Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

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$XXX Congress of the European Society of Cataract and Refractive ...$

CHAPTER 5 ablations. The slit-confocal configuration of the instrument was used to measure the surface height at each point, in an “extended shape” custom measurement mode. Prior to the measurement, the artificial corneas were rotated until the reference mark was oriented along the y-axis of the microscope. The corneal apex, found using the 20x interferometric objective on the same instrument, was used as the origin for the measurements in the curved surfaces. In the case of flat surfaces, the apex was taken as the center of the cylinder (pre-operative) or the center of the ablation (postoperative). All the spherical artificial corneas were measured before and after the ablation. All flat corneas were measured after the ablation. Only a few pre-operative flat corneas were measured, as the deviation from flatness was as low as the precision of the instrument (1 m peak to peak for the whole surface). Fig. 5.2. Single-frame excerpt from video recordings of a refractive surgery procedure in a flat plastic artificial cornea of Filofocon A. 5.2.5. Data analysis 5.2.5.1. Extracting the ablation patterns We wrote routines in Matlab (Mathworks, Nantick, MA, USA) to extract the laser ablation patterns from the shape measurements in flat and spherical surfaces. The topographies measured on ablated flat surfaces should represent the actual spot density profile delivered by the laser. The measurements were only corrected numerically for the tilt found on the non-ablated area. This tilt appears in the measurement because the line of sight of the eye is not necessarily aligned with the microscope optical axis. We found tilts typically lower than 2 degrees. These tilts come from the topography measurement and not from the ablation, where the alignment and centration of the artificial eye were well controlled. We tested the procedure by measuring the same sample at different slight tilting angles, and checking that tilt was correctly compensated by the software. The ablation pattern was also obtained from the ablated spherical corneas by subtracting a base sphere (corresponding to the pre-operative spherical surface) from the measured elevation. We found that subtracting the analytical surface produced a better estimation (less noise) than a point-by-point subtraction of elevation maps. 142
OPTIMIZED LASER PLATFORMS 5.2.5.2. Computing the laser efficiency effects The efficiency effects are described by the ablation efficiency factor K, depending on the incidence angle. For a particular geometry and material (plastic or cornea), the ablation efficiency factor is defined as the ratio of the ablation depth pattern on a spherical surface to the ablation depth pattern on a flat surface: d SPH K , (5.1) d FLAT where d is the ablation depth at each point. The effective ablation depth at each point of a curved surface can be predicted as d eff = d FLAT · K. From a design point of view, 1/K represents the correction factor (one at the center and higher than one at the periphery) by which the intended profile should be multiplied to compensate for the changes in ablation efficiency on curved surfaces. The control algorithm should program d/K, in the laser, in order to obtain the desired pattern d ablated in the material. For accurate estimations, it is essential that both ablations are registered, on the same coordinate center, which was estimated by fitting biconic surfaces to both ablation patterns. The selected radius of the base sphere subtracted from the ablated spherical surface elevation map was also critical in the assessment of the laser efficiency factor, as small discrepancies in the base sphere radius may result in slightly different ablation depths at the apex. As the ablation depth should be identical on flat and spherical surfaces in the center of the ablation, the base sphere radius was slightly refined (in steps of 0.05 mm) until that condition is reached. This refinement only affects the ablation patterns (estimated from spheres) in a few microns, but has a major impact on the fulfillment of the contour conditions (the efficiency factor is one in the center while the non-ablated zones are similar). The refinement needed in the base sphere is compatible with the small changes in the laser intensity observed between ablations and with the expected dilation effects on polymers (Goods et al., 2003) during the measurement. 5.2.5.3. Impact on cornea The nominal ablation depth at the apex -provided by the system’s software- is used to obtain a conversion factor between the ablation depth per pulse on plastic (Filofocon A) and corneal tissue. This factor changes with the laser, as it depends on fluence. The ideal ablation pattern in cornea (not affected by efficiency effects) can then be predicted by directly multiplying the ablation pattern on Filofocon by the plastic/cornea conversion factor. To predict the actual shape of postoperative corneas, it is necessary to consider the geometry-related ablation efficiency factor for corneal tissue, which can be obtained from the ablation efficiency factor measured for plastic. According to the Beer-Lambert law, the etch rate (ablation depth per pulse) at each point of a flat surface should be given by d FLAT =(1/) · ln (F 0 / Fth) where F 0 is the fluence of the laser and and Fth are the absorption coefficient and the ablation threshold of the material respectively. The reflection loss is already implicit in the experimental estimates of ablation properties ( and Fth) as they were obtained from regressions of the ablated depth at different laser fluences on flat surfaces (Dorronsoro et al., 2008b). However, a reflection coefficient R, needs to be considered on non-flat surfaces 143
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Doctoral thesis Corneal Ablation an
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Contents Corneal Ablation and Conta
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2.2.1. Fitting surfaces............
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5.3.3. Ablation efficiency factors
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10.5. DISCUSSION...................
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We thank José Antonio Sánchez-Gil
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BOZR Back Optic Zone Radius BCVA Be
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INTRODUCTION 1.1. MOTIVATION The fr
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INTRODUCTION 1.2. THE OPTICAL SYSTE
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INTRODUCTION Myopia is a very commo
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INTRODUCTION a point focus: the dif
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INTRODUCTION system, and tilt repre
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INTRODUCTION term Z 0 2 stands for
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INTRODUCTION which image was shadow
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INTRODUCTION The defocus Zernike te
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INTRODUCTION 1.4.6.3. Internal Aber
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INTRODUCTION 1.5.2.3. Alternating v
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INTRODUCTION and further-more, at l
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INTRODUCTION alcohol is used to loo
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INTRODUCTION changes in reflectivit
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INTRODUCTION Marcos et al. (Marcos
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INTRODUCTION 1.7.3. Ablation effici
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INTRODUCTION corneal surface may be
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INTRODUCTION Algorithm design: The
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INTRODUCTION contact lenses the fie
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INTRODUCTION 1.10.1. Tear studies A
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INTRODUCTION there are still many u
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INTRODUCTION 6. Evaluation and unde
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METHODS 2Chapter Chapter 2 - Method
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METHODS 2.1. MEASUREMENT OF OPTICAL
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METHODS In both profilometers, cust
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METHODS The measurement method that
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METHODS Fig. 2. 9. Set of points in
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METHODS Slit lamp Scheimpflug Camer
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METHODS representing the deviation
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METHODS 2.2.2.2. Ablation pattern f
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METHODS direction. These calculatio
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METHODS eye through the whole pupil
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METHODS (1993) power thresholds for
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METHODS alignment (frontal-illumina
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METHODS Fig. 2. 24. Example frame f
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METHODS 4) New image processing too
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METHODS Fig. 2. 28. Corrected entra
Page 91 and 92: METHODS Badal lens, corresponds to
Page 93: METHODS Each VA measurement consist
Page 99: REFRACTIVE SURGERY PMMA MODEL 3.1.
Page 102 and 103: CHAPTER 3 optimization of refractiv
Page 104 and 105: CHAPTER 3 These data demonstrate th
Page 106 and 107: CHAPTER 3 regression line, since it
Page 108 and 109: CHAPTER 3 Figure 3.3 shows one of t
Page 110 and 111: CHAPTER 3 Ablation efficiency facto
Page 112 and 113: CHAPTER 3 considering the same set
Page 114 and 115: CHAPTER 3 3.5.3 Impact of correctio
Page 117: ABLATION PROPERTIES OF FILOFOCON A
Page 121 and 122: ABLATION PROPERTIES OF FILOFOCON A
Page 133: ABLATION PROPERTIES OF FILOFOCON A
Page 137: OPTIMIZED LASER PLATFORMS 5.1 ABSTR
Page 140 and 141: CHAPTER 5 pulses. Fluence, repetiti
Page 144 and 145: CHAPTER 5 (Anera et al., 2003): d S
Page 146 and 147: CHAPTER 5 asymmetries also appeared
Page 148 and 149: CHAPTER 5 5.3.4. Ablation patterns
Page 150 and 151: CHAPTER 5 a) b) : 6.5 mm c) Fig. 5.
Page 152 and 153: CHAPTER 5 achieve proper reflection
Page 154 and 155: CHAPTER 5 Both the ablation algorit
Page 157: HYBRID PORCINE/PLASTIC MODEL 6.1. A
Page 160 and 161: CHAPTER 6 6.3.1. Hybrid porcine-pla
Page 162 and 163: CHAPTER 6 is detected. Therefore, i
Page 164 and 165: CHAPTER 6 no preferential orientati
Page 167: ANTERIOR AND POSTERIOR CORNEAL ELEV
Page 171 and 172: ANTERIOR AND POSTERIOR CORNEAL ELEV
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Page 187: SOFT CONTACT LENS FITTING USING MOD
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ON-EYE OPTICAL PERFORMANCE OF RIGID
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SOFT MONOFOCAL AND MULTIFOCAL CONTA
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SOFT MONOFOCAL AND MULTIFOCAL CONTA
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CORRELATION BETWEEN RADIUS AND ASPH
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CONCLUSIONS Conclusions This thesis
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CONCLUSIONS 5We have developed a pr
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CONCLUSIONS LIST OF METHODOLOGICAL
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CONCLUSIONS FUTURE RESEARCH LINES T
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CONCLUSIONS LIST OF PUBLICATIONS AN
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REFERENCES References AIZAWA, D., S
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REFERENCES PHOTODECOMPOSITION (APD)
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REFERENCES DORRONSORO, C., CANO, D.
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REFERENCES GISPETS, J., ARJONA, M.
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REFERENCES KOPF, M., YI, F., ISKAND
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REFERENCES MARCOS, S., DORRONSORO,
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REFERENCES NOVO, A., PAVLOPOULOS, G
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REFERENCES SAKIMOTO, T., ROSENBLATT
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REFERENCES WANG, L., DAI, E., KOCH,
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RESÚMENES pacientes reales fue com
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RESÚMENES experimentales de cirug
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RESÚMENES cada punto de las cornea
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RESÚMENES plástico medidas anteri
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RESÚMENES 8 Evaluación óptica de
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RESÚMENES 9 Prestaciones ópticas
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RESÚMENES 10 Calidad óptica y cal
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RESÚMENES A Correlación entre rad
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CONCLUSIONES clínicos. La descripc
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CONCLUSIONES 12 Las superficies ocu
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CONCLUSIONES IMPLICACIONES DE ESTA
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CONCLUSIONES contribuyen a la compr
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(y muchísimo más difícil). Me da
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