Surgery – Instruments – Light application
Reexamination Certificate
1999-05-21
2001-09-11
Shay, David M. (Department: 3739)
Surgery
Instruments
Light application
C606S010000, C606S005000
Reexamination Certificate
active
06287299
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to a method for laser-beam fluence measurement in laser treatment of biological tissue procedures. The present invention relates in particular to apparatus for measuring spatial distribution of laser-beam energy in a laser ophthalmic surgical procedure wherein corneal curvature is modified by selective photoablation of corneal tissue by means of successive laser pulses incident on the cornea in an overlapping pattern.
DISCUSSION OF BACKGROUND ART
In laser surgical treatment procedures an area to be treated is often irradiated with laser energy in a series of overlapping pulses rather than with a single pulse covering the entire area. This method can provide more accurate and safer dosage, and is also effective when dosage must vary across the treatment area.
In laser ophthalmic surgery systems, in particular those in which a uv-laser beam is used for corneal curvature modification by laser photoablation of corneal tissue, it can be useful to monitor the laser beam. This can be done, for example, for controlling the laser beam or for estimating the extent of photoablation. In early laser ophthalmic surgery systems, treatment pulses were typically centered in the area being treated, variable dosage being obtained by systematically changing the area irradiated by overlapping pulses. Monitoring of laser energy delivered to the target area was restricted to a measurement of the fluence profile of the treatment laser beam.
By way of example, U.S. Pat. No. 4,911,711, issued Mar. 27, 1990, to Telfair et al., discloses a uv-laser photoablation system which includes such a laser beam monitoring device. An operative uv-laser beam of the system is expanded to a relatively large area which terminates with a fixed cornea impingement axis, aligned with the axis of a patient's eye. The intensity profile of the laser beam is one system parameter which is selected to effect a particular corneal curvature modification.
The laser beam monitoring device includes a video camera sensitive to UV radiation. The video camera is coupled with suitable electronic circuitry to monitor the fluence profile of the operative uv-laser beam during a corneal curvature modifying operation. This is done to monitor changes in the laser beam fluence profile. The monitored laser beam fluence profile is used to control optical arrangements for further shaping that profile to ensure that the further-shaped profile stays stable during the operation.
More recent uv-photoablation systems for corneal curvature modification employ pulsed uv-lasers and arrangements for delivering a sequence or succession thousands of relatively low energy pulses in an overlapping pattern over an area of the cornea to be modified. The area of ablation spots created by these pulses is a relatively small fraction of the total area to be treated, for example, five percent or less, and is not centered therein. The extent to which corneal tissue is photoablated at a given site is determined by the total energy that impinges on that site. Examples of such systems are disclosed in U.S. Pat. No. 5,599,340, issued Feb. 4, 1997 to Simon et al. and U.S. Pat. No. 5,520,679, issued May 28, 1996, to Lin.
These types of system present a unique problem in monitoring laser-radiation fluence deposition, as it is the distribution of cumulative energy, on the cornea that determines curvature modification, rather than the energy distribution in the ablating laser beam. In such systems, in fact, the overlapping-pulse pattern compensates for non-uniformity and temporal variations in the energy distribution of the laser beam. An additional problem is presented if the monitored energy distribution is to be relied on as an accurate estimation of a photoablation profile. This is because photoablation is not linearly dependent on absorbed energy. In order to achieve a reasonably accurate estimate of a photoablation profile from an energy distribution measurement, this non-linearity must be taken into account.
It is believed that these problems of cumulative laser-beam energy distribution monitoring for estimation of photoablation have not been addressed in the prior art. A more detailed discussion of these problems, and effective solutions therefor, are presented in the description of the present invention set forth below.
SUMMARY OF THE INVENTION
The present invention is directed to providing laser-beam fluence monitoring apparatus for a system for irradiating a treatment area. The system includes means for generating a succession of pulses of laser radiation, and principal scanning means for directing the laser-radiation pulses toward the treatment-area in an overlapping pattern corresponding to a desired treatment effect. In one aspect, he fluence monitoring apparatus comprises an optical beam-dividing element located between the scanning means and the treatment-area, the beam dividing element arranged to direct a fraction of each of the successive laser-radiation pulses along a first optical path to a monitor plate.
The monitor plate is a plate of a material which emits fluorescent light in response to irradiation by the laser radiation, in an amount proportional to the amount of the laser radiation. A video camera provides an electronic image of the monitor plate. Image processing electronics are arranged for periodically capturing the electronic image and processing the periodically-captured images to provide an integrated electronic image representative of the total laser-beam fluence distribution on the treatment area.
A fluence monitor in accordance with the present invention may be used simply to measure an accumulated radiation dose. Alternatively or additionally the apparatus may be arranged to represent an anticipated effect of that accumulated dose taking into account characteristic response to the laser radiation of material being irradiated.
In one preferred embodiment, fluence monitoring apparatus in accordance with the present invention is used in a system for modifying curvature of a cornea by selective laser photoablation of corneal tissue. The system includes means for generating a succession of pulses of laser-radiation, and principal scanning means for directing the laser-radiation pulses toward the cornea in an overlapping pattern corresponding to a desired pattern of photoablation.
The beam dividing element is arranged to direct a fraction of each of the successive laser-radiation pulses along a first optical path to a monitor plate, and transmit the remaining portion of each of the successive laser pulses along a second optical path to the cornea. The periodically-recorded images provide an integrated electronic image representative of the distribution of laser-beam fluence (total dose of radiation) on the cornea.
In another preferred embodiment of laser-beam fluence monitoring apparatus in accordance with the present invention, the image processor means are programmed to correct integrated, periodically-recorded images for a photoablation threshold energy, below which photoablation is assumed not to occur.
In yet another preferred embodiment of the present invention, wherein the system includes an eye tracker device cooperative with the principle scanning means for compensating for eye-movement during a treatment period, the image processor means are cooperative with the eye tracking device to correct integrated, periodically-recorded images for the action of the scanning means in compensating for eye-movement. In still another preferred embodiment of the present invention, also for use in a system including such an eye-tracker device, a second scanning means is located in the first optical path between the beam-dividing element and the monitor plate. The second scanning means is cooperative with the eye-tracking device for negating eye-movement-correcting actions of the principal scanning means.
REFERENCES:
patent: 4911711 (1990-03-01), Telfair et al.
patent: 5520679 (1996-05-01), Lin
patent: 5599340 (1997-02-01), Simon et al.
patent: 5624437 (1997-04-01), Freema
Austin R. Russel
Sasnett Michael W.
Coherent Inc.
Shay David M.
Stallman & Pollock LLP
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