Beam position monitoring for laser eye surgery

#The present invention relates generally to methods, apparatus and systems for performing laser eye surgery. More particularly, the present invention relates to laser beam position monitoring methods, apparatus and systems for enhancing safety of laser eye surgery systems.

#Photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) employ laser beam delivery systems for directing laser energy to a patient's eye to selectively ablate corneal tissue to reform or sculpt the shape of the cornea and, thereby, to improve vision. Present commercial systems often employ excimer lasers. In a first type of system, positioning of the beam is generally fixed and the beam has a cross-sectional area generally corresponding to an entire surface area of a surgical site on the cornea. Cross-sectional portions of the beam are then sequentially masked or adjusted so as to selectively vary the amount of energy exposure of different portions of the surgical site so as to effect the desired sculpting. This can typically be achieved by using an iris or other exposure control mechanism. While highly effective and relatively easy to control, employing a laser beam having a cross-sectional area generally equal to the area of the treatment or surgical site (typically having a diameter of 5.0 mm to 10.0 mm) often involves the use of relatively large amounts of energy. This is typically relatively expensive, and leads to relatively large laser systems.

#As an alternative to such large beam diameter systems, laser “scanning” systems can be employed for corneal ablation. Such scanning systems typically employ a laser beam having a smaller cross-sectional area, thereby decreasing energy requirements. Accordingly, laser scanning systems delivering laser beams of relatively small cross-sectional area can be more economic to use and normally are of smaller construction than laser systems having larger diameter beams. However, the use of such small beams complicates certain aspects of the treatment protocols required to perform the sculpting. For example, to achieve a desired level of volumetric tissue removal or ablation from the eye, the treatment beam is scanned over or otherwise moved across the eye from one position to a next during the surgical procedure. Movement of the beam is typically achieved through motorized scanning mechanisms, devices, or the like. These scanning mechanisms often regulate the position of an optical element, such as the angle of a mirrored surface, the lateral position of an offset imaging lens or the like, so as to adjust the lateral position of the beam across the treatment site. In a related type of system, the laser beam is scanned over the corneal surface while varying the cross-section of the laser beam.

#FIG. 1 is a simplified block diagram of a laser eye surgery system with which various embodiments of the present invention may be incorporated.

#FIG. 2 is a simplified perspective view of an optical train of a laser surgery system including one embodiment of the present invention.

#FIGS. 3A and 3B are schematic side views of a scanning and monitoring system according to an embodiment of the present invention.

#The present invention provides methods, apparatus and subsystems for monitoring laser beam position in laser eye surgery systems, particularly though not exclusively in laser eye surgery systems of the scanning type. In particular, the present invention provides monitoring methods and systems for verifying laser beam position during the course of corrective surgical ablation of an eye. By verifying that the ablative laser beam has moved to a position corresponding with a predetermined position dictated by a predetermined adjustment sequence or ablation pattern, the system can verify that mechanisms used to scan or move the beam from one position to a next position are functioning properly. This monitoring function enhances safety and efficacy of the laser surgery system by enabling the system to detect malfunctions in, for example, the scanning mechanism, and to interrupt the surgical procedure when an actual laser beam position does not correspond with a corresponding position dictated by the predetermined adjustment sequence.

#By “scanning,” it is meant that an ablative laser beam is moved successively from one lateral position to a next lateral position across the treatment site on an eye so as to expose successive portions of the eye to a predetermined amount or dosage of laser energy. Usually, the laser system will be operated in a pulsed manner and the exposure at any particular position will result from a number of pulses which occur over a very short time period. The corrective eye procedure is normally completed when the ablative laser beam has completed the scanning sequence in accordance with a predetermined ablation pattern.

#Referring now to FIG. 1, a laser eye surgery system 10 with which monitoring methods, apparatus and subsystems of the present invention may be used is similar to a laser eye surgery system described in U.S. Pat. No. 6,322,216 B1, assigned to the assignee of the present invention, the entire contents of which is hereby incorporated by reference. System suitably includes a laser 20 which generates a laser beam 26 that is selectively directed toward eye E by delivery system optics 28, a personal computer workstation 12 coupled to a computer 14 with a tangible medium 21, and horizontal and vertical trackers 11h, 11v, each of which includes a camera 13 and an associated tracking processor 15. Delivery system optics 28 scan beam 26 over eye E according to instructions from computer 14. Computer 14 generally scans beam 26 over eye E by changing the angular position of first and second stage pivot systems 22, 24. In alternative embodiments, the computer may scan the beam by pivoting one or more mirrors using galvanometric motors, or any of a wide variety of alternative scanning mechanisms. Optionally, computer 14 may direct profiling of beam 26 using one or more variable apertures. Further details of laser eye surgery system 10 may be found in U.S. Pat. No. 6,322,216 B1.