Patent Application
20070142827
The present invention is directed to laser surgery on the eye, and, more particularly, to systems and methods for calculating and monitoring a characteristic of a laser beam in real time of a laser ophthalmic surgical device.
In surgery on the eye, the laser system must be calibrated carefully. For example, in refractive laser surgery, typically using an excimer laser system, the volume of material ablated by each laser shot must be calibrated. It is currently known in the art to perform such a calibration measurement on a synthetic material such as a plastic that is selected for being analogous to the human cornea. Then the volume of material ablated per laser shot is calculated by a diameter of an ablation “crater” produced by a given number of laser shots.
The underlying theory of this method is based on a number of assumptions. (1) The interaction of the plastic material and the laser radiation is independent of the number of laser shots; (2) there is a well-known relationship between the volume per shot in the plastic material used for calibration and the volume per shot in the cornea; and (3) the volume per shot is calculated with the assumption that the excimer laser profile is Gaussian and rotationally symmetric.
Other laser system characteristics also must be calibrated. For example, in ultrafast laser systems it is important to determine any changes to the temporal response throughout a surgical procedure.
It would be beneficial to provide a more accurate system and method for performing calibration on a laser surgical system, such as one that could be used in real time during a surgical procedure.
The present invention is directed to a system and method for determining and monitoring a laser ablation volume of a cornea. The method comprises the step of sampling a beam of laser shots from a pulsed treatment laser as a reference portion. A fluence distribution of the beam reference portion is measured, and an ablation volume per laser shot is calculated from the measured fluence distribution. Fluence is defined as the total amount of energy applied per unit area, measured, for example, in joules/cm2.
A system for determining and monitoring a laser ablation volume of a cornea comprises a beam sampler such as a beamsplitter that is positioned to split a beam of laser shots from a pulsed treatment laser into an incident portion, for example, for effecting a treatment, and a sampling, or reference, portion. Means are provided for measuring a fluence distribution of the beam sampling portion and for calculating an ablation volume per laser shot from the measured fluence distribution.
The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.
A description of preferred embodiments of the invention will now be presented with reference to
A beam sampler such as a beamsplitter 16 is positioned to sample laser shots 17 from the treatment laser 11 into a treatment portion 18 and a reference portion 19. In an exemplary embodiment for corneal refractive surgery, for example, the treatment portion 18 will be incident on the cornea 14, and will be referred to as a “corneal portion.” In a method 100 of the present invention, preferably, a calibration is performed prior to the surgery by determining a fluence distribution of the corneal portion 18 (block 101;
Means for measuring a fluence distribution of the beam reference portion, for example, a laser beam fluence profiler 21, is positioned at a corneal plane 22. During surgery, the beamsplitter 16 directs the corneal portion 18 onto the cornea 14 in a predetermined pattern to achieve the predetermined ablation profile, under direction of the controller 13 (block 102). The ablation profile, however, is based upon assumptions on the volume of corneal tissue ablated per shot (VPS), and it is therefore believed beneficial to provide an accurate and periodic, real-time calibration of this value.
In order to provide such a calibration, the reference portion 19 is monitored by the laser beam fluence profiler 21 (block 103) with the use of the following calculations. If the radiance fluence or radiant exposure F0 in a particular point of an excimer laser beam spot at the corneal plane, the depth of ablation per pulse can be calculated using Beer's law, which describes the attenuation of light in an optically absorbing medium:
F=F0·exp[−α·z] (1)
where F0 is the fluence at the separation surface, a is the absorption coefficient, and F is the fluence at a depth z in the absorbent medium.
Assuming that the ablation threshold fluence is known and the laser fluence at a point (x,y) of the ablation area A, the ablation depth at this point is given by:
The ablation volume per laser pulse, or volume per shot VPS, is given by:
and A is the pulse ablation area that can be defined by F(x,y)=FTH.
Therefore, a measurement of the fluence distribution of the pulsed excimer laser 11 at the corneal plane 22, and applying Eq. (3) (block 104), an accurate estimate of the volume per shot is obtained.
Preferably the reference calculation is performed at predetermined intervals during a corneal ablation procedure (block 105). If the VPS value is substantially unchanged from the previous measurement (block 106), no change to the ablation profile is made, and the procedure continues (block 107). If the VPS value is changed substantially (block 106), the system is adjusted (block 108) prior to continuing the procedure (block 107). This process continues until the profile is complete (block 109).
Thus is can be seen that a more accurate and real-time calibration can be performed on the volume per shot, without making an assumption as to the beam profile, and also without making an assumption as to the equivalence between the cornea and a synthetic material.
In the foregoing description, certain terms have been used for brevity, clarity, and understanding, but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such words are used for description purposes herein and are intended to be broadly construed. Moreover, the embodiments of the apparatus illustrated and described herein are by way of example, and the scope of the invention is not limited to the exact details of construction.
Having now described the invention, the construction, the operation and use of preferred embodiments thereof, and the advantageous new and useful results obtained thereby, the new and useful constructions, and reasonable mechanical equivalents thereof obvious to those skilled in the art, are set forth in the appended claims.
