LARGE BEAM SCANNING LASER ABLATION

Abstract: The present invention provides a method for ablating material including directing a laser beam through a mask and through a scanning unit to an area of the material to thereby ablate the material, wherein the scanning unit can scan or be controlled to scan the beam in a predetermined pattern on the material, and an apparatus for laser ablation of material including a laser source for producing a beam of far ultra-violet or infra-red light, a mask, means for directing the beam through the mask and a computer-controlled scanning unit for scanning or being controlled to scan the beam in a predetermined pattern on the material, wherein the beam is directed to an area of the material to be ablated. The mask may be of variable area, and this area may be increased during a single use of the apparatus.

Description: LARGE BEAM SCANNING LASER ABLATION The present invention relates to the laser processing or ablation of materials, and in particular the invention is suitable for use in operations on the corneal tissue of the eye for the correction of myopia, astigmatism and hyperopia, examples of which are refractive correction operations such as photorefractive keratectomy (PRK) and laser in-situ keratomileusis (LASIK) . The invention will be described with reference to these operations though it will be appreciated that other applications are possible and envisaged. The most common laser used for operations on the corneal tissue is the excimer laser operating at a wavelength of 193 nanometres. Whatever the laser source, the laser system needs to control the laser output so that the appropriate shape is etched or ablated into the corneal material. Three distinct systems have evolved to control the laser output necessary to perform this task. The first method uses a large beam capable of ablating a large surface area of between 5 and 10 millimetres. The beam is masked off to limit the area of corneal surface exposed (see, for example, U.S. Patent 4,941,093). The mask size and shape is varied during the procedure to control the shape being ablated. Examples of masks include an iris diaphragm, ablated plastic forming an oval, or parallel blades forming an expanding slit. Sometimes the mask consists of shapes cut in spinning disks. The laser beam may also be rotated continuously using an image rotator about its central axis to smooth the ablated surface. The laser beam, after being shaped by the mask, is sometimes then scanned in a fixed pattern, as described in EP 0 628 298 Al and in Ophthalmic Excimer Lasers : Principles and Practice, edited by McGee, C, Taylor, H.R., Gartry, D., Trokel, S., Martin Dunitz Limited, London (1997) when, for example, hyperopia is being corrected. The other two methods involve scanning the beam across the surface of the material to be ablated. The first method involves scanning a long, narrow slit beam across an aperture of masks similar to that used in the large beam method. The configuration of the shape being ablated into the surface is also controlled in a similar fashion to that used to control the large beam method. The last method involves scanning a small beam with a diameter in the range of 2.5 mm or smaller (see U.S. Patent 5,520,679). The shape being ablated is controlled by having the scanned beam pass over the areas to be ablated more often than those areas where less material is required to be removed. Scanning systems have the advantage that a lower energy laser source is required, with both cost and size advantages. The small beam scanning method also has the advantage that it is easier to control the laser system to ablate any arbitrary shape than it is using masks. Scanning the laser beam also imparts some of the beam smoothing required for these operations. The biggest disadvantage with scanning systems is their inability to maintain a uniform tissue hydration over the area being treated. The ablation rate of tissue is strongly related to its state of hydration. Immediately after exposure to the laser the tissue is warm and dry, and a second laser pulse will ablate more than expected. In the following seconds fluid will well up from deeper tissue so that the surface tissue becomes very hydrated with a layer of fluid on top. In this case the next pulse will ablate much less than expected. Hence it is extremely difficult to create a desired shape, or to predict accurately the profile that a scanning laser will ablate. Large beam systems have an advantage in that they can expose the whole treated area at once and hence they can maintain a uniform tissue hydration level over that area. It can also perform the operation quicker than scanning systems. However, it requires larger and more costly laser sources . The requirement for beam homogeneity is also more important in large beam systems . It is an object of the present invention, therefore, to provide an improved beam control system that can more accurately and/or predictably ablate a desired shape into a material such as tissue in LASIK and PRK operations, while maintaining some or all of the advantages of both large beam systems and the scanning system. Thus, according to the present invention there is provided a method for ablating material including directing a laser beam through a mask and through a scanning unit to an area of said material to thereby ablate said material, wherein said scanning unit can scan or be controlled to scan the beam in a predetermined pattern on said material. Thus, the mask (or aperture therein) is used to control the deposition of laser beam energy onto the material in any pattern. Preferably said mask is a variable mask, having a transparent or transmitting aperture of variable area for admitting or transmitting said beam. Preferably said method includes varying said area. Preferably said method includes increasing said area during a procedure. Preferably said area is initially less than 2.5 mm 2 , and in one embodiment may initially be less than 1 mm 2 . Preferably said area is increased to greater than 5 mm 2 , and in one embodiment to greater than 10 mm 2 . Preferably said laser beam is one of a plurality of laser beams. Preferably said mask is computer controlled. Preferably said mask is an iris diaphragm. The iris diaphragm may have a central hole with a variable diameter for producing beam sizes on said tissue from less than 0.5 mm to 10 mm in diameter. In one embodiment, the central hole has a variable diameter for producing beam sizes on said tissue between 0.5 mm and 6 mm in diameter. After said iris the beam may go through optics to minify or magnify said beam size. In one preferred embodiment, said beam is scanned over said tissue in a plurality of patterns sequentially, and in another preferred embodiment said pattern may be changed during a procedure. The present invention also provides an apparatus for laser ablation of material including a laser source for producing a beam of far ultra-violet or infra-red light, a mask, means for directing said beam through said mask, and a computer-controlled scanning unit for scanning or being controlled to scan the beam in a predetermined pattern on said material, wherein said beam is directed to an area of the material to be ablated. Preferably said mask is a variable mask, having a transparent or transmitting aperture of variable area for admitting or transmitting said beam. In one embodiment, said area is variable from less than 5 mm 2 , and in another embodiment from less than 1 mm 2 . The area may be variable to greater than 5 mm 2 , and in one embodiment to greater than 10 mm 2 . Preferably said laser source is one of a plurality of laser sources . Preferably said mask is an iris diaphragm. Preferably said mask is a computer controlled iris diaphragm. There is still further provided by the present invention a method for ablating human or animal tissue including directing a laser beam through a mask and through a scanning unit to an area of said tissue to thereby ablate said tissue, wherein said scanning unit can scan or be controlled to scan the beam in a predetermined pattern on said tissue. Preferably said tissue is corneal. Preferably said method is used to fully or partially correct defects in eyesight. Preferably said mask is a variable mask. Preferably said laser beam is one of a plurality of laser beams. Preferably said mask is computer controlled. Preferably said mask is an iris diaphragm. The iris diaphragm may have a central hole with a variable diameter for producing beam sizes on said tissue from less than 0.5 mm to 10 mm in diameter. In one embodiment, the central hole has a variable diameter for producing beam sizes on said tissue between 0.5 mm and 6 mm in diameter. Preferably said method includes varying said diameter during a procedure. More preferably said diameter is increased during a procedure . In one specific aspect of the present invention there is provided an apparatus for laser ablation of animal or human tissue including a laser source for producing a beam of far ultra-violet or infra-red light, a mask, means for directing said beam through said mask, and a computer- controlled scanning unit for scanning or being controlled to scan the beam in a predetermined pattern on said tissue, wherein said beam is directed to an area of said tissue to be ablated. Preferably said tissue is corneal. Preferably said apparatus is adapted for the full or partial correction of defects in eyesight. Preferably said mask is a variable mask. Preferably said laser source is one of a plurality of laser sources . Preferably said mask is computer controlled. Preferably said mask is an iris diaphragm. The iris diaphragm may have a central hole with a variable diameter for producing beam sizes on said tissue from less than 0.5 mm to 10 mm in diameter. In one embodiment, the central hole has a variable diameter for producing beam sizes on said tissue between 0.5 mm and 6 mm in diameter. In all of the above aspects of the invention, the laser source or source of the laser beam is preferably a large or compact Argon-Fluoride excimer laser (193 nm) , flash-lamp or laser pumped solid state laser (193 - 215 nm) such as quintupled Nd:YAG laser or a quadrupled Ti: Sapphire laser, Ho:YAG (2.1 micrometres), Er.YAG or Er:glass laser or tunable IR laser. Preferred embodiments of the invention will be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a schematic view of an apparatus according to the present invention. An apparatus for laser ablation is shown generally in Figure 1. The apparatus includes a laser source 1. This laser source produces a laser beam 2 which passes through beam smoothing components 3 before continuing through to a variable mask in the form of an iris diaphragm 4. The beam is then directed toward the scanning unit 5, before passing to the surface of the cornea 7. A computer 6 controls the operation of both the mask and the scanning device. In use the computer 6 controls iris diaphragm 4. In the preferred method of operation, the iris 4 is used to vary the beam diameter - in steps - during a surgical procedure, initially being set to a small iris diameter and hence beam size (with a beam spot diameter of generally between 0.1 mm and 2.5 mm) and increasing to a larger iris setting (to produce a beam spot diameter of between 2.5 mm and 6 mm) . As the beam size is progressively increased, a number of pulses of each desired size, scanned in the selected pattern or path, are applied to the cornea: 1) For spherical corrections, the laser pulses are preferably evenly spaced around a circular path. Each circular path has at least 5 pulses, but more than one circular path may be traced out at each beam size step; 2) For astigmatic corrections, the distribution around the path may be more concentrated in one axis than in another and/or the paths may be made elliptical; 3) For irregular astigmatism corrections, the scanning path will be irregular, the spacing between pulses will be irregular and there may be only 1 pulse per beam size. Where there are more than 5 pulses in a circular path, the pulses are applied in an order such that the scanner transverses the path (be it circular, elliptical or irregular) in no more than about 0.5 s and will continue going around the path until all pulses are fired. Thus, for example, if there are ten pulses to be fired at 10 Hz, every second position is hit on the first pass around the path, and the intermediate positions are hit on the next pass of the laser beam around that path. When the required treated size is larger than the maximum beam size and/or the desired shape cannot be created using an iris alone, the varying sized beam is scanned in an appropriate pattern to produce the desired shape. The beam control may be optimized such that it always scans the largest beam possible so that the treatment time is minimized and the tissue hydration is maintained as uniformly as possible. The apparatus of the present invention therefore provides for accurate ablation, providing an alternative beam control method, while maintaining the advantages associated with two prior methods of laser ablation. Modifications within the spirit and scope of the invention may readily be effected by person skilled in the art. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A method for ablating material including: directing a laser beam through a mask and through a scanning unit to an area of said material to thereby ablate said material, wherein said scanning unit can scan or be controlled to scan the beam in a predetermined pattern on said material . 2. A method for ablating material as claimed in claim 1, wherein said mask is a variable mask, having a transparent or transmitting aperture of variable area for admitting or transmitting said beam. 3. A method for ablating material as claimed in either claim 1 or 2, wherein said method includes varying said area. 4. A method for ablating material as claimed in claim 3, wherein said method includes varying said area in steps, and directing said beam onto said material at each of said steps . 5. A method for ablating material as claimed in either claim 3 or 4, wherein said method includes increasing said area. 6. A method for ablating material as claimed in claim 5, wherein said area is initially less than 5 mm 2 . 7. A method for ablating material as claimed in claim 6, wherein said area is initially less than 1 mm 2 . 8. A method for ablating material as claimed in any one of claims 5 to 7, wherein said area is increased to greater than 5 mm 2 . 9. A method for ablating material as claimed in claim 8, wherein said area is increased to greater than 10 mm 2 . 10. A method for ablating material as claimed in any one of the preceding claims, wherein said laser beam is generated by an Argon-Fluoride excimer laser, flash-lamp or laser pumped solid state laser. 11. A method for ablating material as claimed in any one of claims 1 to 9, wherein said laser beam is generated by a quintupled Nd:YAG laser, a quadrupled Ti : Sapphire laser, a Ho:YAG, Er:YAG or Er: glass laser, or a tunable IR laser. 12. A method for ablating material as claimed in any one of the preceding claims, wherein said laser beam is one of a plurality of laser beams. 13. A method for ablating material as claimed in any one of the preceding claims, wherein said mask is computer controlled. 14. A method for ablating material as claimed in any one of the preceding claims, wherein said mask is an iris diaphragm. 15. A method for ablating material as claimed in claim 14, wherein said iris diaphragm has a central hole with a variable diameter for producing beam sizes on said material from less than 0.5 mm to 10 mm in diameter. 16. A method for ablating material as claimed in claim 15, wherein said iris diaphragm has a central hole with a variable diameter for producing beam sizes on said material of between 0.5 mm and 6 mm in diameter . 17. A method for ablating material as claimed in any one of the preceding claims, including passing said beam through minifying or magnifying optics to minify or magnify the size of said beam, after said beam has passed through said mask. 18. A method for ablating material as claimed in any one of the preceding claims, including changing said pattern during a procedure. 19. A method for ablating material as claimed in any one of the preceding claims, wherein said pattern is one of a plurality of patterns. 20. An apparatus for laser ablation of material including: a laser source for producing a beam of far ultraviolet or infra-red light; a mask; means for directing said beam through said mask; and a computer-controlled scanning unit for scanning or being controlled to scan said beam in a predetermined pattern on said material; wherein said beam is directed to an area of the material to be ablated. 21. An apparatus as claimed in claim 20, wherein said mask is a variable mask, having a transparent or transmitting aperture of variable area for admitting or transmitting said beam. 22. An apparatus as claimed in claim 21, wherein said area is variable from less than 5 mm 2 . 23. An apparatus as claimed in claim 22, wherein said area is variable to less than 1 mm 2 . 24. An apparatus as claimed in any one of claims 21 to 23, wherein said area is variable to greater than 5 mm . 25. An apparatus as claimed in claim 24, wherein said area is variable to greater than 10 mm 2 . 26. An apparatus as claimed in any one of claims 20 to 25, wherein said laser source is an Argon-Fluoride excimer laser, flash-lamp or laser pumped solid state laser. 27. An apparatus as claimed in any one of claims 20 to 25, wherein said laser source is a quintupled Nd:YAG laser, a quadrupled Ti: Sapphire laser, a Ho:YAG, Er:YAG or Er: glass laser, or a tunable IR laser. 28. An apparatus as claimed in any one of claims 20 to 27, wherein said laser source is one of a plurality of laser sources . 29. An apparatus as claimed in any one of claims 20 to 28, wherein said mask is an iris diaphragm. 30. An apparatus as claimed in any one of claims 20 to 29, wherein said mask is computer controlled. 31. A method for ablating human or animal tissue including: directing a laser beam through a mask and through a scanning unit to an area of said tissue to thereby ablate said tissue, wherein said scanning unit can scan or be controlled to scan the beam in a predetermined pattern on said tissue. 32. A method as claimed in claim 31, wherein said tissue is corneal . 33. A method as claimed in either claim 31 or 32, wherein said method is used to fully or partially correct defects in eyesight. 34. A method as claimed in any one of claims 31 to 33, wherein said mask is a variable mask. 35. A method as claimed in any one of claims 31 to 34, wherein said laser beam is generated by an Argon-Fluoride excimer laser, flash-lamp or laser pumped solid state laser. 36. A method as claimed in any one of claims 31 to 34, wherein said laser beam is generated by a quintupled Nd:YAG laser, a quadrupled Ti: Sapphire laser, a Ho:YAG, Er:YAG or Er:glass laser, or a tunable IR laser. 37. A method as claimed in any one of claims 31 to 36, wherein said laser beam is one of a plurality of laser beams. 38. A method as claimed in any one of claims 31 to 37, wherein said mask is computer controlled. 39. A method as claimed in any one of claims 31 to 38, wherein said mask is an iris diaphragm. 40. A method as claimed in claim 39, wherein said iris diaphragm has a central hole with a variable diameter that produces beam sizes on said tissue from less than 0.5 mm to 10 mm in diameter. 41. A method as claimed in claim 40, wherein said iris diaphragm has a central hole with a variable diameter that produces beam sizes on said tissue between 0.5 mm and 6 mm in diameter. 42. A method as claimed in any one of claims 31 to 41, wherein said method includes varying said central hole diameter and hence said beam size during a procedure. 43. A method as claimed in claim 42, wherein said central hole diameter and hence said beam size is varied in steps. 44. A method as claimed in claim 43, wherein said central hole diameter and hence said beam size is increased in steps . 45. A method as claimed in any one of claims 31 to 44, including depositing on said tissue a series of evenly spaced pulses of said beam around said pattern, wherein said pattern is circular, for spherical correction of corneal tissue. 46. An method as claimed in claim 45, wherein said series includes at least 5 pulses . 47. A method as claimed in any one of claims 31 to 44, including depositing on said tissue a series of pulses of said beam around said pattern, wherein said pattern is elliptical and/or said pulses are more concentrated in one portion of said pattern, for astigmatic correction of corneal tissue. 48. A method as claimed in any one of claims 31 to 44, including depositing on said tissue a series of irregularly spaced pulses of said beam around said pattern, wherein said pattern is irregular, for irregular astigmatic correction of corneal tissue. 49. A method as claimed in claim 48, including depositing one beam pulse per mask or beam size. 50. An apparatus for laser ablation of animal or human tissue including: a laser source for producing a beam of far ultraviolet or infra-red light; a mask; means for directing said beam through said mask; and a computer-controlled scanning unit for scanning or being controlled to scan said beam in a predetermined pattern on said tissue; wherein said beam is directed to an area of said tissue to be ablated. 51. An apparatus as claimed in claim 50, wherein said tissue is corneal. 52. An apparatus as claimed in either claim 50 or 51, wherein said apparatus is adapted for the full or partial correction of defects in eyesight. 53. An apparatus as claimed in any one of claims 50 to 52, wherein said mask is a variable mask. 54. An apparatus as claimed in any one of claims 50 to 53, wherein said laser source is an Argon-Fluoride excimer laser, flash-lamp or laser pumped solid state laser. 55. An apparatus as claimed in any one of claims 50 to 53, wherein said laser source is a quintupled Nd:YAG laser, a quadrupled Ti: Sapphire laser, a Ho:YAG, Er:YAG or Er:glass laser, or a tunable IR laser. 56. An apparatus as claimed in any one of claims 50 to 55, wherein said laser source is one of a plurality of laser sources . 57. An apparatus as claimed in any one of claims 50 to 56, wherein said mask is computer controlled. 58. An apparatus as claimed in any one of claims 50 to 57, wherein said mask is an iris diaphragm. 59. An apparatus as claimed in claim 58, wherein said iris diaphragm has a central hole with a variable diameter for producing beam sizes on said tissue from less than 0.5 mm to 10 mm in diameter. 60. An apparatus as claimed in claim 59, wherein said iris diaphragm has a central hole with a variable diameter for producing beam sizes on said tissue variable between 0.5 mm and 6 mm in diameter. 61. An apparatus as claimed in any one of claims 50 to 60, including minifying or magnifying optics located after said mask, wherein said beam may be minified or magnified in size by being passed through said optics. 62. An apparatus as claimed in any one of claims 50 to 61, including means for changing said pattern during a procedure . 63. An apparatus as claimed in any one of claims 50 to 62, wherein said pattern is circular and scanning unit is programmed to control said beam to be directed in a series of evenly spaced pulses around said pattern for spherical correction of corneal tissue. 64. An apparatus as claimed in claim 63, wherein said series includes at least 5 pulses . 65. An apparatus as claimed in any one of claims 50 to 62, wherein said scanning unit is programmed to control said beam to be directed in a series of pulses around said pattern, and said pattern is elliptical and/or said pulses are more concentrated in one portion of said pattern, for astigmatic correction of corneal tissue. 66. An apparatus as claimed in any one of claims 50 to 62, wherein said scanning unit is programmed to control said beam to be directed in a series of irregularly spaced pulses around said pattern, and said pattern is irregular, for irregular astigmatic correction of corneal tissue. 67. An apparatus as claimed in claim 66, wherein said scanning unit is programmed to control said beam to direct one beam pulse per mask or beam size. 68. An apparatus as claimed in any one of claims 50 to 67, wherein said pattern is one of a plurality of patterns.

Inventors: VAN SAARLOOS PAUL PHILLIP, VAN SAARLOOS, PAUL, PHILLIP
Applicants: LIONS EYE INST AUSTRALIA INC, SAARLOOS PAUL PHILLIP VAN, THE LIONS EYE INSTITUTE OF WESTERN AUSTRALIA INCORPORATED, VAN SAARLOOS, PAUL, PHILLIP
Application year: 1998
Main classification: A61F9
IPC codes: A61F9, B23K26