Abstract:
A system and method for creating a line recognition template that replicates an object is provided for use as a control reference during ophthalmic surgery on the object. Creation of the template first requires aligning a plurality of reference points along a central “z” axis, with anatomically measured lengths (Δ“z” n ) between adjacent reference points. Axially-symmetric surfaces can then be traced between selected, adjacent, reference points to create the template. For the present invention, the location of reference points, and the tracing of axially-symmetric surfaces, are based on a cross sectional image of the object for surgery. Preferably, the cross sectional image is obtained using Optical Coherence Tomography (OCT) techniques.

Description:
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/636,582, titled SYSTEM AND METHOD FOR CREATING A CUSTOMIZED ANATOMICAL MODEL OF AN EYE, filed Apr. 20, 2012. The entire contents of Application Ser. No. 61/636,582 are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains generally to systems and methods for creating templates. More particularly, the present invention pertains to systems and methods for creating templates that are useful as a control reference for moving a laser beam&#39;s focal point during ophthalmic surgery. The present invention is particularly, but not exclusively, useful as a system and method for creating templates that will replicate the anatomical object that is to be altered during a laser ophthalmic surgery. 
     BACKGROUND OF THE INVENTION 
     It is axiomatic that any ophthalmic surgical procedure must be accomplished with great accuracy and precision. This is particularly so when a laser system will be used to cut or ablate tissue deep inside an eye. In such cases, it becomes particularly important that there is some operational base reference which can be established to control movements of the laser beam&#39;s focal point during a surgery. 
     Imaging devices, such as those that employ Optical Coherence Tomography (OCT) techniques, have been particularly helpful for providing information that is useful in performing ophthalmic laser surgeries. Nevertheless, OCT imaging techniques, alone, are not always able to provide the degree of precision that is required to establish a discernible and accurate base reference for the control of laser surgeries within the eye. For any number of reasons, an OCT image may lack the sharpness or clarity that is necessary or desired. In the specific case of ophthalmic surgeries, however, the eye itself can be helpful in overcoming these deficiencies. 
     The anatomy of an eye is well known. In particular, for the purpose of establishing a base reference, the eye&#39;s anatomy is unique because, unlike most other body parts, many of its structures are substantially symmetrical. Most importantly, the light refractive, optical elements of the eye are all aligned along a definable central axis, and they are arranged in a known anatomical order. It happens that the central axis can be easily identified for such an arrangement of optical elements, and it can be accurately defined by any of several standard techniques. 
     It is well known that the interface surfaces between different optical structures inside the eye (e.g. the interface surface between the anterior chamber and the anterior capsule of the crystalline lens) can be effectively imaged by OCT. Consequently, the location of an intersection between an interface surface and the central axis can also be accurately established by OCT. Moreover, due to their symmetry on the central axis, the size and extent of the various optical elements in the eye can also be predicted with great accuracy. As recognized by the present invention, such a replication of structural elements can be used, altogether or in part, to establish a base reference for use in an ophthalmic laser procedure. 
     In light of the above, it is an object of the present invention to provide a system and method that will establish a base reference inside an eye for controlling an ophthalmic laser procedure. Another object of the present invention is to provide a system and method for using OCT techniques to establish a base reference that results from an automated anatomical recognition of different optical elements in an eye (i.e. different refractive tissues) and the location of these elements (tissues). Still another object of the present invention is to provide a system and method for establishing a base reference inside an eye that is easy to assemble, is simple to use and is comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a system and method are provided for replicating anatomical features inside an eye. Specifically, this is done to create a template for identifying anatomical structures that can be used as a base reference for controlling a laser system during ophthalmic laser surgery (e.g. a capsulotomy) inside the eye. 
     As required for the present invention, a first step in the creation of a template involves the identification of a central (“z”) axis for the eye. Structurally, this “z” axis needs to be identified such that the optical elements (i.e. light refractive structures) of the eye will be symmetrically aligned along the axis. Next, an OCT device is used to create a cross sectional image of the eye. Importantly, this image is created to include the “z” axis. Once the OCT cross sectional image has been created, and the central “z” axis has been incorporated into the image, various reference points are established along the central “z” axis. Specifically, based on the OCT image, each of the reference points is selected and located where an interface surface between adjacent optical elements in the eye intersects the central “z” axis. In this context, each reference point is specifically related to a particularly identified anatomical surface in the image. The result here is a plurality of contiguous lengths (Δ“z” n ) along the “z” axis that are individually measured between adjacent reference points. These lengths will be anatomically recognized and can be measured with great accuracy. In essence, at this point a template based on the “z” axis has been created that can be used to replicate the optical elements of the eye. 
     A refinement for the present invention can be made by accounting for any tilt there might be between the central “z” axis and the optical elements of the eye. As noted above, the optical elements need to be symmetrically aligned along the central “z” axis. In accordance with the present invention, compensation for a tilt angle “Φ” can be accomplished in either of two ways. In order to compensate for a tilt angle “Φ” (in both ways of compensating for “Φ”) it is first necessary to establish a base reference axis that is substantially oriented in a “z” direction. A base reference point is then located on the base reference axis. Importantly, the base reference point is located on an interface surface inside the eye (object). 
     For one methodology of the present invention, a first axis is identified that is substantially parallel to the base reference axis, and is at a distance “d 1 ” from the base reference axis. A first reference point is then located on the first axis where the first axis intersects the interface surface. The z-location of the first reference point is then compared with an expected z-location for the first reference point to determine a first differential (δ z ′). Similarly, a second axis is identified that is also substantially parallel to the base reference axis, at a distance “d 2 ” from the base reference axis. A second reference point is then located on the second axis where the second axis intersects the interface surface. The z-location of the second reference point is then compared with an expected location for the second reference point to determine a second differential (δ z ″). With this first methodology δ z ′ and δ z ″, along with a measurement of the angle between the respective planes identified by the first and second axes with the base reference axis (e.g. 90° for XZ and YZ planes), are used to measure an angle of tilt (Φ) for the interface surface. The tilt angle Φ can then be used to refine the establishment of the central “z” axis. 
     In another methodology, after a base reference axis is established and a base reference point is located on an interface surface inside the eye (object), a circular path in traced on the interface surface at a distance “r” from the base reference axis. Variations in “z” along the path can then be measured to identify a differential (δ z ) relative to a rotation angle (θ) about the base reference axis. Specifically, in this case, δ z  is measured between a z max  at θ 1  and a z min  at θ 2 . Then, using δ z , θ 1  and θ 2 , the angle of tilt (Φ) for the interface surface can be measured. Like the first methodology, the tilt angle Φ can then be used to refine the orientation of the central “z” axis. 
     A template with greater precision and accuracy can be created by verifying the location of previously selected reference points. To do this, at least one verification point is detected on the “z” axis in the OCT image. Like the original reference points, this verification point will be exactly located on the “z” axis at the intersection of an interface surface with the “z” axis. The location of the verification point in relation to the location of a previously selected reference point is then identified, and measurements are taken to confirm the anatomical identity of the surface that is associated with the reference point. 
     For an operational use of a template that has been created in accordance with the present invention, a replica of the anatomical surface of an optical element can be symmetrically traced between adjacent reference points on the “z” axis. In particular, the replica is based on the location of the selected reference point(s) and on information in the image. As a template, at least one selected reference point, together with the “z” axis, can then be used as a base reference for controlling a movement of a laser beam focal point relative to an identified surface. For example, a first reference point can be located on a posterior surface of an anterior capsule, and a second reference point can be located on the anterior surface of a posterior capsule. Using this template as a control reference, the focal point of the laser beam can then be moved between the anterior capsule and the posterior capsule to perform a capsulotomy. 
    
    
     
       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 line recognition template that has been created in accordance with the present invention; 
         FIG. 2  is a representative cross sectional OCT image of the anterior portion of an eye; 
         FIG. 3  shows the template of  FIG. 1  superposed over a phantom drawing of the OCT image presented in  FIG. 2 ; 
         FIG. 4  is a schematic presentation of the system components that are used with the present invention for a laser ophthalmic surgical procedure; 
         FIG. 5  is a geometric presentation of positional deviations related to a central axis of an eye, which are indicative of a “tilt” of the eye; 
         FIG. 6  is a trace of a circular path around a central axis of an eye that is indicative of a “tilt” of the eye; and 
         FIG. 7  is a graphical presentation of an angular “tilt” of the trace shown in  FIG. 6 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , a line recognition template in accordance with the present invention is shown and is generally designated  10 . As shown, the template  10  essentially includes a central “z” axis  12  that is unique for the present invention in several important respects. Most importantly, the axis  12  is selected primarily for the symmetry it establishes for optical elements of the eye  14 , such as the cornea  16 , the crystalline lens  18  and the capsular bag  20  of the lens  18 . With reference to  FIG. 2  it can be appreciated that each of these optical elements in the eye  14  will be axially symmetric relative to a properly oriented central “z” axis  12 . Moreover, each of these optical elements is anatomically recognizable and they are all arranged in a set anatomical order along the axis  12 . 
     In accordance with the present invention, the proper orientation of the axis  12  is initially established in relation to a cross sectional image of at least the anterior portion of the eye  14 . As shown in  FIG. 2 , this image is preferably made using an imaging device of a type well known in the pertinent art that is capable of OCT imaging. Once the central “z” axis  12  has been properly oriented on the OCT image (see  FIG. 2 ), dimensional references that correspond to optical features of the eye  14  are established along the axis  12 . 
     By way of example, consider the reference point  22  on central “z” axis  12 . With this reference point  22  in mind, it is well known that the anterior surface  24  of cornea  16  can be imaged using OCT techniques. In particular, from the perspective of the refractive differences of the adjacent media, and the capabilities of OCT imaging, the surface  24  can be identified as the interface surface between the environmental air and the cornea  16 . Consequently, the intersection of this surface  24  with the axis  12  can be used to accurately set a location for the reference point  22  on the axis  12 . These same considerations and capabilities can be used to accurately set other reference points. 
     Following from the above disclosure, reference point  26  can be identified and located on the central “z” axis  12  at the intersection of the interface surface  28  with the axis  12 . In this case, surface  28  is the interface between the cornea  16  and the anterior chamber  30  of eye  14  (see  FIG. 2 ). Further, the reference point  32  can be identified and located on the axis  12  at the intersection of an anterior interface surface  34  with the axis  12 . In this case, the anterior interface surface  34  lies between the crystalline lens  18  and the capsular bag  20 , in the anterior portion of the crystalline lens  18 . Still further, the reference point  36  can be identified and located on the axis  12  at the intersection of posterior interface surface  38  with the axis  12 . In this case, the posterior interface surface  38 , in the posterior portion of the crystalline lens  18 , lies between the crystalline lens  18  and the capsular bag  20 . 
     Referring again to  FIG. 1 , it is to be appreciated that the template  10  is established with anatomically accurate dimensional information. Specifically, in this example, the length “Δz 1 ” effectively represents the thickness of cornea  16  along the axis  12  between reference point  22  and reference point  26 . The measured length of “Δz 1 ” is then anatomically determined with reference to the OCT image of eye  14  shown in  FIG. 2 . The length “Δz 2 ” (i.e. the length along axis  12  across the anterior chamber  30  between reference point  26  and reference point  32 ) is similarly determined, as is the length “Δz 3 ” between reference points  32  and  36 . Thus, the orientation of the central “z” axis  12 , together with the placement of the reference points  22 ,  26 ,  32  and  36  on the axis  12  and their corresponding contiguous lengths “Δz 1 ”, “Δz 2 ” and “Δz 3 ”, collectively create the template  10 . The result is a template  10  that can be superposed on an OCT image (see  FIG. 3 ) and used as a control reference for ophthalmic surgical procedures, such as a capsulotomy. 
     For a refinement of the present invention, it is to be understood that a template  10  which is created in accordance with the above disclosure, can be verified for accuracy, if desired. To do so, a verification point  40  is selected. In this case, the verification point  40  shown in  FIG. 1  is only exemplary, and it is taken to be the most anterior intersection of the capsular bag  20  with the central “z” axis  12 . The corresponding verification distance “Δv 1 ” (i.e. the thickness of the capsular bag  20 ) can then be measured to determine whether the distance “Δv 1 ” and its location on the axis  12  will verify the accuracy of template  10 . 
     In another aspect of the present invention, selected reference points can be used to trace the outline of optical elements in the eye  14 . For example, in  FIG. 1  a traced outline of the crystalline lens  18  is shown by the dotted line  42 . As shown, the dotted line  42  represents extensions of the interface surfaces  34  and  38 , between the reference points  32  and  36  that replicate the boundary of the crystalline lens  18 . Again, this is only exemplary as it will be appreciated that similar extensions between other reference points can be made to replicate other optical elements in the eye  14 . 
     In an operation of the present invention, the template  10  is superposed onto an OCT image of the eye  14  as shown in  FIG. 3 . As mentioned above, this is done to establish a base reference that can be used by the computer/controller  44  for laser control purposes. As envisioned for the present invention, the template  10  (i.e.  FIG. 3 ) is based on images of the eye  14  obtained from the imaging unit  46 . As indicated in  FIG. 4 , the computer/controller  44  can the effectively control the laser unit  48  to guide and control a laser beam  50  during an ophthalmic surgical operation in the eye  14 . 
     In another aspect of the present invention, the location and orientation of the central “z” axis  12  can be refined in accordance with either of two methodologies. For the first methodology, reference is directed to  FIG. 5  where the reference point  32 , on interface surface  34 , is used as an example. In this example, the reference point  32  is shown located on the base central axis  12 . An axis  52  is then identified that is substantially parallel to the base central axis  12 , and is located at a distance “d 1 ” from the base central axis  12 . Further, based on imaging techniques as disclosed above, a reference point  54  is located on the axis  52  that is actually located on the interface surface  34 . The computer/controller  44  ( FIG. 4 ) then compares the actual location of the reference point  54  with an expected location for the reference point  54 ′ to determine a first differential (δ z ′). 
     With further consideration of the first methodology, an axis  56  is identified which is also substantially parallel to the base central axis  12 , and is located at a distance “d 2 ” from the base central axis  12 . Further, a reference point  58  is located on the axis  56  which is also on the interface surface  34 . As was done with reference point  54 , the computer/controller  44  compares the location of the reference point  58  with an expected location for the reference point  58 ″ to determine a second differential (δ z ″). The computer/controller  44  then uses δ z ′ and δ z ″ to measure an angle of tilt (Φ) for the interface surface  34 , and incorporates the tilt angle Φ into its control over an operation of the laser unit  48 . 
     For a second methodology that can be alternatively employed to refine the orientation and location of the base central axis  12 ,  FIG. 6  indicates that a circular path  60  can be identified that encircles the base central axis  12 . Importantly, the path  60  lies on an interface surface (e.g. interface surface  34 ). Thus, the reference point  32  will be located on the base central axis  12 , with the path  60  on the interface surface  34  at a distance “r” from the base central axis  12 . The computer/controller  44  is then used to measure variations in “z” along the path  60 . Specifically, these variations are measured relative to a rotation angle (θ) that is taken about the base central axis  12 . The result of these measurements is shown in  FIG. 7 . As will be appreciated by the skilled artisan, the measurements from  FIG. 7  can then be used by the computer/controller  44  to identify a differential (δ z ) which is measured between a z max  at θ 1  and, 180° later, a z min  at θ 2 . The computer/controller  44  then uses δ z , θ 1  and θ 2  to measure an angle of tilt (Φ) for the interface surface  34 , and incorporates the tilt angle Φ into its control over an operation of the laser unit  48 . 
     While the particular System and Method for Creating a Customized Anatomical Model of an Eye 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.