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T Bek

Publications and source records attributed to T Bek.

At least 55 records · Page 3Linked to original sources
PubMed1996

Evaluating surgically induced astigmatism by Fourier analysis of corneal topography data.

PURPOSE: To evaluate surgically induced astigmatism using Fourier harmonic series analysis of corneal topography data. SETTING: Aarhus Kommunehospital, Aarhus University, Denmark. METHODS: We evaluated the results of 46 phacoemulsifications with a 4 or 6 mm scleral tunnel sutureless incision based on the axis of the steepest meridian. We performed conventional keratometry and corneal topography before and up to 1 month after surgery. Using Fourier analysis, the corneal topographic images were broken into spherical power, regular astigmatism, and nonregular astigmatism for individual or aggregate analysis of surgically induced astigmatism. The induced refractive change (average of the difference between preoperative and postoperative corneal topographies) was analyzed and normalized according to the surgical meridian and to right/left eye. RESULTS: Regular astigmatism calculated by Fourier analysis of mires from the keratometer zone correlated well with conventional keratometry readings. Surgery induced a localized flattening in the superior region and a with-the-rule regular astigmatism component in the central area. CONCLUSION: Surgically induced corneal topography changes can be analyzed by Fourier series harmonic analysis, allowing aggregate data to be broken into optically meaningful quantities.

Adult↗
PubMed1996

Vascular occlusion in diabetic retinopathy. A qualitative and quantitative histopathological study.

The retinal vessels from seven diabetic patients and from six age-matched normal controls were studied qualitatively and quantitatively using various histological staining techniques. In diabetic patients the walls of retinal arterioles and capillaries showed significantly more staining than normals for periodic acid Schiff (neutral glycoproteins), Sirius red (connective tissue), and for Alcian blue at pH 2.6, pH 5.8 and at pH 5.8 combined with MgCl22 in concentrations less than 0.9 M (acid mucopolysaccharides). In the retina from diabetic patients there was no difference between the number of capillaries staining with these dyes in areas of vascular occlusion, and in adjacent control areas. Furthermore, in areas of vascular occlusion, the material accumulated centrally to occlude the lumen of ghost vessels did not stain with any of the dyes used. A homogenous material, accumulated in the outer retina in areas of vascular occlusion in the retina from diabetic patients, only stained with Alcian blue at pH 5.8 combined with MgCl2 in concentrations less than 0.4 M, suggesting a different molecular composition from the Alcian blue material accumulated in the retinal vascular walls. The findings are in accordance with the knowledge that basement membranes of retinal vessels are thickened in diabetes mellitus. However, the findings also indicate that basement membrane thickening cannot fully account for vascular occlusion in diabetic retinopathy.

Aged↗
PubMed1996

Glycoprotein deposition in vascular walls of diabetic retinopathy. A histopathological and immunohistochemical study.

The association between periodic acid Schiff staining and immunoreactivity to laminin, fibronectin, vitronectin, and type VI collagen was studied qualitatively and quantitatively in the retinal vascular bed from 7 eyes of 5 diabetic patients and from 5 eyes of 5 normal persons. In the retina from diabetic patients the number of arterioles showing immunoreactivity to vitronectin, the number of venules showing immunoreactivity to type VI collagen, and the number of both arterioles and venules showing immunoreactivity to laminin and fibronectin, was higher than in normals. There was no difference between the number of capillaries showing periodic acid Schiff staining and immunoreactivity to laminin, fibronectin, and vitronectin when comparing areas of vascular occlusion with adjacent control areas. However, the number of capillaries displaying immunoreactivity to type VI collagen was higher in control areas than in areas of vascular occlusion in diabetic patients and in normal controls. Staining with periodic acid Schiff correlated topographically with immunoreactivity to laminin and fibronectin, but not with immunoreactivity to vitronectin and type VI collagen. In areas of vascular occlusion there was seen no immunoreactivity or histological staining corresponding to the material accumulated to occlude the ghost vessels.

Aged↗
PubMed1996

Precise correlation of histopathological and fluorescein angiographic morphology using retinal vascular casting.

The histopathology of three eyes obtained post mortem from 2 patients with age-related macular degeneration was correlated with the pre mortem fluorescein angiographic morphology. A precise point-by-point correlation between histopathology and the corresponding angiographic appearance was ensured by using the cast retinal vascular system as a pattern of reference. The study showed that both the photoreceptors, the pigment epithelium, and substances accumulated between the retinal and the choroidal vascular systems, may have a blocking effect on choroidal background fluorescence as seen on fluorescein angiograms. Furthermore, it is confirmed that fluorescein angiographic hyperfluorescence may be due to a lack of blocking of the choroidal fluorescence because of a window defect in the retinal photoreceptor layer and/or the pigment epithelium.

Aged↗
PubMed1996

Quantitative assessment of fixational eye movements by scanning laser ophthalmoscopy.

A new method for quantifying fixational eye movements by scanning laser ophthalmoscopy was developed and the method was evaluated in ten normal persons. Video sequences of the ocular fundus obtained during fixation were recorded, and linear movements of the fovea between successive video frames were transformed to angular movements of the eye by an algorithm that takes into account the individual optical properties of the eye. A computer program was developed to automatically calculate the amplitude, the direction, and the duration of the angular movements together with the precision of each of these estimates. Two types of eye movements could be recognized in normal persons; a fast type (saccades) which was initiated and terminated within one video frame (20 msec), and a slow type (drifts) which lasted more than 8 video frames (160 msec). The mean amplitude of the fast movements (0.41 degrees) was significantly higher than the mean amplitude of the slow movements (0.31 degrees). The methods was found to be suitable for quantifying fixational eye movements in clinical trials while simultaneously visualizing the ocular fundus. However, by refining the determination of the foveal position on the SLO images the precision of the method can be further improved.

Adult↗
PubMed1995

Quantitative assessment of lateral interaction as determined by computerized quantitative layer-by-layer perimetry.

BACKGROUND: Quantitative layer-by-layer perimetry is a psychophysical technique which assesses lateral interaction in human vision. In prior designs of this technique the test procedure was time-consuming and quantitative assessment of the results was not possible. In order to shorten test duration and provide quantitative experimental data, a new computerized version of the technique has recently been developed. METHODS: A mathematical model for describing lateral interaction as assessed by computerized quantitative layer-by-layer perimetry was developed, and experimental data from the testing of 18 normal persons were fitted to the model. Two descriptive parameters of, respectively, lateral stimulation and lateral inhibition were for each test point related to the differential light sensitivity and to the eccentricity in the visual field. RESULTS: The two parameters describing lateral stimulation could not be reliably estimated. However, the two parameters describing lateral inhibition showed, respectively, a positive correlation with the differential light sensitivity in the visual field and a significant decline with increasing eccentricity in the visual field. CONCLUSION: The technique and the mathematical model employed are suitable for quantitative assessment of lateral inhibition in human vision.

Adult↗
PubMed1995

Clinical pathology and retinal vascular structure in the Bardet-Biedl syndrome.

A comparative study of clinical pathology and retinal vascular structure is described as studied by vascular casting in an eye of a patient with the Bardet-Biedl syndrome. At the time of examination the eye had been almost blind for at least 4 years. The histopathological examination showed a largely uniform loss of the outer retinal layers. The gross pathological examination of the cast ocular fundus showed three distinct zones, an inner zone inside the temporal vascular arcades where retinal vessels had been cast, a mid peripheral zone with bone spicules, and a peripheral zone with neither cast vessels nor bone spicules. The findings are discussed in relation to possible pathophysiological mechanisms involved in the development of retinal dystrophy in the Bardet-Biedl syndrome.

Female↗
PubMed1995

Fourier analysis of video-keratographic data. A tool for separation of spherical, regular astigmatic and irregular astigmatic corneal power components.

Topographic analysis of the cornea and computation of changes in corneal topography is difficult because of the lack of a standardized concept for analysing topographic information. We have used Fourier series harmonic analysis to decompose the often very complex information given in topographic images (TMS-1 video-keratograph). The polar data values arising from each mire were separated into spherical (average power), regular astigmatic (2' harmonic) and non-regular astigmatic components. The non-regular astigmatic components were further divided into a decentration component (1' harmonic) and a higher order irregular refractive component (root mean square (r.m.s.) of the residual between original data and the sum of spherical, 1' and 2' order components). Algorithms for utilizing the Fourier analysis technique on subjective refractive and on keratometric changes in regular astigmatism were also derived. Averages of the separate refractive components were calculated for groups of normal (right and left eye of 25 patients), keratoconic (13 eyes), and transplanted eyes (20 eyes), and average topographic maps were reconstructed and imaged. Changes in the separate topographic refractive components were compared with subjective refractive changes in eyes that had undergone combined myopic-astigmatic excimer laser ablation (8 eyes). Fourier series analysis was found superior to conventional keratometry in predicting spherical and regular astigmatic changes in subjective refraction. Fourier series analysis seems to be a powerful tool to decompose the information of complex topographic corneal images into rational optical components.

Algorithms↗
PubMed1994

Transretinal histopathological changes in capillary-free areas of diabetic retinopathy.

The histopathology of capillary-free areas in diabetic retinopathy was studied in 27 lesions from 12 human donor eyes. Localised capillary-free areas were identified on casts of the retinal vascular system, and on subsequent histological sections of these areas the casted vessels acted as points of reference to accurately combine the histological appearance of the retina with the vascular morphology as seen on the cast. All studies lesions showed inner retinal changes with loss of ganglion cells, attenutation of the inner nuclear layer and hyalinization of the retinal capillary network. In 21 of the studied lesions a homogeneous eosinophil substance was seen accumulated between the photoreceptor outer segments and the retinal pigment epithelium corresponding to the areas with inner retinal changes. The findings may help explain the blocking of choroidal fluorescence in capillary-free areas as seen on fluorescein angiograms in diabetic retinopathy. The study suggests future guidelines for further illumination of the pathophysiology of proliferative diabetic retinopathy.

Aged↗
PubMed1993

Optimal test parameters for computerized quantitative layer-by-layer perimetry.

Quantitative layer-by-layer perimetry is a psychophysical technique believed to assess the functional characteristics of individual retinal layers. In the initial design of the technique, the test procedure was too time consuming for the testing of large field point patterns, and statistical treatment of the measured threshold values was difficult. Therefore, to minimize these limitations, a new design of quantitative layer-by-layer perimetry was developed by incorporating the technique into the existing hardware environment of the computerized perimeter Humphrey Field Analyzer. The present paper describes the theoretical considerations and experimental findings underlying a selection of optimal stimulus parameters to provide a basis for statistical treatment of measured threshold values, and to minimize test duration, while obtaining an acceptable estimate of the threshold level with minimum variance.

Adult↗
PubMed1993

Three-dimensional structure of human retinal vessels studied by vascular casting.

A new casting technique for studying the three-dimensional structure of the human retinal vascular system on post mortem tissue was developed. The technique was designed to provide an overview presentation of the retinal vasculature such that localized areas with vascular pathology could be identified for further analysis of three-dimensional structure. The paper presents qualitative characteristics of three-dimensional structure in the normal human retinal vascular bed, to provide a basis for future studies on vascular impairment in retinal disease.

Capillaries↗
PubMed1992

Quantitative layer-by-layer perimetry adapted to the Humphrey Field Analyzer. I. Design of methodology and instrumentation.

Quantitative layer-by-layer perimetry is a psychophysical technique believed to assess response characteristics of individual retinal layers. This paper presents a new design of this technique, implemented into the existing hardware environment of the computerized perimeter Humphrey Field Analyzer. The technique was optimized for a more simple, fast and reproducible testing of larger visual field point patterns. The paper deals with the design of instrumentation and with methodological aspects of this adaptation of the technique.

Algorithms↗
PubMed1992

Quantitative layer-by-layer perimetry adapted to the Humphrey Field Analyzer. II. Centre-surround interaction produced by sustained surround stimulation.

In this paper the practical applicability of a new design of quantitative layer-by-layer perimetry is confirmed by studying centre-surround interaction produced by sustained surround stimulation in preliminary trials on four normal individuals. The results are discussed in relation to established laws for retinal light perception, and some practical aspects of the employed test design and the chosen test variables are discussed.

Adult↗
PubMed1992

Quantitative layer-by-layer perimetry adapted to the Humphrey Field Analyzer. III. Centre-surround interaction produced by transient surround stimulation. Area characteristics of the transient stimulus pattern.

In this paper some qualitative aspects of centre-surround interaction produced by transient surround stimulation are investigated, employing a new design of quantitative layer-by-layer perimetry. Using a rotating windmill pattern, a transient response, believed to be organized in the inner plexiform layer of the retina, is assessed in four normal individuals. The paper considers area characteristics of the stimulus pattern used to produce the transient response, and optimal dimensions for this pattern are proposed.

Adult↗
PubMed1992

Localized retinal morphology and differential light sensitivity in diabetic retinopathy. Methodology and clinical results.

The thesis presents a technique designed to allow a comparison of retinal function as assessed by computerized perimetry with retinal morphology as seen on photographs of the ocular fundus, including results from the practical application of this technique in the study of diabetic retinopathy. The basis of the technique is an optical algorithm that allows angular distances in the visual field to be transformed to match linear distances on fundus photographs and fluorescein angiograms. The visual field data is superimposed onto the corresponding retinal morphology on the photograph on the basis of two points of reference. The fixation point in the visual field is superimposed onto the foveola on the photograph of the ocular fundus, and the blind spot in the visual field is superimposed onto the optic disc on the photograph. In the practical application of this technique for the study of diabetic retinopathy, visual field scotomata were found corresponding to areas displaying signs of retinal vascular impairment in the form of vascular occlusion, while no relation was found between visual field scotomata and breakdown of the blood-retina barrier as studied on fluorescein angiograms. Furthermore, visual field scotomata were found to correspond to areas peripheral from retinal neovascularizations, a finding supporting the hypothesis that the neovascularizations develop because of stimulation from vasogenic factors released from ischaemic and hypoxic retinal tissue. Visual field scotomata were also found in relation to retinal cotton wool spots. These scotomata were localized, and not accurately extended, which could be expected if the retinal nerve fiber layer had been damaged. Finally, some visual field scotomata could not be related to any visible funduscopic or angiographical morphology. It is concluded that pathological changes in the inner retinal vascular supply may lead to impairment of visual function in diabetic retinopathy, but that also other mechanisms not manifested in a morphologically visible way, are involved. A further investigation of the pathophysiology leading to visual impairment in diabetes mellitus should focus on these unknown factors. A possible approach could be the development of new techniques for studying pathophysiological mechanisms in specific retinal layers, and especially the layers supplied by the external vascular supply to the retina from the choroidal circulation.

Diabetic Retinopathy↗
PubMed1991

Localised scotomata and types of vascular occlusion in diabetic retinopathy.

In 13 patients with diabetic retinopathy who had localised scotomata in the central visual field that could not be ascribed to haemorrhages, cotton wool spots or hard exudates, visual field data obtained by computerised perimetry were correlated with the corresponding retinal morphology as seen on fundus photographs and fluorescein angiograms. It was attempted to relate the retinal areas corresponding to the scotomata to areas of distribution from the central retinal artery. In six cases localised scotomata could be correlated with focal areas of angiographical non-perfusion in the macular area. When these areas of non-perfusion were traversed by a patient vessel, the retinal zone adjacent to this vessel was found to have retained light sensitivity. Two localised scotomata could be correlated with retinal areas peripheral from occluded arterioles, but no scotoma was found which could be related th retinal venous occlusion. In five cases scotomata occurred, however, which could not be related to areas of distribution from the central retinal artery. It is concluded that factors other than retinal vascular occlusion probably are involved in causing impairment of neurosensory function in diabetic maculopathy.

Adult↗