Abstract:
A system and method are provided wherein an operational characteristic of a laser beam is identified. A predetermined ophthalmic reference datum is also identified. The identified laser beam characteristic is then used in its relationship with the reference datum for guidance and control of the laser beam&#39;s focal point. In operation, the laser beam&#39;s focal point is moved through eye tissue while minimizing any deviations of the operational characteristic of the laser beam from the reference datum.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/535,841, filed Sep. 16, 2011 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains generally to systems and methods for performing ophthalmic laser surgery which results from the Laser Induced Optical Breakdown (LIOB) of selected tissue inside an eye. More particularly, the present invention pertains to systems and methods for performing LIOB wherein the laser beam path passes through different types of eye tissue, with each tissue type having a different threshold for LIOB. The present invention is particularly, but not exclusively, useful as a system and method for performing ophthalmic surgery wherein an operational characteristic of a laser beam is dimensionally identified, and the identified characteristic is then referenced with an imaged datum, to position the laser beam&#39;s focal point for an intended LIOB result in selected tissue of the eye. 
       BACKGROUND OF THE INVENTION 
       [0003]    Each laser beam will always have certain physical characteristics that are unique to that particular beam. In the specific case of a pulsed laser beam, apart from the wavelength of the light, laser beam characteristics will include: the location of the laser beam&#39;s focal point on the beam path; the cross section area of the laser beam at selected points (i.e. stations) along the beam path; and the energy level of the laser beam. A collective consequence of these characteristics is that each laser beam will have a determinable fluence (i.e. energy density) at each station along its path. In the case of a focused laser beam, this fluence will change inversely with changes in the beam&#39;s cross sectional area. In particular, the fluence of a focused light beam will increase as the cross sectional area of the beam decreases. Insofar as a Laser Induced Optical Breakdown (LIOB) of tissue in the eye is concerned, this change in the fluence of a laser beam can become of considerable importance. 
         [0004]    Anatomically, it is well known that the eye has many different types of tissue, and that each of these tissue types is in direct contact with at least one other type of tissue. It is also well known that all of the various type tissues of the eye are susceptible to alteration (e.g. photoablation) by LIOB. Further, the threshold for LIOB will vary from tissue to tissue, and the occurrence of LIOB will depend on the fluence of the beam as it passes through the particular tissue. In overview, all of these factors lead to at least three separate operational considerations. For one, in an eye, the interface surface between adjacent, different type tissues is detectable by known imaging techniques, such as Optical Coherence Tomography (OCT), Scheimpflug, confocal, two-photon, laser (optical) range finding, or acoustical (non-optical) imaging. For yet another, the interface image can be used as a reference datum for laser guidance and control purposes. For yet another, it is often desirable in many ophthalmic procedures to perform LIOB on only one type of tissue, without causing collateral damage to other types of tissue. 
         [0005]    In light of the above, it is an object of the present invention to perform Laser Induced Optical Breakdown (LIOB) on selected tissue inside an eye, wherein the laser beam path passes through different types of eye tissue. Another object of the present invention is to perform ophthalmic surgery wherein an operational characteristic of a laser beam is identified, and is referenced with a datum, to precisely position the laser beam&#39;s focal point in the eye for an intended LIOB result in selected tissue. Still another object of the present invention is to provide a laser system for performing ophthalmic surgery which is easy to use, simple to manufacture and comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with the present invention, a system and method are provided for moving the focal point of a laser beam through eye tissue. Specifically, this is done for the purpose of performing Laser Induced Optical Breakdown (LIOB) on the tissue during a surgical procedure. As envisioned for the present invention, the path of the focal point will be through tissue that is inside the eye. Consequently, the target tissue for LIOB will be in a layer of underlying tissue, and the laser beam must necessarily pass through a layer of overlying tissue before it gets to the underlying target tissue. It typically happens that the overlying tissue and the underlying tissue will have different thresholds for LIOB. It is, nevertheless, desirable, and perhaps essential, that LIOB occur in only the target (i.e. underlying) tissue. 
         [0007]    With this in mind, an operational concern for the present invention is that LIOB may be inadvertently performed on the overlying tissue. This is particularly problematic when the LIOB threshold of the overlying tissue is below the LIOB threshold of the underlying (target) tissue. In such a case, if the focal point of the laser beam is too close to the interface surface that is between the overlying tissue and the underlying (target) tissue, it can happen that the energy density (fluence) of the laser beam will exceed the LIOB threshold of the overlying tissue. As indicated above, this is to be avoided. 
         [0008]    Structurally, a system in accordance with the present invention includes a laser unit for generating a laser beam with ultrashort pulses (e.g. femtosecond, picosecond or short nanosecond). Also, it includes an optical assembly for focusing the laser beam along a beam path. For example, the optical assembly may include scanners, adaptive optics, or optics with a variable numerical aperture. Importantly, this laser beam will have determinable cross sectional dimensions at respective stations along the beam path. Stated differently, depending on the energy in the laser beam, and the adaptive optics that is being used for the system, the laser beam will be dimensioned to have a determinable profile. Further, based on this profile, the energy density (i.e. fluence) of the laser beam at selected stations along the beam path can be determined. 
         [0009]    In addition to the laser unit, the system also includes a detector for identifying a reference base inside an eye of a patient. For purposes of the present invention, this reference base is preferably an interface surface that is identified inside an eye, and is located between an overlying tissue and an underlying tissue. For purposes of the present invention, the interface surface can be established between different tissues such as the cornea/aqueous, aqueous/trabecular meshwork, aqueous/lens, lens/vitreous, corneal tissues, lens tissues and retinal tissues. As noted above, the overlying tissue will have an LIOB threshold, “T 1 ”, and the underlying tissue will have a different LIOB threshold, “T 2 ”. Preferably, the detector will be an optical device that identifies the reference base (interface surface) using any of various well known imaging techniques. More specifically, imaging techniques envisioned for the present invention include optical, interferometric and ultrasound techniques. Further, these techniques may be employed by appropriately using Optical Coherence Tomography (OCT), wavefront analysis, confocal microscopy, Scheimpflug, two-photon imaging, or laser (optical) range finding devices. 
         [0010]    A computer is also included in the system of the present invention and it will be used for controlling an operation of the laser unit in accordance with a predetermined computer program product. Thus, the computer controls the movement of the laser beam&#39;s focal point. In particular, these movements may be in geometric and/or non-geometric patterns that include spirals, lines, rasters, circles, planes and cylinders. 
         [0011]    The computer is also used to select a station on the beam path having a specified energy density (fluence). As envisioned for the present invention, the identification of a station involves its location on the beam path, as well as the cross sectional area of the laser beam at that location. Thus, for a beam having a particular energy, the energy density (fluence) of the beam at a particular station can be determined. This selection of a station for the present invention is important for at least two reasons. For one, the selected station will have an energy density (fluence) that is below the “T 1 ” LIOB threshold for the overlying tissue. For another, the selected station can be determined as being at a distance “d” upstream from the focal point of the laser beam. 
         [0012]    An exemplary application of the present invention involves the cornea of an eye. In this example, the overlying tissue is the epithelium of an eye and the underlying surface is the stroma of the eye. Accordingly, the interface surface is against a posterior surface of the epithelium (e.g. Bowman&#39;s membrane). In one mode of operation, the computer maintains the distance “d” at a constant value in order to create a flap of stromal tissue having a substantially uniform thickness. Such a flap could be used, for example, as part of a LASIK procedure. In certain circumstances, a constant stromal thickness for such flaps, or even a predetermined stromal thickness pattern for such a flap, may be desirable in its own right. Thus, “d” can be established, according to the requirements of a particular application, to create patterns for stromal tissue that have predetermined thicknesses. For example, such applications may include LASIK procedures (as noted above), the creation of stromal pockets, and the creation of constant thickness flaps. In other modes of operation, the distance “d” can either be continuously minimized, or otherwise arbitrarily established to avoid causing unwanted LIOB. Within the eye, it will be appreciated that the interface surface may be established between any two different types of tissue. For instance, it may be established between tissues in the lens of an eye, between tissues in the retina of an eye, or between tissues in the sclera. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    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: 
           [0014]      FIG. 1  is a schematic of the components of a system in accordance with the present invention; 
           [0015]      FIG. 2  is a schematic of the operational characteristics of a focused laser beam; and 
           [0016]      FIG. 3  is an illustration of a laser beam and its focal point positioned relative to an overlying tissue layer and an underlying tissue layer during an operation of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    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 laser unit  12 , a detector  14 , and a computer  16 . Collectively, these components of the system  10  will cooperate with each other to direct a laser beam  18  from the laser unit  12  and toward an eye  20  for the purpose of performing laser surgery on the eye  20 . 
         [0018]    For the present invention, the laser unit  12  preferably comprises what is commonly referred to as a “femtosecond laser.” Specifically, this means that the laser beam  18  which is generated by the laser unit  12  will be pulsed, and that pulses in the laser beam  18  will be of ultrashort duration (e.g. 500 fs). Further, the laser beam  18  needs to be generated with an energy level in each pulse that will cause Laser Induced Optical Breakdown (LIOB) in selected tissues of the eye  20 . Thus, in general, the laser beam  18  could range from femtosecond, picosecond, or short nanosecond pulse lasers that emit their radiation in the infrared, visible, or ultraviolet wavelength range. 
         [0019]    The detector  14  of system  10  can be a device of any type known in the pertinent art that is capable of creating two or three dimensional images of tissue structures inside the eye  20 . Preferably, the detector  14  employs interferometric techniques and is an Optical Coherence Tomography (OCT) device that can create three dimensional images of interface surfaces that are identified between two different types of adjacent eye tissue. 
         [0020]    As envisioned for system  10 , the computer  16  will be connected with the laser unit  12 , and with the detector  14 , substantially as shown in  FIG. 1 . With these connections, the computer  16  uses imaging information from the detector  14 , along with programmed input to the computer  16  that is provided by the operator of system  10 , for moving the laser beam  18 . In particular, closed loop, feedback control techniques are used by the computer  16  for the purposes of guiding and controlling the laser unit  12 . 
         [0021]    It is an important aspect of the present invention that the guidance and control of the laser unit  12  be precise, and that it be effective for the accomplishment of an intended LIOB result. In general, such control can be relatively straightforward when only homogeneous tissue is involved. There are, however, many locations in an eye  20  where LIOB may be useful, but different types of tissue are in close proximity to each other. On this point, recall that different tissues in the eye  20  have different thresholds for LIOB, and they have different refractive properties. 
         [0022]    When considering the respective LIOB thresholds of different tissue types, the following hypothetical is helpful. If the laser beam  18  is set with an energy level that will alter one tissue having a relatively high LIOB threshold “T 2 ”, it is possible that another tissue with a lower LIOB threshold “T 1 ” can be unintentionally affected by the laser beam  18 . This is particularly problematic at the interface between different tissues, and it is a situation that is obviously to be avoided. Further, it is known that beam convergence needs to be reduced the further posterior in the eye one focuses a laser beam, hence while one may avoid unintended LIOB in the corneal epithelium simply by using a highly convergent beam, this design option becomes increasingly unavailable as one moves deeper into the eye. 
         [0023]    Apart from avoiding an unwanted outcome, the fact that different tissues in the eye  20  have different optical properties can be operationally exploited. This is so because the optical differences between adjacent tissues create an identifiable interface surface that can be located with great accuracy and precision. In particular, a detector  14  (e.g. an OCT device) that is capable of imaging the interface between different tissues in the eye  20  can provide useful information for the guidance and control of a laser unit  12 . In both instances (i.e. the avoidance of unwanted tissue damage at or near a tissue interface, and the exploitation of the interface as a reference for guidance and control purposes), the operational characteristics of the laser beam  18  are important. 
         [0024]    In  FIG. 2  the laser beam  18  is shown directed along a beam path  22  to a focal point  24 . In this case, when the laser beam  18  is being focused to a focal point  24 , the boundary  26  of the laser beam  18  will be conically-shaped, and it will be inclined at an angle θ relative to the beam path  22 . These geometric operational characteristics of the laser beam  18  can be established by the laser unit  12 , as required. In the event, a consequence of this geometry is that for a given energy level in the laser beam  18 , the beam  18  will have a lower fluence  28  (energy density) at an upstream station  30  on the beam path  22 , and a higher fluence  32  (energy density) at a downstream station  34  on the beam path  22 . Note: as used for the present invention, the word “fluence” means an energy density. In this context, the locations of stations  30  and  34  can be selected points on the beam path  22 , as desired. Importantly, it will then happen that for a given value of energy in the laser beam  18 , along with the location of the focal point  24  and the inclination angle “θ” of the laser beam  18 , the fluence  32  at station  34  and its distance “d” from the focal point  24  can be determined. Similarly, the fluence  28  at station  30  and its distance “d” from the focal point  24  can be determined. 
         [0025]    An exemplary application for the system  10  that involves LIOB of tissue in eye  20  is shown in  FIG. 3 . Specifically,  FIG. 3  shows the epithelium  36  and the stroma  38  of the eye  20 , with the focal point  24  of laser beam  18  positioned in the stroma  38 . As shown, the focal point  24  is at a distance “d” in a posterior direction from an interface surface  40  that is identified between the epithelium  36  and the stroma  38 . In this example, the stroma  38  is to be altered by LIOB. The laser beam  18 , however, must first pass through the epithelium  36  and the threshold for LIOB of the epithelium  36  (T 1 ) is less than the threshold for LIOB of the stroma  38  (T 2 ) [i.e. T 2 &gt;T 1 ]. This then creates a situation wherein an unwanted LIOB of tissue in the epithelium  36  is a possibility. Accordingly, if the fluence  32  in laser beam  18  (see  FIG. 2 ) corresponds with the LIOB threshold (T 1 ) for tissue of the epithelium  36 , the focal point  24  must be in the stroma  38  at or beyond the distance “d” from the interface surface  40  to avoid LIOB of the epithelium  36 . Further, by using the detector  14  to monitor movements of focal point  24  in the stroma  38 , the interface surface  40  can be used as a reference datum to maintain the focal point  24  at or beyond the interface surface  40  with great accuracy and precision. 
         [0026]    For additional applications of the present invention, wherein the relative LIOB thresholds of adjacent tissues are not a concern (e.g. the LIOB threshold of upstream tissue is greater than that of the downstream tissue) the operational characteristics of the laser beam  18  can still be used for guidance and control purposes. Specifically, the fluence (e.g. fluence  28  and  32 ) at stations (points or locations) on the beam path  22  (e.g. stations  30  and  34 ) can be identified as desired. The corresponding distances “d′” and “d” can be established for operational purposes. Again, the detector  14  can be used to identify a suitable reference datum (e.g. an interface surface such as the surface  40 ), and this reference datum can be appropriately used for guidance and control of the laser beam  18 . For instance, the creation of an extremely thin flap (not shown) on the eye  20 , having a substantially constant thickness, can be created by performing LIOB in the stroma  38  at the distance “d” from the reference surface  40 . In the event, safety margins can be included into the distance “d”. 
         [0027]    For implementing the above, a computer program product comprising program sections is provided for respectively: directing a laser beam  18  along a beam path  22  through an overlying tissue to a focal point  24  in the underlying tissue, wherein the overlying tissue has a threshold for Laser Induced Optical Breakdown (LIOB), “T 1 ”, and the underlying tissue has a threshold for LIOB, “T 2 ”; for identifying an interface surface  40  between the overlying tissue and the underlying tissue; and for positioning the focal point  24  of the laser beam  18  at a distance “d” from the interface surface  40 , wherein T 1  is not equal to T 2  (T 1 ≠T 2 ). In its implementation, such a computer program product can be used to create flaps of substantially constant thickness (not shown) or incisions of variously intended configurations. Further, a computer program product can be used to adjust the numerical aperture of laser unit  12  to avoid LIOB in underlying tissue when T 2  is greater than T 1 . 
         [0028]    While the particular System and Method for Moving the Focal Point 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.