Abstract:
A system and method are provided for establishing precise locations for a focal point of a laser beam within a predetermined scanning range during ophthalmic laser surgery. An important aspect of the present invention is the use of a tolerance for deviation of the laser beam&#39;s focal point from the laser beam path. The purpose of the tolerance is to ensure that the surgical procedure is effective and that collateral damage to non-targeted tissue does not occur. The present invention accounts for deviations caused by various factors during a procedure. A computer is provided to ensure that the cumulative effect of all deviations maintains the focal point within the tolerance.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains generally to systems and methods for performing ophthalmic laser surgery. More particularly, the present invention pertains to systems and methods for performing an ophthalmic laser surgical procedure that accounts for optical distortions introduced by the anatomy of the eye and by the system components that are required for performing laser surgery. The present invention is particularly, but not exclusively, useful as a system and method for establishing precise locations for a focal point within a predetermined tolerance that is established for a scanning range which is required to conduct a particular ophthalmic laser procedure. 
       BACKGROUND OF THE INVENTION 
       [0002]    Femtosecond laser technology has been adapted for use with various ophthalmic laser surgical procedures. Until recently, the use of femtosecond lasers on the eye has focused primarily on the cornea. As the use of femtosecond lasers becomes more advanced, other areas of the eye that lie beyond the cornea are now being targeted by procedures that use femtosecond lasers. As appreciated by skilled artisans, often laser systems such as picosecond and UV lasers can be similarly employed. In the event when areas beyond the cornea are targeted, establishing precise locations for the focal point of the laser becomes less precise due to deviations in focal point location caused by: (1) the location in the eye where the procedure is being conducted; (2) system components required for the procedure; and (3) the interaction of the system with the eye during the procedure. Furthermore, these deviations in focal point location can cause the laser procedure to be less effective, or cause damage to areas of the eye that are not being targeted by the procedure. 
         [0003]    For any laser surgical procedure, attention must be paid to deviations in focal point location. Different surgical procedures target different areas of the eye, and targeted areas of the eye may be more prone to deviations because of factors like depth within the eye or the anatomical structure of the targeted area. Deviations can also occur based on distortions of the eye that occur due to contact of the eye by the laser unit during the procedure. 
         [0004]    System components required for a laser surgical procedure can also produce deviations in focal point location. One important component required by a laser system for positioning a laser beam focal point is an algorithm that is used by a computer to produce a reference datum. Such a reference datum is needed for accurate movement and placement of the focal point during the procedure. Typically, each algorithm will introduce a deviation because of the level of detail it provides for the reference datum. This will vary depending on the algorithm that is selected. Another component that can produce deviations is the lens or lenses in the optical unit. In particular, during a procedure, the lens or lenses must follow a very precise path to accurately focus the laser to a focal point. Further, imprecise or inaccurate mechanical responses of the laser system (e.g. inaccurate lens movement) will also introduce deviations in focal point location. Other deviations in focal point location can be caused by distortions in the eye which are caused when a patient interface is used to facilitate a connection between the laser unit and the eye. 
         [0005]    One way to account for the deviations that may be caused by the factors discussed above is to establish a tolerance. In the context of the present invention, the tolerance is an acceptable margin for error in the location of the focal point during a procedure. This tolerance is selected to allow for slight deviations to occur that will not affect the quality of the laser procedure being performed or damage non-targeted areas of the eye. 
         [0006]    In light of the above, it is an object of the present invention to provide systems and methods for establishing precise locations for a focal point of a laser. Another object of the present invention is to provide systems and methods for establishing precise locations for a focal point that accounts for deviations caused by the interaction of a laser system with the eye. Still another object of the present invention is to provide systems and methods establishing precise locations for a focal point of a laser beam within a predetermined scanning range that are easy to use and comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with the present invention, a system and method are provided for precisely positioning the focal point of a laser beam during laser ophthalmic surgery. Initially, the particular surgical procedure to be performed is selected and identified. For the present invention it is envisioned that such a procedure may be performed anywhere in an eye where a laser beam can be effectively focused (examples include, but are not limited to, the cornea, the crystalline lens, the trabecular meshwork, and the retina). Based on the requirements of the selected procedure, a tolerance for deviation of the laser beam&#39;s focal point from the laser beam path during the procedure is determined and established. For instance, it may be necessary to keep the laser beam&#39;s focal point beyond a certain distance from an identified tissue interface. Specifically, this restriction may be necessary in order to prevent unwanted collateral damage to a non-targeted tissue, or because the application demands high precision. In each case, as implied above, such considerations go to establishing the tolerance for the particular procedure. 
         [0008]    It is understood by the present invention that various factors can affect the operation of a laser system and, consequently the placement of its laser beam&#39;s focal point. Of particular concern here is the extent to which these factors, individually and collectively, will cause the focal point to deviate from an intended beam path during an operational procedure. In the event, whatever deviations in focal point placement may be introduced, their cumulative effect must not exceed the limitations of the predetermined tolerance. For the present invention, the factors which can create focal point deviations that are of particular interest include: 1) the mechanical response of the optical components in a laser system during the focusing and placement of a laser beam&#39;s focal point; 2) the accuracy of the algorithm that is to be used by a laser system for operational guidance and control of a laser beam&#39;s focal point; and 3) distortions of the eye that may be caused when a patient interface is used to bring a laser unit into contact alignment with an eye. 
         [0009]    Structurally, the system of the present invention includes a laser unit for generating a laser beam, and it includes a moveable lens that is mounted on a rail for focusing the laser beam to a focal point. Also included is a computer for defining a path for movement of the focal point in an x-y-z space during surgery. Specifically, movement of the focal point will be within the scanning range that is required by the surgical procedures and, most importantly, the position of the focal point will be confined to within an established tolerance. 
         [0010]    In order to minimize deviations in out-of-tolerance focal point movements that may be introduced by the laser unit itself, an arrestor is selectively positioned on the rail to fix a start position for the lens. An actuator then moves the lens along the rail from the start position in response to instructions from the controller. In particular, the start position is selected to keep required lens movements close to the start position. This is done to thereby minimize out-of-tolerance deviations that may otherwise be caused when lens movements are farther from the start position and less controllable. As envisioned for the present invention, additional arrestors can be positioned along the rail, as desired, to minimize focal point deviations in selected areas of the scanning range. 
         [0011]    Another structural component of interest for the present invention is an optical imaging device that can be used to produce an image of the x-y-z space required by the ophthalmic procedure. Once created, the image is inputted into an algorithm to establish a reference datum for movement of the focal point along a defined path in the x-y-z space. As is well known, different types of imaging devices, and different algorithms have different levels of accuracy and precision. With this in mind, the present invention requires selection of an algorithm that establishes a reference datum which will effectively maintain precise locations for the focal point within the predetermined tolerance. Preferably, the ophthalmic imaging device for the present invention will be an Optical Coherence Tomography (OCT) scanner or a Hartmann-Shack sensor. In addition to an OCT scanner and a Hartmann-Shack sensor, other types of imaging devices appropriate for use with the present invention include: a topographic imaging unit, a Scheimpflug imaging unit, a confocal imaging unit, a two-photon imaging unit, a laser range finding imaging unit, and a non-optical imaging unit. 
         [0012]    Still another structural component that is often used in a laser system which may cause a laser focal point to deviate from its intended focal point is a patient interface. Specifically, patient interfaces are sometimes used to establish an interaction between the laser unit and an eye of a patient that will stabilize the eye during a surgical procedure. In use, however, patient interfaces can distort the eye and thereby introduce deviations in focal point placements. Typical patient interfaces, in order of improving operational effect (i.e. less distortion), include an applanation lens, a concave lens, and a water-filled lens. Like the other structural components mentioned above, the patient interface needs to be selected to maintain the focal point within the predetermined tolerance. 
         [0013]    In an alternate embodiment, instead of using a single lens for positioning the focal point of the laser beam along the z-axis, two lenses are used. More specifically, the system can include two lenses mounted for movement on the rail, with a first (proximal) lens mounted on the rail nearer to the laser unit and a second (distal) lens mounted on the rail further from the laser unit. For this embodiment, the distal lens can be selectively positioned at one of a plurality of distal start points. Specifically, each of these distal start points corresponds to a particular ophthalmic procedure. For example, when the distal lens is positioned at a selected start point, the system will be configured for moving the laser beam&#39;s focal point to treat tissue in a specific portion of the eye (e.g. cornea, crystalline lens or retina). Preferably, throughout the procedure, the distal lens remains stationary and the proximal lens moves along the rail. This structural cooperation moves the focal point of the laser beam in the manner required for the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    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: 
           [0015]      FIG. 1  is a schematic of the components of a system in accordance with the present invention; 
           [0016]      FIG. 2A  is an illustration of the relationship of the scanning range, the tolerance, and z-value for the present invention; 
           [0017]      FIG. 2B  is an illustration of the cumulative effect of deviations caused by various factors; 
           [0018]      FIG. 3  is a flowchart of the operation of the present invention; 
           [0019]      FIG. 4  is a schematic of an alternate embodiment of components of a system in accordance with the present invention when two lenses are used; and 
           [0020]      FIG. 5  is a diagram of the effect a second lens can have on different scanning ranges. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Referring initially to  FIG. 1 , an ophthalmic laser system in accordance with the present invention is shown and is generally designated  10 . As shown, the system  10  includes a computer  12 , an imaging unit  14 , and a laser unit  16 . Taken together, these components of the system  10  will cooperate with each other to direct a laser beam  18  from the laser unit  16  and toward an eye  20  for the purpose of performing laser surgery on the eye  20 . 
         [0022]    Various other components illustrated in  FIG. 1  are also required for the present invention. It can be seen that laser beam  18  is directed from the laser unit  16  to a lens  22  that is mounted on a rail  24 . After passing through the lens  22 , the beam  18  is directed to a focal point  26  in the eye  20 . As shown, the lens  22  is affixed to a sled  28  via a connecting rod  30  for movement during laser surgery. For movement of the lens  22  along the rail  24 , an actuator  32  is provided that is electronically connected between the lens  22  and the computer  12 , which provides movement instructions to the actuator  32 . Two mechanical stops, an arrestor  34  and a second arrestor  36 , are also provided to limit the movement of the lens  22  along the rail  24 . In addition to limiting movement in this way, the arrestor  34  and the second arrestor  36  may also be used as calibrated reference points for movement of the lens  22 . This is accomplished by moving the lens  22  into a position where the sled  28  contacts the arrestor  34  or the second arrestor  36  prior to the start of, or any time during, a procedure. Thus, for some types of procedures, the calibrated reference point is also a start point for the procedure. Moreover, for some types of procedures, such as procedures that do no require the accuracy afforded by a calibrated reference point, it may not be required to move the lens  22  against the arrestor  34  or  36  at the start of, or during, the procedure. The arrestor  34  is a shaft formed onto the laser unit  16  and extending in a horizontal direction away from the laser unit  16 . For the second arrestor  36 , it is envisioned to be a wheel stop with a shape like the one shown in  FIG. 1 . Or, the second arrestor  36  can also take the shape of the arrestor  34 . At any point during the procedure, the distance of the lens  22  from the arrestor  34  can be measured to have a value of “L”  38 . This measurement is accomplished by the interaction of a sensor (not shown) connected to the lens  22  or sled  28  that reads a plurality of incremental reference lines formed onto the rail  24 . 
         [0023]    In  FIG. 1 , a patient interface  40  is also shown in contact with the eye  20 . As described earlier, the patient interface  40  can be any type appropriate for use for the particular procedure. Three types of patient interface  40  that are suitable for use with the present invention are: an applanation lens, a concave lens, and a water-filled lens. When choosing a particular patient interface  40 , an operator will consider the type of procedure being performed, as well as the effect of the lens  22  on the anatomical structure of the eye  20  of a patient. 
         [0024]    Now referring to  FIG. 2A , the relationship between a scanning range  42  and tolerance  44  is shown. It can be seen that the scanning range  42  includes a start point  46 . The lens  22  begins at the start point  46  and moves in a z-direction along the z-axis  48  to focus the laser beam  18  to locations on a defined optical path. In  FIG. 2A , two exemplary locations for the focal point  26  on the defined path are shown and generally designated  50   a  and  50   b . In  FIG. 2A , location  50   a  is closer to the start point  46 , which means location  50   a  is closer to the laser unit  16  than location  50   b . In order to locate the focal point  26  at location  50   a  and  50   b  during a procedure, the computer  12  instructs the actuator  32  to move the lens  22  to a particular position on the rail  24  that will focus the laser beam  18  at the appropriate location  50   a ,  50   b . For the exemplary scanning range  42  shown in  FIG. 2A , the tolerance  44  is the same for both locations  50   a  and  50   b . This is because the selected tolerance  44  always remains substantially the same throughout any selected scanning range  42 . Different tolerances  44  are only used when multiple scanning ranges are used during a procedure, which is not the case illustrated in  FIG. 2A . 
         [0025]    Again referring to  FIG. 2A , each location  50   a ,  50   b  has an associated deviation distance  52 , with location  50   a  having a deviation distance  52   a , and location  50   b  having a deviation distance  52   b . These deviation distances  52   a ,  52   b  represent total deviations that account for deviations caused by any factor. As shown, deviation distance  52   b  has a larger magnitude than deviation distance  52   a . This occurs because the lens  22  is further from the start point  46 . And, the further the lens  22  is moved from the start point  46 , the less will be the accuracy of the focal point position of the laser beam  18 , and the greater will be the deviation distance  52 . Despite the difference in magnitude, both deviation distances  52   a ,  52   b  are within the tolerance  44  for the scanning range  42 . Additionally, arrow  56  is provided to show that the focal point  26  for the scanning range  42  can move in any forward and backward along the z-axis  48 . 
         [0026]    Referring now to  FIG. 2B , the cumulative aspect of a total deviation is illustrated. For a selected ophthalmic procedure, several factors induce deviations in the location of the focal point  26  during the procedure. As shown, the cumulative effect of all deviations must still maintain the focal point  26  within the tolerance  44 . In  FIG. 2B , three deviations are shown: (1) deviation  58  for deviations induced by the selected algorithm; (2) deviation  60  for deviations caused by the inaccurate movement of the lens  22 ; and (3) deviation  62  for deviations caused by the selected patient interface  40 . As illustrated, each of the three deviations  58 ,  60 ,  62  has a unique value, and when added together, the sum of the three deviations  58 ,  60 , and  62  is deviation  63 . As shown, deviation  63  is less than the tolerance  44  (deviation  58 +deviation  60 +deviation  62 &lt;T). 
         [0027]    In  FIG. 3 , a flowchart is used to demonstrate the operation of the present invention. To begin, a procedure is selected in action block  64 . This procedure can be any type of procedure that requires the use of a femtosecond laser, and the procedure can occur at any depth in the eye  20 . When a procedure is selected, an associated protocol is also selected that will include, at a minimum, the appropriate scanning range  42  required for the procedure. With the tolerance  44  determined, the computer  12  establishes an optical path for the focal point  26  in action block  68 . Once the optical path has been determined, the computer  12  conducts an analysis to ensure that the focal point  26  remains within the tolerance  44  due to deviations induced by the path as shown in inquiry block  70 . If the focal point  26  is within the tolerance  44 , an algorithm is selected in action block  72 . If the focal point  26  is not within the tolerance  44 , the computer  12  calculates whether the focal point  26  can be brought into tolerance  44  at inquiry block  74 . If the focal point  26  can be brought into tolerance  44 , then the actuator  32  is adjusted to incorporate a new optical path at action block  76 . If the computer  12  determines that the focal point  26  cannot be brought into tolerance  44  at inquiry block  74 , the decision is made whether to restart the procedure at inquiry block  78 . When the procedure is restarted, the system  10  is reconciled at action block  80  and a new tolerance  44  is determined at action block  66 . If a decision is made at inquiry block  78  to not restart the procedure, then the procedure is stopped at action block  82 . 
         [0028]    Continuing the procedure after an optical path has been determined to be within the tolerance  44 , an algorithm is selected at action block  72 . This algorithm is used to produce a reference datum that is used to guide the focal point  26  during the procedure. It will be appreciated that, if two or three positions on a corneal surface are measured, only second-order Zernike polynomial coefficients can be accurately calculated. That is, the spherical shape or the cylindrical shape can be determined. If ten points on a corneal surface are measured, then third-order Zernike polynomial coefficients can be calculated. If fifteen points on a corneal surface are measured, then fourth-order Zernike polynomial coefficients can be calculated. That is, defocus, spherical aberration, second order astigmatism, coma, and trefoil can be calculated. After the algorithm is selected, the computer  12  then determines whether the focal point  26  is within the tolerance  44  due to deviations induced by the algorithm at inquiry block  84 . It should be noted that the computer  12  in inquiry block  84  also must account for deviations caused by the optical path (See action block  68 ). If the focal point is within the tolerance  44 , the planning of the procedure continues. If it is not, the computer  12  again determines whether the focal point  26  can be brought into tolerance  44 , and if it can be brought into tolerance  44  at inquiry block  86 , the algorithm is modified at action block  88 . The decision to either restart or stop the procedure is the same as described earlier with inquiry block  78  and action blocks  80  and  82 . 
         [0029]    Once it has been determined that the focal point is in tolerance at inquiry block  84 , a patient interface  40  is selected or detected at action block  90 . Once the patient interface  40  is selected or detected, the computer  12  determines whether the focal point  26  remains within the tolerance  44  due to deviations caused by the patient interface  40  at inquiry block  92 . At inquiry block  92 , the computer  12  is still accounting for deviations caused by the optical path and the algorithm (See blocks  68  and  84 ). If the focal point  26  is within the tolerance  44 , then the procedure is conducted as depicted in action block  94 . If the focal point  26  is not within the tolerance  44 , the computer  12  again determines whether it can be brought within the tolerance  44  at inquiry block  96 . If the focal point  26  can be brought within the tolerance  44 , then a new patient interface  40  is selected at action block  98 . If the focal point  26  cannot be brought into tolerance  44  at inquiry block  96 , a decision is again made at block  78  to restart or stop the procedure. The follow-on steps to inquiry block  78  are the same as disclosed previously. 
         [0030]    Referring now to  FIG. 4 , an alternate embodiment for the system of the present invention is shown and is generally designated  100 . As shown, in addition to the components disclosed above for the system  10 , the system  100  includes an additional lens  102 . More specifically, the lens  102  is mounted on a sled  104  for coaxial movement on rail  24 , relative to the lens  22 . Movement of the lens  102  on rail  24  is provided by an actuator  106  that interconnects the lens  102  with the computer  12 . In this arrangement, the lens  22  is sometimes hereinafter referred to as the “proximal lens  22 ” and the lens  102  will then be referred to as the “distal lens  102 ”. 
         [0031]      FIG. 4  also shows that the lens  102  can be moved, and selectively stopped, at any of three different arrestors (i.e. arrestor  108 , arrestor  110  and arrestor  112 ). As envisioned for the present invention, the arrestors  108 ,  110  and  112  are positioned in alignment along the rail  24  to establish respective start points for the lens  102 . As indicated in  FIG. 4 , regardless which arrestor (i.e.  108 ,  110  or  102 ) may be used with the lens  102 , the lens  102  is intended to cooperate in combination with the lens  22  from the selected start point. The significance of this is best appreciated with reference to  FIG. 5 . 
         [0032]    As shown in  FIG. 5 , when the distal lens  102  is positioned at the arrestor  108 , a cooperative interaction of the distal lens  102  with the proximal lens  22  will move the focal point  26  of the laser beam  18  within a scanning range  114 . The import here is that the scanning range  114  is effective for ophthalmic procedures which are to be performed in the front (i.e. cornea) of the eye  20 . Similarly, when the distal lens  102  is positioned at the arrestor  110  (e.g. lens  102 ′), the cooperative interaction of the distal lens  102 ′ with the proximal lens  22  will move the focal point  26 ′ of the laser beam  18 ′ within a scanning range  116 . In this case, the scanning range  116  needs to be effective for surgeries deeper in the eye  20  (e.g. crystalline lens). Likewise, when the distal lens  102  is positioned at the arrestor  112  (e.g. lens  102 ″), the cooperative interaction of the distal lens  102 ″ with the proximal lens  22  will move the focal point  26 ″ of the laser beam  18 ″ in a scanning range  118  for procedures performed deep in the eye  20  (e.g. retina). 
         [0033]    For each of the above described scenarios (i.e. respective scanning ranges  114 ,  116  and  118 ) it will be appreciated that either the proximal lens  22 , or the distal lens  102 , can be moved from their respectively selected start points to move focal point  26  within a selected scanning range  114 ,  116  or  118 . In each case, the respective start points for lenses  22  and  102  will be established by an arrestor. Specifically, lens  22  will operate relative to the arrestor  34 , and lens  102  will operate relative to whichever arrestor  108 ,  110  or  112  is to be used for a selected procedure. During an operation of the system  100 , however, only one of the lenses, lens  22  or lens  102  will be moved to vary the location of the focal point  26  within the particularly selected scanning range  114 ,  116  or  118 . 
         [0034]    While the particular System and Method for Controlling the Focal Point Locations of a Laser Beam 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.