromal Wavefront-Guided Ablation as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.