Retinal assessment using optical coherence tomography.

Over the 15 years since the original description, optical coherence tomography (OCT) has become one of the key diagnostic technologies in the ophthalmic subspecialty areas of retinal diseases and glaucoma. The reason for the widespread adoption of this technology originates from at least two properties of the OCT results: on the one hand, the results are accessible to the non-specialist where microscopic retinal abnormalities are grossly and easily noticeable; on the other hand, results are reproducible and exceedingly quantitative in the hands of the specialist. However, as in any other imaging technique in ophthalmology, some artifacts are expected to occur. Understanding of the basic principles of image acquisition and data processing as well as recognition of OCT limitations are crucial issues to using this equipment with cleverness. Herein, we took a brief look in the past of OCT and have explained the key basic physical principles of this imaging technology. In addition, each of the several steps encompassing a third generation OCT evaluation of retinal tissues has been addressed in details. A comprehensive explanation about next generation OCT systems has also been provided and, to conclude, we have commented on the future directions of this exceptional technique.

The placido wavefront sensor and preliminary measurement on a mechanical eye.

PURPOSE: The hardware and software of a novel wavefront sensor was developed (The sensor presented here is patent pending.). It has the same principal of the Hartmann-Shack (HS) and other sensors that are based on slope information for recovery of wavefront surface, but a different symmetry, and does not use individual microlenses. This polar symmetry might offer differences during practical measurements that may add value to current and well-established "gold standard" techniques. METHODS: The sensor consists of a set of concentric "half-donut" surfaces (longitudinally sectioned toroids) molded on an acrylic surface with a CCD located at the focal plane. When illuminated with a plane wavefront, it focuses a symmetric pattern of concentric discs on the CCD plane; for a distorted wavefront, a nonsymmetric disc pattern is formed (similar to images of a placido-based videokeratographer). From detection of shift in the radial direction, radial slopes are computed for a maximum of 2880 points, and the traditional least-squares procedure is used to fit these partial derivatives to a set of 15 conventional OSA-VSIA Zernike polynomials. Theoretical computations for several synthetic surfaces containing low-order aberration (LOA) and high-order aberration (HOA) were implemented for both the HS and the new sensor. RESULTS: Root mean square error (RMSE) in microns when theoretical data was taken as control, for HS sensor and new sensor, was 0.02 and 0.00003 for LOA (defocus, astigmatism) and 0.07 and 0.06 for HOA (coma, spherical, and higher terms), respectively. After this, practical preliminary measurements on a mechanical eye with a 5-mm pupil and 10 different defocus aberrations ranging from -5 D to 5 D, in steps of 1 D, were compared between sensors. RMSE for difference in measurements for HS and new sensor for sphere, cylinder, and axis, was 0.13 D, 0.07 D, and 11. Measurements were taken only on defocus aberrations. Qualitative images for astigmatism are shown. DISCUSSION: Although practical in vivo tests were not conducted in this first study, we also discuss certain possible alignment differences that may arise as a result of the different symmetry of the new sensor. To take any conclusive assumption regarding the accuracy and/or precision of this new sensor, when compared with other well-established sensors, statistically significant in vivo measurements will need to be conducted.

Occult chorioretinal anastomosis in age-related macular degeneration: a prospective study by optical coherence tomography.

PURPOSE: To investigate macular optical coherence tomography (OCT) features in patients with chorioretinal anastomosis (CRA) and drusen, as well as their correlation with the Gass occult-CRA hypothesis. DESIGN: Prospective observational case series. METHODS: setting: Tertiary ophthalmic referral center. study population: All patients with biomicroscopic evidence of CRA and drusen consecutively evaluated between February 2003 and March 2004. observation procedures: Third-generation OCT evaluation and stereoscopic angiographic studies. main outcome measures: Macular morphologic features at baseline and at 12 weeks. RESULTS: Twenty eyes with CRA and drusen were identified in 11 patients, seven women (63.6%) and four men (36.4%) ranging in age from 69 to 82 years (median, 79 years). Focal elevation of the retinal pigment epithelium was seen in eyes with stage 1 (pre-clinical) CRA. Small hyperreflective signals at the level of the elevated retinal pigment epithelium were seen in stage 2 CRA. In stage 3 CRA, a hyperreflective "mound" at the level of the elevated retinal pigment epithelium was seen in association with a thickened retina. In stage 4 CRA sub-retinal pigment epithelium fluid accumulation was present, and complete disorganization of the macular region was observed in stage 5 CRA. Macular changes were observed in eight eyes (40%) at follow-up, with all but one CRA lesion progressing one stage. CONCLUSION: Morphologic features and changes demonstrated by OCT suggest that fibrovascular detachment of the retinal pigment epithelium followed by development of occult CRA are the initial events occurring in eyes with CRA in age-related macular degeneration. Our findings may support the evolutionary CRA staging system proposed by Gass.

Optical coherence tomography 3: Automatic delineation of the outer neural retinal boundary and its influence on retinal thickness measurements.

PURPOSE: To investigate the automatic delineation of the outer limits of the macular neural retina, by using the optical coherence tomography (OCT)-3 built-in software, and to determine its influence in assessing retinal thickness in the normal macula. METHODS: Retrospective analysis of the OCT3 data at a tertiary-care referral center was performed to study the automatic delineation of the outer neural retina boundary generated by the OCT built-in software. In parallel, a cross-sectional study was designed to compare retinal thickness measurements obtained at specific macular regions of nine normal eyes by the automatic measurement tool with those obtained using a manual-caliper-assisted technique. RESULTS: OCT data from 121 eyes were evaluated. Two parallel, linear highly reflective layers (HRL) were visible at the level of the outer retinal boundary in normal macular regions. Disappearance of the inner and maintenance of the outer HRL was noted in the presence of eye conditions affecting the external retinal layers. The automated software delineation for the outer retinal border was primarily guided by the presence of the inner HRL, whereas the correlation of the OCT findings with the expected clinical and angiographic features on eyes presenting specific macular conditions pointed toward a deeper retinal pigment epithelium-retina interface occurring at the level of the outer HRL. There was a statistically significant difference between the retinal thickness in specific normal macular regions obtained by the automatic measurement tool and the caliper-assisted technique in which the outer retinal border delineation was based on the outer HRL (P = 0.008, Wilcoxon signed rank test). CONCLUSIONS: Incorrect delineation of the outer neural retina boundary is occurring with the automated retinal thickness measurement tool of the OCT3 software. At specific regions of the normal macula, retinal thicknesses were significantly underestimated due to such misalignment.

Videokeratoscopes for dioptric power measurement during surgery.

PURPOSE: To evaluate 2 versions of a computerized surgical videokeratoscope that measures a central region of the cornea of approximately 4.0 to 6.0 mm in diameter to provide information on dioptric power and astigmatism. SETTING: Grupo de Optica Oftalmica Universidade de São Paulo, and Departamento de Oftalmologia da Escola Paulista de Medicina, São Paulo, Brazil. METHODS: The first videokeratoscope system with 10 Placido rings is based on a 15 000 fiber-optic illuminated disk attached to the objective lens of a surgical microscope. With this system, the light intensity can be adjusted during surgery for better contrast of the Placido image. The second system with 14 Placido rings is based on a neon light source behind the Placido disk. With both systems, a 480 x 640 pixel resolution charge-coupled device camera and an IBM-compatible personal computer frame grabber were used. Image processing was used for boundary detection of the rings. An axial curvature algorithm based on the spherical surfaces was used to calculate the dioptric power for each examination. Measurements of 4 real spherical surfaces and 4 simulated aspheric surfaces (ellipsotoric surfaces with different apical radii and different shape factors) were performed. RESULTS: Twenty corneas of 10 healthy volunteers were measured on both videokeratoscope systems and an EyeSys System 2000 corneal topographer. The root- mean-square error for the spherical and simulated aspheric surfaces was, respectively, less than 0.20 diopter (D) and 1.30 D for the 10-disk videokeratoscope system and less than 0.16 D and 1.40 D for the 14-disk system. The mean deviation in corneal measurements with both systems was 0.09 mm for the radius of curvature, 0.51 D for dioptric power, and 5 degrees for cylinder. CONCLUSIONS: The results indicate that the 2 surgical videokeratoscopes are sufficiently precise to aid the anterior segment surgeon in reducing residual astigmatism in cataract surgery and keratoplasty.

Videokeratograph (VKS) for monitoring corneal curvature during surgery

Purpose: We have developed an instrument for computerized surgical videokeratography. A corneal central region of approximately 7.00 mm in diameter may be analyzed, providing the surgeon with information of the power and the astigmatism. Methods: The system is based on a fiber optic Placido disc projecting cone, which is attached to the objective lens of aZeiss compatible surgical microscope. At the beam splitter we installed a monochromatic high resolution camera. A frame grabber is installed on a PC and images are digitized at a 480x640 resolution. Image processing is used for edge detection of rings. Results: Calibrating curves based on 4 spherical surfaces were generated and approximately 3600 points are calculated for each examination. Preliminary measurements on 10 healthy corneas were compared with those of an EyeSys System 2000 Corneal Topographer. Mean deviation was 0.05 for radius of curvature, 0.24 D for the power and 5.0 degrees for the cylinder. Conclusions: This surgical V KS, with some hardware and software improvements, may be used to reduce residual astigmatisms in conventional cataract and keratoplasty. It could also be used to gather preoperative data in corneal topography assisted LASIK.

Processamento digital de imagens refletidas pelo filme lacrimal da superfície anterior da córnea / Digital processing of images reflected from the lachrymal film of the anterior corneal surface

Neste trabalho foi implementado um algoritmo para processamento de imagens refletidas pela fina lâmina lacrimal da superfície anterior da córnea humana. Este processamento é uma importante etapa em instrumentos oftálmicos para análise da da topografia da córnea. Um anteparo em forma de cone, com discos concêntricos pintados em sua superfícies, é posicionado diante da córnea do paciente. A córnea neste caso comporta-se como um espelho convexo esférico. Os raios de luz provindos do anteparo geram uma imagem virtual localizada atrás da córnea. Atrás do cone alinhado com o eixo deste existe um sistema óptico de aumento e uma câmera CCD. A imagem virtual é ampliada e focalizada na matriz deste CCD, cujo sinal é enviado a uma placa de aquisição de imagens ("frame-grabber") instalada em um IBM-PC compatível. As imagens digitalizadas contêm as informações essenciais para que as curvaturas sobre a superfície possam ser calculadas. Nosso algoritmo se mostrou bastante satisfatório na tarefa de extração de bordas destas imagens, e permitiu o mapeamento das curvaturas destas córneas, até mesmo para olhos bastante claros ou córneas irregulares. O desvio médio para superfícies teste foi de 50mm (0,02 pixels), 95mm (0,04 pixels)para olhos claros, e 72mm (0,03 pixels) para olhos escuros. Os resultados deste algoritmo foram satisfatórios para todas as superfícies teste e para córneas in vivo sob condições adversas de iluminação

Refrator ocular automático / Ocular autorefrator

Um sistema de medidas automáticas e objetivas de erros refrativos oculares (miopia, hipermetropia e astigmatismo), utilizando tecnologia laser, foi desenvolvido. O sistema consiste em projetar, através de um laser de diodo (lambida=850nm), um alvo luminoso (um anel) no fundo do olho do paciente e os feixes de luz que emergem do olho e atingem um detector CCD matricial são analisados quanto às suas vergências. Os feixes emergentes do olho testado são divididos em seis partes (três meridianos) e são analisados dois a dois pelo CCD. A distância entre as duas imagens formadas em cada meridiano fornece o poder de refração naquele meridiano. Com os poderes de refração dos três meridianos (0 graus, 120 graus e 240 graus) é possível determinar o erro refrativo ocular. Comparativamente com os sistemas por nós anteriormente desenvolvidos, este sistema utiliza o mesmo princípio básico de medidas, porém a forma do alvo de medida e o detector utilizados, resultaram num sistema menos complexo para o alinhamento ótico e com menor introdução de erros. Medidas em olhos artificiais e em olhos humanos foram realizadas apresentando excelente concordância com os resultados obtidos em consultórios oftalmológicos e estão dentro da precisão requerida para estes tipos de medidas (0,125di e 5 graus).

An automatic and objective system for measuring ocular refractive errors (myopia, hyperopia and astigmatism) was developed using laser technology. The system consists of projecting a light target (a ring), using a diode laser (À=850nm), at the fundus of the patient's eye. The light beams scattered from the retina are analyzed by a CCD detector (matrix) regarding their vergence. The beams which emerge from the tested eye are divided into six portions (three meridians) and are analyzed in pairs by the CCD. The distances between the two images in each meridian provide the refractive power in that particular meridian. As the refractive power is obtained for the fr meridians coº' 120° e 240°), it is possible to determine the ocular refractive error. This system uses the sarne basic principle for detecting the refractive errors as our previous ones 123, but the new measuring target and the detector have provided a less complex system for the optical alignment avoiding most of the measurement errors introduced in the other system. Measurements in artificial eyes and in human eyes were done and they are in good agreement with the retinoscopic measurements and also they are as precise as these kinds of measurement require (0,125di and 5°).

Detecçäo de bordas de imagens refletidas pela superfície anterior da córnea / Edge detection of the images reflected by anterior surface of the cornea

Desenvolveu-se neste trabalho um método para detecção de bordas de imagens refletidas pela superfície anterior da córnea, quando iluminada por anéis concêntricos baseando-se em técnicas de processamento de imagens.

A method for edge detection from images reflected off the anterior surface of the cornea was developed, based an image processing technics.

Sistema de análise topográfica da córnea humana / System of topographical analysis of human corneal

Um sistema computadorizado de análise do contorno da córnea foi desenvolvido. Anéis iluminados são projetados na córnea e a imagem refletida é processada, gerando como resultado mapas coloridos com códigos de cor.

A computerized system which analyses the contour of the cornea was developed. Illuminated rings are projected on the subject and the reflected image is processed. As a result, color-coded maps are generated

Análise biométrica ocular por interferometria com laser de baixa coerência / Pcular biometric analysis by interferometry with low coherent laser

Os autores apresentaram resultados preliminares de uma nova técnica para paquimetria e biometria oculares utilizando inferferometria com laser de baixa coerência. Testes iniciais mostram um método que reúne três características importantes; alta resoluçäo, fácil operacionalidade e custo razoável. Discute-se ainda a potencialidade do mesmo para obtençäo de informaçäo concernente á estrutura da retina.