Surgery – Instruments – Light application
Reexamination Certificate
2002-04-23
2003-12-23
Pham, Hoa Q. (Department: 2877)
Surgery
Instruments
Light application
C606S010000, C250S252100
Reexamination Certificate
active
06666855
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is generally directed to methods, systems, and apparatus for laser calibration and eye tracker camera alignment. In particular, the present invention relates to methods and systems for measuring laser energy, shape, and dimensions of a laser beam from a laser beam delivery system, particularly opthalmological surgery systems, and aligning eye tracking cameras used in conjunction with such laser systems that measure a position of the eye during laser eye surgery.
Laser-based systems are now used in opthalmological surgery on corneal tissues to correct vision defects. These systems use lasers to achieve a desired change in corneal shape, with the laser removing thin layers of corneal tissue using a technique generally described as ablative photodecomposition to alter the cornea's refractive power. Laser eye surgery techniques are useful in procedures such as photorefractive keratotomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), and the like.
In such laser-based systems and methods, the irradiated flux density and exposure time of the cornea to the laser radiation are controlled so as to provide a surface sculpting of the cornea to achieve a desired ultimate surface change in the cornea. To that end, ablation algorithms have been developed that determine the approximate energy density that must be applied to remove a certain depth of tissue from the cornea. At ultraviolet wavelengths, for example, a cumulative energy density of about 1 joule/cm
2
will typically ablate corneal tissue to a depth of about one micron when applied in a series of pulses of about 40 to 400 millijoules/cm
2
. Accordingly, the ablation algorithms are tailored for each procedure depending on the amount and the shape of corneal tissue which will be removed to correct a particular individual's refractive error.
In order to properly use these laser ablation algorithms, the laser beam delivery system typically should be calibrated. Calibration of the laser system helps ensure removal of the intended shape and quantity of the corneal tissue so as to provide the desired shape and refractive power modification to the patient's cornea. For example, deviation from a desired laser beam shape or size, such as the laser beam exhibiting a non-symmetrical shape or an increased or decreased laser beam diameter, may result in tissue ablation at an undesired location on the patient's cornea which in turn leads to less than ideal corneal sculpting results. As such, it is beneficial to know the shape and size profiles of the laser beam so as to accurately sculpt the patient's cornea through laser ablation. In addition, it is usually desirable to test for acceptable levels of system performance. For example, such tests can help ensure that laser energy measurements are accurate. Ablations of plastic test materials are often performed prior to laser surgery to calibrate the laser energy and ablation shape of the laser beam delivery system. Although such laser ablation calibration techniques are fairly effective, in some instances, alternative methods for laser energy and beam shape calibration may be advantageous.
A variety of integrated structures have been proposed for both scanning of a laser beam across the corneal tissue and tracking of eye movements. Tracking of the eye during laser eye surgery has been proposed to avoid uncomfortable structures which attempt to achieve total immobilization of the eye. Tracking further compensates for eye movement during a treatment procedure so that the intended portion of the eye may be accurately ablated. An exemplary two camera off-axis eye tracker for laser eye surgery is described in U.S. Pat. No. 6,322,216 B1, assigned to the assignee of the present application, the full disclosure of which is incorporated herein by reference. In this system, first and second cameras or image capture devices are oriented toward the eye. An energy delivery system laterally deflects an energy stream toward the corneal tissue along a first and second axis in response to movement of the eye sensed by the first and second image capture devices. Alignment of such image capture devices may be facilitated by a jig plate.
In light of the above, it would be desirable to provide improved methods, systems, and apparatus for calibrating laser energy, laser beam shape, and/or laser beam dimensions from a laser eye surgery system. It would be particularly desirable if such improvements enhanced calibration accuracy without significantly increasing the overall system cost and complexity. It would be further desirable if such methods, systems, and apparatus further allow for eye tracker camera alignment so that laser calibration and camera alignment may be conveniently and effectively carried out utilizing a single, reusable apparatus. At least some of these objectives will be met by the methods, systems, and apparatus of the present invention described hereinafter.
BRIEF SUMMARY OF THE INVENTION
The present invention provides methods, systems, and apparatus for calibrating a laser ablation system, such as an excimer laser system for selectively ablating a cornea of a patient's eye. The invention also facilitates alignment of eye tracking cameras (which are often used in conjunction with such laser systems) that measure a position of the eye during laser eye surgery. In particular, the present invention provides methods and systems which measure laser energy, laser beam shape, and/or laser beam dimensions with enhanced calibration accuracy without significantly increasing the overall system cost and complexity. Moreover, the present invention allows for laser calibrations and eye tracker camera alignment to be effectively and conveniently carried out.
In a first aspect of the present invention, a method for calibrating laser energy from a laser eye surgery system comprises transmitting or reflecting a laser beam suitable for ablation of corneal tissue from a surface, such as a galvanometric mirror having a reflecting surface or a beam splitter, and scanning the laser beam across a calibration fixture having a feature, such as an opening, reference-edge, or artificial pupil. Sample laser energy is separated from the beam at the surface during the scanning and measured. Laser energy transmitted past or directed at the feature during the scanning is measured. A calibration of the laser system is then determined by comparing the energy measurements. Energy measurements during the scanning are typically made by energy sensors, such as a photodetectors, light detectors, energy meters, and like detectors that are positioned near, adjacent to, or behind the surface or calibration fixture.
Calibration of the laser system is determined by comparing a ratio of the measured sample laser energy and the measured laser energy directed at the feature to a predetermined tolerance. If the ratio is within the predetermined tolerance, calibration of the laser system further comprises independently comparing the measured sample laser energy to a first threshold range and the measured laser energy directed at the feature to a second threshold range. Calibration of the laser system is complete if the measured sample laser energy is within the first threshold range and the measured laser energy directed at the feature is within the second threshold range. In a passing calibration, the calibration fixture may then be removed from a treatment plane and the laser beam directed towards a patient's cornea for ablating the cornea with the calibrated system. However, if the measured sample laser energy is outside the first threshold range or the measured laser energy directed at the feature is outside the second threshold range, a fault is indicated in the laser system, such as flawed delivery system optics or a flawed laser.
The laser beam may be transmitted from several different positions on the surface. In the case where the ratio is outside the predetermined tolerance, each ratio of the measured sample laser energy an
Shimmick John K.
Somani Seema
Yee Kingman
Pham Hoa Q.
Townsend and Townsend / and Crew LLP
Visx, Inc.
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