A general simulation framework has been developed for characterising eye motion artefacts in the ophthalmic imaging of ocular media. The optimisation of the scanning strategy for an optical coherence tomography of the anterior eye chamber has been presented as the first example of an application for the simulator.
Nowadays UV-cross-linking is an established method for the treatment of keraectasia. Currently a standardized
protocol is used for the cross-linking treatment. We will now present a theoretical model which predicts the
number of induced crosslinks in the corneal tissue, in dependence of the Riboflavin concentration, the radiation
intensity, the pre-treatment time and the treatment time. The model is developed by merging the difussion
equation, the equation for the light distribution in dependence on the absorbers in the tissue and a rate equation
for the polymerization process. A higher concentration of Riboflavin solution as well as a higher irradiation
intensity will increase the number of induced crosslinks. However, performed stress-strain experiments which
support the model showed that higher Riboflavin concentrations (> 0.125%) do not result in a further increase in
stability of the corneal tissue. This is caused by the inhomogeneous distribution of induced crosslinks throughout
the cornea due to the uneven absorption of the UV-light. The new model offers the possibility to optimize the
treatment individually for every patient depending on their corneal thickness in terms of efficiency, saftey and
treatment time.
Cross-linking of the cornea is a new curative approach to re-increase the mechanical stability of corneal tissue that is
progressively decreasing as a result of a corneal disease such as keratoconus or pellucid marginal degeneration. The
new procedure might have the potential to reduce the need for invasive corneal transplantation. The aim of the treatment is to create additional chemical bonds inside the corneal stroma by means of a photopolymerizer and UV light at 365 nm. Two different potential damage mechanisms must be considered: the UV-irradiation alone and the action of the photochemically induced free radicals (photochemical damage). In this study damage thresholds from the literature were compared to the treatment parameters currently used in corneal cross-linking and aspects of the design of a UV illumination system for corneal cross-linking were discussed with respect to the safety of the procedure. It was shown that the currently used UVA dose density of 5.4 mJ/cm2 is below the known damage thresholds of UVA for the corneal endothelium, lens, and retina. All these safety considerations assume an optically homogeneous irradiation. Optical inhomogeneities such as hot spots may lead to localized supra-threshold irradiation with consecutive damage to the corneal endothelium which represents the most endangered structure. Some authors have used direct illumination of the cornea by means of UV-LEDs, which bares the risk of creating too high intensities. Therefore, clinically used light sources must guarantee a perfect homogeneity of the irradiance across the beam area. The illumination system presented here provides good homogeneity and shows a very high tolerance towards variations in treatment distance which was shown to cause dangerous hot spots when direct LED illumination is used.
The effectiveness of the corneal ablation process in refractive surgery is mostly evaluated by indirect measures of vision or optical quality such as post-operative refraction or wavefront aberrometry. Yet, the effective amount of corneal tissue removed in the treatment can only be determined by correctly overlapping a pre- and a post-operative topography measurement. However such an overlap is not trivial due to the discrepancy in the centration axes used in the measurement and the treatment, as well as due to the shift of ocular axes through the treatment or tilt between the two surfaces. We therefore present two methods for overlapping pre- and post-operative topographies for the purpose of extracting an effective corneal ablation profile. Method one uses a 3-dimensional profile matching algorithm and cross-correlation analysis on surface rings outside the optical zone of the topographies. Method two employs a surface normal matching routine to align the two surfaces along their common ablation axis. The profile matching method implies the problem that it requires measurement data outside of the optical zone which was found to be uncertain with placido-disk-based topographers. Method number two is more simple and implies the advantage of using measurement data within the optical zone. For regular profiles the extracted ablation profiles showed a very good match with the planned ones. Surprisingly, even for highly irregular profiles of topography-guided laser treatments the method delivered reasonable overlaps when being compared to the planned profiles. Analysis of the effective tissue removal yields valuable information on the quality of the ablation process.
Based on eye movement data, we present a study on the effect of various laser and eye-tracking parameters on the optical outcome after scanning spot refractive surgery. Numerical simulations of the entire ablation process were performed on a schematic model eye under variation of the following parameters: ablation depth per pulse, laser spot size, eye tracker latency and magnitude of refractive correction. Three-dimensional ray tracing through an analytical model eye featuring the ablated corneal front surface enabled evaluation of the resulting optical quality. The modulation transfer function (MTF) was calculated to rate the difference in optical quality between an ideal (movement-free) treatment, and treatments performed with an eye-tracker working with a certain latency. For all the calculations it was assumed, that the laser repetition rate remains constant at 250 Hz. It was shown, that the contrast transfer can decrease significantly with increasing latency of the eye-tracker. For constant laser and tracking parameters, this decrease was found to be more significant for higher myopic corrections. It was further shown, that treatments performed with smaller spot sizes and smaller ablation depths per pulse are more sensitive to tracking latency. Assuming a certain eye tracker latency, the most stable results are obtained for large beam diameters and high central ablation depths per pulse. Latencies below 10 ms would allow for a reduction of the beam diameter to 0.50 mm as well as for ablation depths as small as 0.50 microns.
Purpose: Corneal topography data expressed as corneal aberrations are frequently used to report corneal laser surgery results. However, the optical image quality at the retina depends on all optical elements of the eye such as the human lens. Thus, the aim of this study was to investigate the correlations between the corneal and total wavefront aberrations and to discuss the importance of corneal aberrations for representing corneal laser surgery results. Methods: Thirty three eyes of 22 myopic subjects were measured with a corneal topography system and a Tschernig-type wavefront analyzer after the pupils were dilated to at least 6 mm in diameter. All measurements were centered with respect to the line of sight. Corneal and total wavefront aberrations were calculated up to the 6th Zernike order in the same reference plane. Results: Statistically significant correlations (p < 0.05) between the corneal and total wavefront aberrations were found for the astigmatism (C3,C5) and all 3rd Zernike order coefficients such as coma (C7,C8). No statistically significant correlations were found for all 4th to 6th order Zernike coefficients except for the 5th order horizontal coma C18 (p equals 0.003). On average, all Zernike coefficients for the corneal aberrations were found to be larger compared to Zernike coefficients for the total wavefront aberrations. Conclusions: Corneal aberrations are only of limited use for representing the optical quality of the human eye after corneal laser surgery. This is due to the lack of correlation between corneal and total wavefront aberrations in most of the higher order aberrations. Besides this, the data present in this study yield towards an aberration balancing between corneal aberrations and the optical elements within the eye that reduces the aberration from the cornea by a certain degree. Consequently, ideal customized ablations have to take both, corneal and total wavefront aberrations, into consideration.
The effect of lateral and torsional misalignments of the ablation on the postoperative optical outcome was theoretically investigated based on measured wavefront aberration data from 130 normal eyes. Simulations included lateral decentrations and rotations around the longitudinal axis of the eye (torsion). The optical quality of the simulated refractive correction was rated by means of the root-mean-squared residual wavefront error. The accuracy for lateral centration in order to achieve the diffraction limit at a pupil size of 8 mm in 95% of the investigated eyes should be 50 microns or better. However, an accuracy of 450 microns was found to be enough to guarantee that none of the investigated eyes would suffer from a decreased optical performance after surgery. Alignment would have to be performed with a torsional precision of approximately 1 deg or better in order to achieve the diffraction limit in 95% of the measured normal eyes for an 8-mm pupil, whereas an accuracy of 15 deg is required to obtain at least some improvement of the optical quality in all the examined eyes. The accuracy needed for torsional alignment increases compared to pure sphero-cylindrical treatments when additional correction of the higher-order aberrations is aspired.
The use of modern erbium: yttrium–
aluminum–garnet (YAG) laser systems in opthalmic microsurgery requires
a precise knowledge of the size and dynamics of the laser
induced vapor bubbles. The aim of this work was to clarify the possibilities
of controlling the vapor bubble shape and size by using an
optimized fiber tip geometry for various ophthalmic applications with
the erbium: YAG laser. Methods. The mid-infrared radiation of freerunning
erbium: YAG laser was coupled optically into means of different
low OH2 quartz fiber tips to investigate the vapor bubble formation
in water by high-speed photography. The core diameter of four
fiber tips ranged from 200 up to 940 mm. Fourteen fiber tips were
polished at an angle graduated from 10° to 70° over the full core
diameter (seven fiber tips) and over the half core diameter (seven fiber
tips). Three fiber tips were produced to have a curvature at the distal
end with curvature radii of 160, 230, and 420 mm. Results. The shape
as well as the size of erbium: YAG laser induced vapor bubbles can be
controlled systematically by using adequate fiber tip geometries. In
detail, the used different angles and curvatures demonstrate that the
propagation direction of the vapor bubbles can be estimated by optical
modeling considering Snell’s law and the Fresnel laws at a quartzair
boundary. Beside this, the size of a vapor bubble can be predetermined
by choosing ideal fiber tip geometries to reduce or increase the
radiant exposure at the distal end of the quartz fiber tip. Conclusions.
The good possibility of controlling the shape and size of vapor
bubbles offers a wider range of new applications, especially in ophthalmic
microsurgery such as erbium YAG laser vitrectomy.
The aim of this work was to study the optical aberrations of higher order in a normal population and to answer the question on the optical quality of the human eye. Therefore, the optical aberrations of 130 eyes (90 individuals) have been measured by means of a wavefront measuring devices of Tscherning's-type. The pupil of each measured eye was dilated to at least 7 mm in diameter and wavefront sensing was performed with respect to the line of sight. The optical aberrations are expressed in terms of Zernike coefficients up to the 6th order, root-means-squared wavefront errors and critical pupil size. The main finding of this paper is that the `average eye' has only minimal wavefront errors indicating that the construction of the human eye, in principle, provides excellent optics exceeding the Marechal- criterion only by a factor of 2.1 (pupil diameter 5 mm). However, such minimal aberrations are achieved in only 6% of the individual eyes examined in this study. In conclusion, the `averaged human' eye has nearly a perfect optic, but the individual eye provides poor optical quality.
Higher-order optical errors of the human eye are often responsible for a reduced visual acuity in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can result from operations in the eye involving optical structures. The presented wavefront analyzer bases on Tscherning's aberroscope. A collimated laser beam (532 nm, 10 mW) illuminates a mask with a regular matrix of 0.3 mm diameter holes which forms a bundle of thin parallel rays. These rays are focused by a lens in front of the eye that their intraocular focus point is located in a certain distance in front of the retina generating a corresponding pattern of light spots on it. According to the existing ocular optical errors, this spot pattern is more or less distorted in comparison to the mask matrix. For a 6 mm pupil diameter 68 retinal spots are plottable for the assessment of optical aberrations. The retinal spot pattern is imaged onto the sensor of a low-light CCD video camera by indirect ophthalmoscopy. The deviations of all spots from their ideal regular positions are measured by means of a PC, and from these values the intraocular wave-front aberration is computed in the form of Zernike polynomials up to the 6th order.
Purpose: The aim of this work was to study the effect of currently used ablation profiles along with eccentric ablations on the increase of higher order aberrations observed after PRK. Material and Methods: The optical aberrations of 10 eyes were tested before and after PRK. Refractive surgery was performed using a ArF-excimer laser system. In all cases, the ablation zone was 6 mm or larger. The spherical equivalent of the correction was ranging from -2.5 D to -6.0 D. The measured wavefront error was compared to numerical simulations done with the reduced eye model and currently used ablation profiles as well as compared with experimental results obtained from ablation on PMMA balls. Results: The aberration measurements result in a considerable change of the spherical- and coma-like wavefront errors. This result was in good correlation with the numerical simulations and the experimental results. Furthermore, it has been derived that the major contribution on the induced higher order aberrations are a result of the small decentration (less than 1.0 mm) of the ablation zone. Conclusions: Higher order spherical- and coma-like aberrations after PRK are mainly determined by the decentration of the ablation zone during laser refractive surgery. However, future laser systems should use efficient eye-tracking systems and aspherical ablation profiles to overcome this problem.
Purpose: The goal of this presentation is to discuss the use of the Light Shaping Beam Homogenizer in an optical system for scanning-spot PRK.
Methods: The basic principle of the LSBH is the transformation of any incident intensity distribution by light scattering on an irregular microlens structure z = f(x,y). The relief of this microlens structure is determined by a defined statistical function, i.e. it is defined by the mean root-squared tilt σ of the surface relief. Therefore, the beam evolution after the LSBH and in the focal plane of an imaging lens was measured for various root-squared tilts. Beside this, an optical setup for scanning-spot PRK was assembled according to the theoretical and experimental results.
Results: The divergence, homogeneity and the Gaussian radius of the intensity distribution in the treatment plane of the scanning-spot PRK laser system is mainly characterized by dependent on root-mean-square tilt σ of the LSBH, as it will be explained by the theoretical description of the LSBH.
Conclusions: The LSBH represents a simple, low cost beam homogenizer with low energy losses, for scanning-spot excimer laser systems.
Background: The formation of evaporation bubbles and pressure waves during Erbium:YAG laser vitrectomy might cause intraocular damages.
Methods: In water, the formation of the evaporation bubbles was observed by high-speed photography. The output energy of the quartz tip ranges from 5 to 50 mJ and the laser pulse duration from 50 μsec to 300 μsec. The dynamic of the evaporation bubbles were investigated for different diameters, various angles and radii of the quartz fiber tip. Furthermore, the spread out of the evaporation bubbles was observed for various geometries of the microsurgery probe. The induced stress waves were measured with a PVDF-hydrophone.
Results: The evaporation bubble size increases semi-logarithmic with the pulse energy and reduces with the increase of the pulse duration. The diameter of the tip has no significant influence in the vapor bubble size. The expansion of the vapor bubble can be controlled by the geometry of the tip. The spread out of the vapor bubble can reduced by a slit geometry of the aspiration hole. The maximum pressure amplitude as found to be < 2 MPa.
Conclusions: The evolution of evaporation bubbles and the induced pressure amplitudes from the microsurgery probe can be minimized for Erbium:YAG laser vitrectomy.
The Erbium:YAG laser emitting at a wavelength of 2,94 micrometer have been promised as an alternative laser for the ArF-excimer laser (193 nm) in photorefractive keratectomy (PRK). This report discusses the limitations of laser parameters such as wavelength, energy density and pulse duration for the ablation of the cornea. In addition, the melting process during ablation on the corneal surface roughness may play a role.
Purpose: The increasing interest in a homogeneous Gaussian light beam profile for applications in ophthalmology e.g. photorefractive keratectomy (PRK) requests simple optical systems with low energy losses. Therefore, we developed the Light Shaping Beam Homogenizer (LSBH) working from UV up to mid-IR. Method: The irregular microlenses structure on a quartz surface was fabricated by using photolithography, chemical etching and chemical polishing processes. This created a three dimensional structure on the quartz substrate characterized in case of a Gaussian beam by random law distribution of individual irregularities tilts. The LSBH was realized for the 193 nm and the 2.94 micrometer wavelengths. Simulation results obtained by 3-D analysis for an arbitrary incident light beam were compared to experimental results. Results: The correlation to a numerical Gaussian fit is better than 94% with high uniformity for an incident beam with an intensity modulation of nearly 100%. In the far field the cross section of the beam shows always rotation symmetry. Transmittance and damage threshold of the LSBH are only dependent on the substrate characteristics. Conclusions: considering our experimental and simulation results it is possible to control the angular distribution of the beam intensity after LSBH with higher efficiency compared to diffraction or holographic optical elements.
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