ES2313806B1 - LASER CALIBRATION PROCEDURE CONSIDERING PULSE SOLUTION. - Google Patents

Abstract

Laser calibration procedure considering the pulse overlap applicable to calibrate lasers Gaussian pulse. This procedure allows to improve any type of calibration as it considers the energy over-exposure that points receive surrounding the area where the laser acts in a given moment. The principles on which this procedure is based are easy to carry out in practice: first it is calculated the "overlap coefficient", which will depend on the laser characteristics This coefficient should be applied in all cases in which this laser is used which, in the practice, involves multiplying this overlap coefficient by the ablation algorithm with which that laser works.

Description

Laser calibration procedure considering pulse overlap.

Technical sector

Laser instrumentation used in Ophthalmology.

State of the art

The purpose of refractive surgery is to eliminate corneal tissue by photoablation to correct defects refractive (myopia, farsightedness, astigmatism). Also, in the In recent years, the aim is to correct optical aberrations (which degrade image quality); therefore, accuracy with the The modeling of the cornea is performed every time must be greater. In in recent years it has been achieved, for example, that the spot of laser is getting smaller, but if you don't correct the overlapping effect that we describe in this patent, these increasingly advanced instruments will always have the limitation that we present, which, has an important influence on The final corneal shape.

Currently, the laser systems used in refractive surgery they are calibrated, for example, following a procedure in which an area of a plate is removed from polymethylmethacrylate (PMMA) and then it is studied if that ablation is the  expected or not (ES 2 136 727). This calibration mode is not a very precise method because the analysis of the treated area is done in set, and not point to point. Making a patent review that we found for this type of calibration, we found that in the procedure described, the final form of the treated cornea, considered as an extensive surface (US6666855, ES 2 136 727).

According to the searches performed, they have not been found patents or descriptions of commercial houses in the that the overlapping factor of Gaussian pulses be considered in the same corneal area within the treated area.

Explanation of the invention. Object of the invention

Calibrate the emission of lasers used in refractive surgery so that we know each corneal point treated the overlapping of the Gaussian pulses emitted in the surrounding areas so as not to remove more tissue from the strictly necessary at each point.

Summary of the Invention

The object of the invention is a process to optimize the calibration of lasers used in the Present in refractive surgery clinics.

The problem posed by other methods is that at make different pulses affect different points of the cornea laser, there are always areas of it in which the pulse corresponding to that area and also pulses from nearby areas, that reach that point by the pulse geometry (Gaussian) (Figure 1).

The procedure we present allows you to do a much more precise calibration, in which the actual intensity of energy that receives each point of the cornea is known and therefore, It can be adjusted to the real needs of ablation. This form, we prevent the same corneal point from being exposed to more energy of the really necessary, which has as a consequence greater tissue removal This procedure can be applied. to all types of lasers (Gaussian or top-hat) employees in clinical practice. The principles on which it is based This procedure are easy to carry out in practice. He Summary procedure to follow is as follows: first place we calculate what we have called "coefficient of overlap ", this coefficient will correspond to a laser determined. Once the coefficient is known, it must be applied in all cases in which that particular laser is applied (use clinical, industrial, etc.), which in practice implies multiply this overlap coefficient by the algorithm of ablation with which that laser works.

Brief description of the illustrations

Figure 1: In this figure we observe the effect overlapping two pulses at the same point, P, of the corneal surface, S. This point P, receives corresponding energy to the two Gaussian pulses represented (\ Psi_ {1}) and \ Psi_ {2}), although in reality, if we consider the point P three-dimensionally, I would be receiving energy from each of the pulses emitted around it, so the effect would not be negligible. D is the size of the laser spot.

Figure 2: Mean corneal difference between experimental and theoretical data simulated from 3 values of a . For each value of a, the data were simulated with and without ( c = 1) the correction factor of the Gaussian pulse overlap. The data includes the standard error. D is the number of diopters to correct.

Detailed description of the invention

The calibration procedure we present It depends exclusively on the characteristics of the laser. It is about of determining the actual creep of the laser that reaches each point of the cornea On the one hand, energy corresponding to pulses will arrive that affect that specific region, but at the same time point, an amount of energy will come as a result of the form Gaussian pulse emitted by the laser (Figure 1). So in function of laser characteristics exclusively we can determine and measure these energies and correct the ablation to ensure that the same corneal area does not receive more energy from the strictly necessary.

We will detail the development below mathematician followed to get the expression provided by the overlap coefficient that we will use to improve the calibration of lasers. Radiant exposure (energy per area) of the laser is directly related to the depth of ablation by pulse through the Lambert-Beer law:

one

where d p is the pulse ablation depth, m is the luminous efficiency profile, F 0 is the radiant exposure (energy per illuminated area) and F th represents the radiant threshold exposure so that There is ablation. F 0 is a constant that depends on the type of laser, usually varies between 120 mJ / cm2 and 400 mJ / cm2, with F th = 50 mJ / cm2 } To consider the effect-overlap of Gaussian pulses, we will have to include a correction factor in the Lambert-Beer law. In a Gaussian pulse, the incident exposure (creep) is given by:

2

where x , y are spatial coordinates perpendicular to the optical axis, w is the beam radius and F 0 is the maximum exposure, which is limited by a spot size of D mm.

The different corneal regions are overexposed since they receive extra energy when nearby points are treated (see figure 1). Therefore we can calculate the photo-ablation caused at one point determined that it is due to the ablation in the points surrounding:

3

This correction factor would account for the overlapping effect ( s ) at one point of the pulses received in areas near the cornea. The limit of the integral r 'is conditioned by the size of the spot of the laser D and by the surface r where the ablation occurs. So r 'will be given by the minimum value of D / 2 or r . We can verify that s will be = 1 when the beam is uniform ( w = \ infty, top-hat type pulse), which would indicate that there is no overlap. Therefore, this correction factor can be applied to all lasers currently used in photorefractive surgery clinics of any type (Gaussian or top-hat).

Solving equation (3) we obtain:

4

Any ablation algorithm used by lasers used in clinic, when multiplied by this correction factor s (which depends exclusively on the size of the beam, radiant exposure and spot size), will be considering the overlapping effect of the emitted pulses.

If we calibrate the laser following this procedure, that is, by multiplying the ablation algorithm with which it works, by the value obtained for this overlapping coefficient s , the prediction of the corneal power after the ablation is significantly greater than if we do not consider it.

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Example Preferred Embodiment

To calibrate a given commercial laser, the corresponding overlap coefficient s must be calculated, for this the radiant exposure F 0 (energy per area) of that laser (with an ultraviolet energy detector) must be measured, and calculated r '(minimum value of the spot size divided by two ( D / 2 )) (the spot size is also measured) and the area of the corneal area that we consider. w is a characteristic parameter of each laser. F th is known for corneal tissue (50 mJ / cm 2).

Thus, the commercial house itself can calculate the Overlap coefficient of each laser and take it into account time to schedule it for clinical application, so that improve the prediction of the ablation algorithm used, once calibrated in this way.

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Example of application

To verify that apply this adjustment factor it's really an improvement in data prediction Experimental in refractive surgery, you can simulate a application of this to a certain ablation algorithm. For it we will consider this overlap coefficient s together with another known (ec 5) that accounts for the reflection losses that are produced when the laser affects the cornea and losses energy when the laser moves away from the optical axis, and therefore the incidence is no longer normal:

5

where, in this expression, R is the post-surgical corneal radius, p is the pre-surgical corneal asphericity, and is the height of incidence and a = 1 / 1n ( F 0 / F th).

To consider the overlap correction s, together with the adjustment factor (\ rho), we must multiply both factors (ecs. 4 and 5), so that any algorithm of ablation, which is corrected to consider these effects, will remain multiplied by s':

6

To calculate the practical influence of s 'in refractive surgery we can calculate the predicted corneal radius using this correction coefficient s' and compare with experimental results. The corneal power (\ varphi) is calculated:
var = Δ n / R where R is the 10 radius and Δ n = 0.375 the difference between the refractive index of the air and the cornea. Let's apply ec. (6) to a particular algorithm. Many of the ablation algorithms that are used are the property of the authors, but we do know the Munnerlyn algorithm, on which non-personalized refractive surgery or standard surgery is based, and is given by the following equation:

7

where D is the number of diopters to be corrected and d is the diameter of the corneal area treated. The corneal surface can be represented by a conicoid:

8

where R is the radius of curvature and p represents the p-factor or asphericity factor, with the optical axis on the z axis and where x and y are spatial coordinates.

To determine the expected corneal radius after an ablation given by s' z (y) , we apply a mathematical procedure and find the following expressions for the radius after surgery:

9

We observe that the value of R 'depends on R , s , d and D. In our simulation we consider 3 different values of a : a = 0.48, 0.62 and 1.14, which correspond to different incident exposure values and an overlap correction factor of (from (4)): s = 0.93, 0.90 and 0.83 respectively.

These predicted theoretical results using the correction coefficients were compared with real values of operated subjects. Data from 141 operated nearsighted eyes were considered, which were measured using the Munnerlyn ablation algorithm (ec. 7). Figure 2 shows the difference that appears between experimental and theoretical results. For each value of the prediction it was made considering and without considering ( s = 1) the overlap correction factor. We observe that the prediction of the corneal radius (the most important refractive parameter) is much better when we consider this overlap factor.

TABLE 1

10

Table 1 shows the differences. means between experimental and theoretical values for power corneal (\ Delta \ varphi). The theoretical data has been simulated with and without the correction factor that considers the overlap of Gaussian pulses

^ c: The simulation includes the factor of overlap correction.

*: Significant differences are found (p <0.05) for corneal power although they are lower in all the cases in which we consider the correctness of the overlap.

The results found for corneal power show the importance of using this overlap correction factor in ablation algorithms. The mean differences in corneal power between theoretical and experimental data using the overlapping factor or do not vary between 0.15 D and 0.35 D for a = 0.48 and a = 1.14, respectively. These differences can significantly influence the visual function of the observer, as we know that small blur values impair visual acuity.

In view of this data it can be seen as consider a correction factor that quantifies the effect of overlapping Gaussian pulses during corneal ablation can significantly improve data prediction corneal (corneal power).

Claims (1)

1. Method of optimizing the calibration of lasers used in photorefractive surgery, characterized by reprogramming the laser ablation software by multiplying the algorithm defined in said software by the value obtained for the overlap coefficient, s , determined by eleven where r 'is given by half the size of the laser spot (characteristic of each laser); w is the size of the laser beam (characteristic according to the type of laser); F 0 is the radiant exposure of the laser and F th represents the radiant threshold exposure for ablation (in the specific case of corneal tissue ablation F th = 50 mJ / cm 2) ).