Abstract:
A method for ocular surgery requires use of a delivery system for generating and guiding a surgical laser beam to a focal point in a treatment area of an eye. Additionally, a contact device is employed for using the eye to establish a reference datum. Further, an optical detector is coupled to the beam path of the surgical laser to create a sequence of cross-sectional images. Each image visualizes both the reference datum and the focal point. Operationally, a computer then uses these images to position and move the focal point in the treatment area relative to the reference datum for surgery.

Description:
This application is a divisional of application Ser. No. 11/625,213, filed Jan. 19, 2007, which is currently pending. The contents of application Ser. No. 11/625,213 are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains generally to methods for performing ocular surgery. More particularly, the present invention pertains to computer-controlled laser surgical systems. The present invention is particularly, but not exclusively, useful as a method that incorporates optical coherence tomography (OCT) techniques for the purpose of imaging both a treatment area and a reference datum, to control laser beam focal point movements within the treatment area during a surgical operation. 
     BACKGROUND OF THE INVENTION 
     When using a laser beam to perform ocular surgery, the precise movement of the laser beam&#39;s focal point through the tissue to be altered is absolutely imperative. Specifically, focal point position accuracies within about ten microns (10 μm) are preferable. To do this, the desired path for the laser beam&#39;s focal point must have a precisely defined start point. And, the laser beam&#39;s focal point must then be moved along the prescribed path. Although this can be accomplished in some situations with open loop control (i.e. having the laser beam focal point follow a pre-programmed path), in many other situations it may be more desirable to incorporate a closed loop feedback control system. Unlike open loop systems, closed loop feedback control systems provide continuous monitoring and corrections for deviations of the focal point. In either case, movements of the laser beam&#39;s focal point must be accomplished in the context of a reference datum. 
     An important requirement for any closed loop feedback control system is the need to accurately identify an appropriate error signal. As implied above, this error signal must be measurable. Thus, a reference datum is required from which the error signal can be measured. Once the error signal is identified, control of the system&#39;s performance is made by system adjustments that will nullify, or at least minimize, the error signal. Stated differently, deviations (i.e. error signals) from desired performance parameters must be determinable and maintained below an acceptable minimum. For the specific case involving feedback control of a surgical laser&#39;s focal point during ocular surgery, a reference datum that is anatomically related to the eye undergoing surgery needs to be selected. Further, knowledge of the location of the laser beam&#39;s focal point relative to the reference datum, and thus relative to a path through the eye, is also required. 
     Anatomically, the eye includes various tissues that may be beneficially altered by laser surgery. These include: the cornea, the crystalline lens, and the retina. Importantly, a thorough knowledge of the geometry of these ocular elements, and of their geometrical relationship to each other, is essential for successful surgery. All of this, of course, cannot be done by merely an external examination of the eye. With this limitation in mind, one method for imaging the interior of an eye involves optical coherence tomography (OCT) techniques. Fortunately, these techniques are well known to skilled artisans (e.g. See U.S. Pat. No. 6,004,314 which issued to Wei et al. for an invention entitled “Optical Coherence Tomography Assisted Surgical Apparatus”). Specifically, for purposes of the present invention, OCT can be employed to identify an appropriate eye-based reference datum for conduct of the laser surgery. Further, OCT provides a means for visualizing a treatment area inside the eye, while laser surgery is being performed. Although OCT techniques may be preferred, it will be appreciated by the skilled artisan that other imaging techniques might be used for the purposes of the present invention. Specifically, imaging techniques such as confocal microscopy, or second harmonic generation microscopy, may be employed. 
     In light of the above, it is an object of the present invention to provide a method for directing a surgical laser beam onto tissue in a treatment area of an eye of a patient, wherein control of the laser beam is based on cross-sectional views of the eye obtained by employing OCT techniques. Another object of the present invention is to provide a method for directing a surgical laser beam onto tissue in a treatment area of an eye of a patient wherein an eye-based reference datum can be selected that is most appropriate for the particular surgical operation that is to be performed. Still another object of the present invention to provide a method for directing a surgical laser beam onto tissue in a treatment area of an eye of a patient that is easy to implement, is relatively simple to manufacture, and is comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method is provided for performing ocular surgery. In particular, this surgery is accomplished by directing a laser beam onto tissue in a treatment area of a patient&#39;s eye; and it requires identifying a reference datum that is related to the eye. For purposes of the present invention, this reference datum can be either the anterior surface of the cornea, the posterior surface of the cornea, a surface area on the crystalline lens, or the retina. To identify the reference datum, the present invention employs an optical detector that creates images using optical coherence tomography (OCT) techniques. Specifically, the detector is used to create cross-sectional views of the eye that include images of both the reference datum and of the treatment area where the tissue that is to be altered by laser surgery is located. 
     Along with the optical detector, the apparatus of the present invention includes a beam delivery system. Specifically, the beam delivery system has a laser source for generating the surgical laser beam, and it has appropriate optical elements for directing the laser beam from the laser source to the treatment area. Included in these optical elements is a scanner that is able to move the laser beam in orthogonal x, y and z directions. Also, the delivery system includes a lens for focusing the laser beam to a focal point in the treatment area. As intended for the present invention, the surgical laser beam that is generated by the beam delivery system comprises a sequence of femtosecond pulses having a wavelength that is approximately one thousand nanometers (λ s =1,000 nm). Preferably, the apparatus also includes a contact lens that can be placed against the anterior surface of the patient&#39;s eye, to stabilize the eye during surgery. Further, the contact lens can also establish an interface at the anterior surface between the eye and the apparatus that may be used as a reference datum. 
     A computer (i.e. a data processor) is electronically connected to both the beam delivery system and to the optical detector. With these connections, the computer is able to compare the location of desired focal points in the treatment area (based on pre-planned data for the surgery) with actual focal points. Deviations of actual focal points from desired focal points (i.e. error signals) can thus be identified. Using well known closed loop feedback control techniques, the delivery system is then adjusted to nullify or minimize the error signals. Consequently the system can be controlled to have its focal point follow a predetermined path through the treatment area. Alternatively, the system can be operated in an open-loop mode. If so operated, the focal point is moved to follow the predetermined path through the treatment area without any further adjustments. In the open-loop mode, however, it is still important to use the optical detector to establish an appropriate start point for the path of the focal point. 
     As indicated above, an important aspect of the present invention is its use of the optical detector to generate cross-sectional views of the treatment area. As envisioned for the present invention, such views can be sequentially made in real time. Further, they can be made from different perspectives, based on different cross-section planes through the eye. With these capabilities, the cross-sectional views can be used for control of the system, and they can also provide the operator with a three dimensional visualization of the treatment area. With this capability, it is envisioned that manual control over movements of the focal point in the treatment area is possible for the present invention. When used, manual control may either augment the computer control mentioned above, or provide an alternative to the computer control. 
    
    
     
       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 drawing of an apparatus for performing ocular surgery in accordance with the present invention; 
         FIG. 2  is a top plan view of an eye as would be seen along the line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a cross-section view of an eye as seen along the line  3 - 3  in  FIG. 2 ; and 
         FIG. 4  is an enlarged cross-section view of the cornea of the eye shown in  FIG. 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , an apparatus for performing ocular surgery in accordance with the present invention is shown and is generally designated  10 . As shown, the apparatus  10  includes a laser source  12  for generating a surgical laser beam  13 . For the present invention, the surgical laser beam  13  preferably includes a sequence of femtosecond pulses having a wavelength of approximately one thousand nanometers (λ s =1,000 nm).  FIG. 1  also implies that the apparatus  10  includes a scanning unit  14  that will allow the surgical laser beam  13  to be moved in orthogonal x, y and z directions. Relay optics  16  transfer the surgical laser beam  13  in a manner well known in the pertinent art, and a focusing lens  18  is used to focus the surgical laser beam  13  to a focal point  20 . 
     As indicated in  FIG. 1 , the focal point  20  may be selectively established in the tissue of a patient&#39;s eye  22 . A contact lens  24  that is mounted on the apparatus  10  by way of connections (not shown) is also shown positioned on the eye  22 . Further,  FIG. 1  indicates the surgical laser beam  13  will follow along a beam path  26  as it progresses from the laser source  12  to its focal point  20  in the eye  22 . For this purpose, turning mirrors  28  and  30  can be employed to establish the beam path  26 , as desired. 
     Still referring to  FIG. 1 , it will be seen that the apparatus  10  includes an optical detector  32  and a computer (data processor)  34 . More specifically, the computer  34  is connected via a line  36  to the optical detector  32 , and it is connected to the laser source  12  via a line  38 . Together, these components (i.e. laser source  12 , optical detector  32 , and computer  34 ) effectively control the apparatus  10  during ocular surgery. 
     As envisioned for the present invention, and stated above, the optical detector  32  uses optical coherence tomography (OCT) techniques to create cross-section views of the eye  22 . Importantly, these views include images of specific anatomical features of the eye  22 . Moreover, optical detector  32  creates these views (with images) in a way that allows the images to be used by the computer  34  for control of the laser source  12 . To better appreciate this function, refer to  FIG. 2 . 
     In  FIG. 2 , the eye  22  is seen in a top plan view; and it is shown with end-on indications of several reference planes  40 ,  42  and  44 . The present invention envisions these planes  40 ,  42 , and  44  will be generally parallel to the optical axis of the eye  22  and will extend through the eye  22 . The planes  40 ,  42  and  44 , however, are only exemplary, and their importance is best appreciated by cross referencing  FIG. 2  with  FIG. 3 . Specifically,  FIG. 3  is representative of a cross section view of the eye  22  as seen in the single plane  40 . The fact that other cross section views of the eye  22  are possible (i.e. the perspectives of planes  42  and  44 ), allows OCT images to be collectively considered for a three-dimensional presentation of the interior of the eye  22 . On the other hand, an individual image from any particular plane (e.g. plane  40 ,  42  or  44 ) will, by itself, provide valuable information for the use and operation of apparatus  10 . 
     With specific reference now to  FIG. 3  it will be seen that the cross section view presented (i.e. plane  40 ) specifically reveals several anatomical features of the eye  22 . These include: the anterior surface  46  of the cornea  48 , the posterior surface  50  of the cornea  48 , the crystalline lens  52 , and the retina  54 . Further, this cross section view also shows details of the contact lens  24 , if used. Thus, the interface between contact lens  24  and the anterior surface  46  of cornea  48  can be identified. At this point it is to be noted that less than an entire cross section view (e.g. as shown in  FIG. 3 ) can be used for the purposes of the present invention. For example, an image emphasizing the cornea  48  or the retina  54  may be needed. Further, it is also to be noted that, particular information from an image (e.g. plane  40 ) can be substantiated or verified by comparing it with images from other planes (e.g. planes  42  or  44 ). 
     For purposes of disclosure, the interface between contact lens  24  and the anterior surface  46  of the cornea  48  is hereafter referred to as a reference datum  56 . It must be appreciated, however, that this reference datum  56  is only exemplary. Other anatomical features of the eye  22  can be alternatively used for the same purposes, and perhaps more effectively, depending on the requirements of the particular ocular surgery being performed. 
     Returning for the moment to  FIG. 1 , it will be seen there are two functional embodiments of the apparatus  10  that are envisioned for the present invention. The primary difference between the two embodiments is determined by the location where optical detector  32  is coupled onto the beam path  26 . For both embodiments this coupling is accomplished where the diagnostic beam, used by the optical detector  32  for OCT imaging, joins the beam path  26  of the surgical laser beam  13 . 
     For a preferred embodiment of the present invention, the diagnostic laser beam (represented by the dashed line  58  in  FIG. 1 ) is coupled onto beam path  26  by a dichroic mirror  60 . As shown, the dichroic mirror  60  is positioned downstream from the scanning unit  14 . In this case, the diagnostic laser beam  58  does not pass through the scanning unit  14 . Accordingly, for this preferred embodiment, the optical detector  32  needs to include its own scanning unit (not shown). 
     For an alternate embodiment of the present invention, the diagnostic laser beam (represented by the dotted line  62  in  FIG. 1 ) is coupled onto beam path  26  by a dichroic mirror  64  that is located upstream from the scanning unit  14 . In this case, the optical detector  32  can use the same scanning unit  14  that is being used for the surgical laser beam  13 . As an operational consideration, the diagnostic laser beam  58 ,  62  for both embodiments will have a wavelength of approximately one thousand three hundred nanometers (λ d =1,300 nm). The implication here is the embodiment wherein the diagnostic laser beam  58  is coupled downstream from the scanning unit  14 , may be preferable. This is so in order to avoid the additional refinements that are required for scanning unit  14  and the relay optics  16  when two different wavelengths use the same optical elements. 
     OPERATION 
     In the operation of the apparatus  10  of the present invention, a predetermined path  66  for the focal point  20  of surgical laser beam  13  is established for ocular surgery in a treatment area  68  of the eye  22  (see  FIG. 4 ). An image (e.g.  FIG. 3 ), or a partial image thereof, is made using the optical detector  32 . Importantly, the image (partial image) needs to include both the reference datum  56  (only exemplary) and a visualization of the treatment area  68 . The focal point  20  of the surgical laser beam  13  can then be directed toward a start point  70  that is selected in the context of the reference datum  56  (cross reference  FIG. 3  with  FIG. 4 ). 
     Open loop control of the focal point  20 , as it is moved through the treatment area  68 , can be achieved by merely moving the focal point  20  along the predetermined path  66  in accordance with pre-programmed instructions in the computer  34 . Whenever an open-loop mode of operation is used, however, it is important that the start point  70  be accurately established, and the path  66  be precisely pre-programmed. This will require that a desired focal point  72  coincide with the start point  70 , and that the path  66  be properly oriented in the treatment area  68 . As envisioned for the present invention, the coincidence of the desired focal point  72  with the required start point  70  can be accomplished using information from the optical detector  32 . Thus, using the start point  70 , and a predetermined definition of the path  66 , the apparatus  10  can be operated in an open-loop mode to perform the desired ocular surgery. On the other hand, closed loop control may be more appropriate for the particular ocular surgery being performed. In this case, the optical detector  32  is activated to provide continuous updates of cross-section images from the eye  22 . As indicated in  FIG. 4 , information contained in such cross-section images will include position data, relative to the reference datum  56 , of both an actual focal point  20 ′, and a desired focal point  72  on the path  66 . The positional difference “Δ” between the points  20 ′ and  72  will then represent an error signal that can be used for appropriate adjustments of the apparatus  10 . According to well known procedures and techniques (i.e. closed loop feedback control techniques), adjustments to the apparatus  10  can be input from the computer  34  that will either nullify or minimize “Δ” to maintain the focal point  20  on path  66  for a successful completion of the ocular surgery. 
     While the particular System and Method for Precise Beam Positioning in Ocular Surgery 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.