Luis Alberto Vieira de Carvalho
Other affiliations: University of California, Berkeley, Federal University of São Paulo, Spanish National Research Council ...read more
Bio: Luis Alberto Vieira de Carvalho is an academic researcher from University of São Paulo. The author has contributed to research in topics: Zernike polynomials & Corneal topography. The author has an hindex of 12, co-authored 41 publications receiving 574 citations. Previous affiliations of Luis Alberto Vieira de Carvalho include University of California, Berkeley & Federal University of São Paulo.
TL;DR: The feasibility of correcting higher-order aberrations and improving visual performance with customized soft contact lenses in keratoconic eyes while compensating for the static decentration and rotation of the lens is demonstrated.
Abstract: Higher-order aberration correction in abnormal eyes can result in significant vision improvement, especially in eyes with abnormal corneas. Customized optics such as phase plates and customized contact lenses are one of the most practical, nonsurgical ways to correct these ocular higher-order aberrations. We demonstrate the feasibility of correcting higher-order aberrations and improving visual performance with customized soft contact lenses in keratoconic eyes while compensating for the static decentration and rotation of the lens. A reduction of higher-order aberrations by a factor of 3 on average was obtained in these eyes. The higher-order aberration correction resulted in an average improvement of 2.1 lines in visual acuity over the conventional correction of defocus and astigmatism alone.
TL;DR: The results suggest that, for surfaces such as that present after RK, in keratoconus, or after keratoplasty, even more than 36 terms may be necessary to obtain minimum accuracy requirements.
Abstract: Purpose Zernike polynomials have been successfully used for approximately 70 years in many different fields of optics. Nevertheless, there are some recent discussions regarding the precision and accuracy of these polynomials when applied to surfaces such as the human cornea. The main objective of this work was to investigate the absolute accuracy of Zernike polynomials of different orders when fitting several types of theoretical corneal and wave-front surface data. Methods A set of synthetic surfaces resembling several common corneal anomalies was sampled by using cylindrical coordinates to simulate the height output files of commercial videokeratography systems. The same surfaces were used to compute the optical path difference (wave-front [WF] error), by using a simple ray-tracing procedure. Corneal surface and WF error was fit by using a least-squares algorithm and Zernike polynomials of different orders, varying from 1 to 36 OSA-VSIA convention terms. Results The root mean square error (RMSE) ranged-from the most symmetric corneal surface (spherical shape) through the most complex shape (after radial keratotomy [RK]) for both the optical path difference and the surface elevation for 1 through 36 Zernike terms-from 421.4 to 0.8 microm and 421.4 to 8.2 microm, respectively. The mean RMSE for the maximum Zernike terms for both surfaces was 4.5 microm. Conclusions These results suggest that, for surfaces such as that present after RK, in keratoconus, or after keratoplasty, even more than 36 terms may be necessary to obtain minimum accuracy requirements. The author suggests that the number of Zernike polynomials should not be a global fixed conventional or generally accepted value but rather a number based on specific surface properties and desired accuracy.
TL;DR: In this paper, Liou et al. implemented a simulation of five well-known schematic eyes available in the literature, including the Helmholtz-Laurance, Gullstrand, Emsley, Greivenkamp and Liou & Brennan, and compared the optical performance of these different models using different quantitative optical quality parameters.
Abstract: Current ophthalmic technology allows the manipulation of eye components, such as anterior cornea and lens, of the human eye with a considerable precision and customization. This technology opens up the possibility of exploiting some characteristics of the eye in order to improve the methods of correcting optical aberrations. Moreover, product development and research for the eye-care professional has reached very high standards, since there is nowadays software available to design and simulate practically any mechanical or optical characteristic of the product, even before it is thrown into production line. Although quite similar in the general form, different human eye models simulate the image formation by considering different property combinations in the constitutive elements of the eye structure (such as refraction index and surface curvatures), producing retinal images that resemble very closely those of the biological eye. Using optical design software, we have implemented a simulation of 5 well-known schematic eyes available in the literature. These models were the Helmholtz-Laurance, Gullstrand, Emsley, Greivenkamp and Liou & Brennan. The optical performance of these different models was compared using different quantitative optical quality parameters. The model of Liou and Brennan, contains features of the biological eye that were not considered in previous models, as the distribution of a gradient refraction index and a decentered pupil. Furthermore, it has great reliability since it takes into account the mean value of empirical measurements of the in vivo eye in order to define size and parameters such as anterior and posterior curvature of cornea, lens, axial length, etc. Comparisons between the MTF (Modulation Transfer Function), spot diagrams and ray fan showed the difference in image quality between eye models, and the Strehl Ratio was also used as a parameter of comparison. A careful comparison between the different models showed that the first four schematic eyes have better optical quality than what is expected for the general and healthy emmetropic in vivo eye. Liou and Brennan schematic eye is the one that most closely resembles the in vivo biological eye. Therefore, in applications, such as research or product development for customized vision correction, which must consider optical properties intrinsic to the biological eye, we recommend this latter model; for applications that do not require refraction-limited performance, most of the other models should be a good approximation.
TL;DR: In this paper, the stability properties of vector linear difference equations of the form x(t) = ∑kj=1 Ajx(t − rj), 0 < r1 < r2 < … < rk < ∞.
Abstract: This paper studies some stability properties of vector linear difference equations of the form x(t) = ∑kj=1 Ajx(t − rj), 0 < r1 < r2 < … < rk < ∞. This is done through the direct method of Liapunov, by means of a quadratic functional defined in an appropriate L2 space. The derivative of this functional along the equation, nevertheless, is defined in a finite dimensional euclidean space. The positive definiteness of certain matrices involved, together with the bounded real lemma of system theory, furnishes useful criteria to determine stability properties of the given equation. In particular, a criterion is given to determine stability with respect to the delays ri.
TL;DR: A finite element model of the cornea is developed, that tries to predict keratoconus-like behavior and its evolution based on material properties of the Corneal tissue, and shows that changes in local material properties and intraocular pressure are intrinsically related to ker atoconus pathology and its shape/curvature.
Abstract: PURPOSE: The ability to predict and understand which biomechanical properties of the cornea are responsible for the stability or progression of keratoconus may be an important clinical and surgical tool for the eye-care professional. We have developed a finite element model of the cornea, that tries to predicts keratoconus-like behavior and its evolution based on material properties of the corneal tissue. METHODS: Corneal material properties were modeled using bibliographic data and corneal topography was based on literature values from a schematic eye model. Commercial software was used to simulate mechanical and surface properties when the cornea was subject to different local parameters, such as elasticity. RESULTS: The simulation has shown that, depending on the corneal initial surface shape, changes in local material properties and also different intraocular pressures values induce a localized protuberance and increase in curvature when compared to the remaining portion of the cornea. CONCLUSIONS: This technique provides a quantitative and accurate approach to the problem of understanding the biomechanical nature of keratoconus. The implemented model has shown that changes in local material properties of the cornea and intraocular pressure are intrinsically related to keratoconus pathology and its shape/curvature.
TL;DR: In this article, optical coherence tomography is used for high-resolution, noninvasive imaging of the human retina, including the macula and optic nerve head in normal human subjects.
Abstract: Objective: To demonstrate optical coherence tomography for high-resolution, noninvasive imaging of the human retina. Optical coherence tomography is a new imaging technique analogous to ultrasound B scan that can provide cross-sectional images of the retina with micrometer-scale resolution. Design: Survey optical coherence tomographic examination of the retina, including the macula and optic nerve head in normal human subjects. Settings Research laboratory. Participants: Convenience sample of normal human subjects. Main Outcome Measures: Correlation of optical coherence retinal tomographs with known normal retinal anatomy. Results: Optical coherence tomographs can discriminate the cross-sectional morphologic features of the fovea and optic disc, the layered structure of the retina, and normal anatomic variations in retinal and retinal nerve fiber layer thicknesses with 10- μm depth resolution. Conclusion: Optical coherence tomography is a potentially useful technique for high depth resolution, cross-sectional examination of the fundus.
TL;DR: The best concise account of the basic mathematical aspects of control has been brought completely up to date while retaining its focus on state-space methods and its emphasis on points of mathematical interest.
Abstract: The best concise account of the basic mathematical aspects of control has been brought completely up to date while retaining its focus on state-space methods and its emphasis on points of mathematical interest. The authors have written a new chapter on multivariable theory and a new appendix on Kalman filtering, added a large number of new problems, and updated all the references. This book will continue as a fundamental resource for applied mathematicians studying control theory and for control engineers and electrical and mechanical engineers pursuing mathematically oriented studies. From reviews of the first edition: \"Excellent....Strongly recommended.\"--Bulletin of the International Mathematical Association. \"Could hardly be bettered.\"--Times Higher Education Supplement
TL;DR: Cone packing density in the living human retina decreases as a function of age within the foveal center with the largest difference being found at the most central measurement site.
Abstract: As the first stage of vision, the photoreceptors provide the spatial information to higher stages of visual processing. Packing density and arrangement of photoreceptors are related to the development, function, and evolution of the visual system.1–4 The human cone photoreceptors distribution has the following features2,5,6: (1) a high peak cone packing density at the foveal center; (2) a rapid decrease in packing density within the central 2 mm of retina and then a more gradual decrease farther away; and (3) isodensity contours of cone photoreceptor distribution that are elongated along the horizontal axis, sometimes referred to as a cone streak.2 Previous studies of the effect of aging on the retina suggest that normal aging is accompanied by photoreceptor changes. Both rod7 and cone8 outer segments have been reported to become disorganized with aging, especially when they are close to the fovea. Cone photopigment is also reduced with age4,9 and Swanson et al.10 reported that the decreased photopigment is largest in the central 1° of the retina, suggesting a change in foveal architecture with age, which is supported by findings of changes in both photopigment and macular pigment distributions.11 However, the question of whether there is loss of cone photoreceptors with aging in the human retina is still controversial. Pandajonas et al.12 found a loss of 37% of rods and 18% of cones extrapolated to a lifespan of 100 years. Gao et al.13 and Curcio et al.14 found that loss of rods occurs with age but did not find a decrease in cone density, although variability among individuals was high.2 A limitation of these histologic approaches is that these measurements have been possible only ex vivo, which requires experimental manipulation of tissue before the measurements. The best data can only be collected in very fresh tissue, which is hard to obtain, and thus the total sample of data is relatively small and the age distribution sampling is variable. In recent years, adaptive optics has enabled high-resolution retinal imaging, sufficient for measuring the cone packing density in vivo.5,6,15–20 In the present study, we used state-of-the-art adaptive optics scanning laser ophthalmoscopes (AOSLO),21,22 to measure the variation in cone photoreceptors in two normal populations that differed in age. We used the rapid acquisition capability to systematically collect data along four retinal meridians from near the fovea to approximately 12° retinal eccentricity. The cone packing density was compared as a function of retinal eccentricity, meridian, and age.
TL;DR: HC radius, inverse concave radius, DA ratio, and DefA ratio were shown to be suitable parameters to evaluate in vivo corneal biomechanics due to their independence from IOP and their correlation with pachymetry and age.
Abstract: Purpose To evaluate the influence of pachymetry, age, and intraocular pressure in normal patients and to provide normative values for all dynamic corneal response parameters (DCRs) provided by dynamic Scheimpflug analysis. Methods Seven hundred five healthy patients were included in this multicenter retrospective study. The biomechanical response data were analyzed to obtain normative values with their dependence on corrected and clinically validated intraocular pressure estimates developed using the finite element method (bIOP), central corneal thickness (CCT), and age, and to evaluate the influence of bIOP, CCT, and age. Results The results showed that all DCRs were correlated with bIOP except deflection amplitude (DefA) ratio, highest concavity (HC) radius, and inverse concave radius. The analysis of the relationship of DCRs with CCT indicated that HC radius, inverse concave radius, deformation amplitude (DA) ratio, and DefA ratio were correlated with CCT (rho values of 0.343, -0.407, -0.444, and -0.406, respectively). The age group subanalysis revealed that primarily whole eye movement followed by DA ratio and inverse concave radius were the parameters that were most influenced by age. Finally, custom software was created to compare normative values to imported examinations. Conclusions HC radius, inverse concave radius, DA ratio, and DefA ratio were shown to be suitable parameters to evaluate in vivo corneal biomechanics due to their independence from IOP and their correlation with pachymetry and age. The creation of normative values allows the interpretation of an abnormal examination without the need to match every case with another normal patient matched for CCT and IOP. [J Refract Surg. 2016;32(8):550-561.].
TL;DR: The visual limitation in keratoconus could be explained by different alterations that occur in these corneas and allowed development of a new grading system for this condition.
Abstract: Purpose To evaluate the clinical features of keratoconus taking into consideration anterior corneal aberrations, internal astigmatism, and corneal biomechanical properties and to define a new grading system based on visual limitation. Setting Vissum Corporation, Alicante, Spain. Design Retrospective case series. Methods This multicenter study comprised consecutive keratoconic eyes with no previous ocular surgery or active ocular disease. Visual, refractive, corneal topography, and pachymetry outcomes were analyzed. Internal astigmatism was calculated by vectorial analysis. Corneal aberrations and corneal biomechanics characterized by the Ocular Response Analyzer were evaluated in some eyes. Correlations between clinical data and a linear multiple regression analysis for characterizing the relationship between visual limitation and objective clinical data were performed. Results This study comprised 776 eyes of 507 patients (age range 11 to 79 years) The mean keratometry (K) correlated significantly with logMAR corrected distance visual acuity (CDVA) ( r = 0.591, P r = 0.497, P r = −0.647, P r ≥0.603, P r ≥0.444, P r 2 = 0.69, P P Conclusion The visual limitation in keratoconus could be explained by different alterations that occur in these corneas and allowed development of a new grading system for this condition. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.