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Biomedical subjects

D Schweitzer

Publications and source records attributed to D Schweitzer.

At least 19 recordsLinked to original sources
PubMed2005

[Fundus spectrometry in age-related maculopathy].

BACKGROUND: Spectroscopic methods permit the non-invasive detection of fundus pigments by the wavelength-dependent absorption of fluorescence as well as by the fluorescence lifetime. From the relative concentrations of haemoglobin and oxyhaemoglobin, the oxygen saturation can be calculated. The onset of age-related maculopathy might be delayed by a high optical density of xanthophyll. The detection of alterations in fundus autofluorescence points to age-related pathomechanisms (accumulation of lipofuscin, formation of connective tissue). The detection of autofluorescence of redox-pairs of coenzymes results in information about metabolic states at the cellular level, and might make possible an early detection of age-related changes when they are still reversible. METHOD: The evaluation of reflectance spectra, detected by imaging ophthalmo-spectrometry, results in the calculation of oxygen saturation or in the optical density of xanthophyll or of melanin. Fluorescence spectra can be measured also by this technique. For the 2-dimensional determination of the distribution of xanthophyll, a very simple method was developed, requiring fundus illumination by one wavelength only. In the detection of time-resolved autofluorescence, the fluorescence lifetime is used for the determination of endogenous fluorophores. RESULTS: As result of comparing studies between ARM patients and healthy subjects, the consumption of retinal oxygen was increased already in the children of ARM patients. An increasing optical density of xanthophyll was determined after lutein supplementation. Differences in fluorescence lifetime were determined between ARM patients and healthy subjects, but their interpretation requires investigations of cell or of organ model cultures. CONCLUSIONS: The described methods permit in vivo basic investigations of ARM and can be considered as impulses for the development of diagnostic devices.

Adult↗
PubMed2005

[Limits of the confocal laser-scanning technique in measurements of time-resolved autofluorescence of the ocular fundus].

GOAL: The 2-dimensional measurement of the time-dependent autofluorescence has in combination of confocal laser scanner technique and time correlated single photon counting the potential for the investigation of the metabolism state at the eye ground. It was to be examined, to what extent these measurements are influenced through the auto-fluorescence of the crystalline lens. MATERIAL AND METHOD: The time-dependent auto-fluorescence was measured on eyes of 21 patients with age-related macular degeneration (AMD) and of 26 healthy subjects in 40 degrees fundus images. The experimental set-up used, was developed at the eye clinic of the University of Jena. In this scanning laser ophthalmoscope, the fundus was excited at 446 nm by pulses of 100 ps full width at half maximum and 40 MHz repetition rate. The auto-fluorescence was detected with a time resolution of 25 ps for wavelengths > 500 nm. The dynamic auto-fluorescence of each pixel was approximated by a tri-exponential model function. In order to determine the influence of the fluorescence of the crystalline lens, healthy eyes and eyes were compared with implanted artificial intra-ocular lenses. For comparison the frequency distributions were used, which the decay times t1, t2 and t3 in the fundus images had been calculated with. RESULTS: Clear differences were shown in the frequency distributions of the fluorescence lifetimes between AMD-patients and healthy subjects, It resulted, however, from a crucial analysis that the fluorescence of the crystalline lens is not suppressed by the confocal laser scanning principle sufficiently. In particular the long lifetime t3 is overlapped by the fluorescence of the crystalline lens of about 4.6 ns. To the complete suppression of the influence of the lens fluorescence in measurements of the time-resolved fundus auto-fluorescence it is suggested to combine the confocal laser scanning technique with the principle of the aperture diaphragm division. CONCLUSION: In order to be able to determine the capability of measurements of the time-dependent fundus auto-fluorescence as diagnostic tool exactly, the influence of the crystalline lens is to reduce through the combination of confocal laser scanning ophthalmoscopy with the principle of the aperture diaphragm division diversely.

Fluorescein Angiography↗
PubMed2004

[Alterations in autofluorescence decay time in the fundus after oxygen provocation].

BACKGROUND: Alterations in the equilibrium of redox pairs of co-enzymes give information about the metabolic state in the citric acid cycle as well as in the respiratory chain. Fluorescence properties are different between the reduced and oxidised states of co-enzymes so that a change of the oxygen partial pressure can be sensitively detected in the tissue. Therefore, it was investigated whether the autofluorescence of co-enzymes is detectable in the living fundus. METHOD: The provocation of metabolism was realised by inspiration of 100% oxygen. The time-resolved autofluorescence was detected by a single photon counting technique. The decay behaviour of autofluorescence was approximated by a bi-exponential model. For evaluation of the results, histograms of decay times tau(1) and tau(2) were calculated in defined ranges around the macula and optic disc before, during, and after inhaling oxygen. RESULTS: The calculated decay times corresponded in the macula to the decay times of FAD and NADH(+) given in the literature. Connective tissue in the optic disc also showed fluorescence. CONCLUSIONS: Changes in the histograms of decay rates demonstrate that provocation of metabolism is detectable by measurement of time-resolved autofluorescence. This method reveals the evaluation of metabolism at the cellular level as a new diagnostic possibility.

Adaptation, Physiological↗
PubMed2004

[Spectral separation in ocular fundus autofluorescence images in patients suffering from age-related macular degeneration].

INTRODUCTION: Alterations in the intrinsic ocular fundus fluorescence are under discussion particularly in connection with age-related macular degeneration. However, further knowledge of the chemical substrate of fluorescence is necessary. METHOD: Ocular fundus autofluorescence was observed using a fundus camera. The fluorescence emission was recorded using a 3-chip CCD camera. For comparison, the fluorescence spectra of lipofuscin, A2-E, FAD, NADH(2) and AGE's were recorded by the Jena ophthalmo-spectrometer. RESULTS: In contrast to the homogeneous intrinsic fluorescence of a normal fundus, the fluorescence images of patients suffering from AMD showed remarkable local differences. The detected fluorescence spectra showed remarkable overlaps. DISCUSSION: We introduced a new technique enabling the detection of spectral differences in images of ocular fundus autofluorescence. Besides the fluorescence of lipofuscin, we found a green one in patients suffering from nonexudative AMD.

Adult↗
PubMed2003

[Determination of the concentration distribution of macular pigment from reflection and fluorescence images].

OBJECTIVE: The macular pigment xanthophyll protects the macula in two ways: firstly, it absorbs hazardous blue light and secondly, it acts as a radical scavenger. A low concentration of xanthophyll may be regarded as a risk factor for age-related macular degeneration (AMD). Therefore, we investigated a simple method to determine the xanthophyll concentration at the fundus which is suitable for patient screening. METHOD: The local distribution of xanthophyll density was determined from monochromatic blue reflection images and autofluorescence images of the fundus in 18 healthy volunteers (mean age: 23.9 years). The significance of the parameters maximal, global, and mean concentration were compared. RESULTS: The maximal optical density of xanthophyll determined from reflection images was found to be 0.29+/-0.08 (mean for all test persons) which is in good agreement with literature data. The total xanthophyll concentration which is proportional to the maximal density, appeared to be appropriate to describe a person's overall xanthophyll status. Because of the low intensity of autofluorescence images, these are less useful for the determination of the xanthophyll concentration. CONCLUSIONS: Because of it's simplicity, the determination of xanthophyll concentration as described here can be performed by every ophthalmologist using a fundus camera and is, therefore, suitable as a screening method.

Adolescent↗
PubMed2002

[Intrastomal refractive surgery with ultra-short laser pulses. Results from initial in vitro experiments].

PURPOSE: To investigate the possibilities and techniques for intrastromal ablation of the cornea by the use of ultra-short laser pulses. METHOD: A laser-induced plasma with a diameter of 8 mu was generated 100-300 mu below Bowmans membrane by focussing of a Ti:Saphir laser (lambda=805 nm, pulse width: 115 fs, energy: 0.6-3.3 microJ). Scanning the laser beam enabled arbitrary transsections in the corneal stroma. All experiments were performed in porcine eyes 1-4 h post-mortem. RESULTS: Two-dimensional sections were achieved applying a minimal laser energy of 0.6 microJ at a separation of the single laser effects of 10 micrometer. A 150 micrometer thick disc was excised 150 micrometer below Bowmans membrane and extracted through a keratotomy performed by the laser. Sharp-cut edges were observed by slit-lamp biomicroscopy. CONCLUSIONS: Ultra-short laser pulses are suitable for precise intrastromal ablation.

Animals↗
PubMed2002

[Time-correlated measurement of autofluorescence. A method to detect metabolic changes in the fundus].

The detection of metabolic changes opens the possibility for intervention of reversible pathological alterations. Measurements of oxygen saturation are limited to the blood vessel system. Detection of alterations in oxygen concentrations are up to 3 orders of magnitude more sensitive by autofluorescence of coenzymes than by measurement of oxygen saturation. Because of limited transmission of the ocular media no specific excitation of endogenous fluorophores can be realised. For this reason it was investigated if the fluorescence lifetime after pulse excitation can be detected at the human fundus. Applying a laser scanner ophthalmoscope and mode-locked Ar(+) laser as well as time-correlated single photon counting, lifetime images of the living fundus were obtained. In mono-exponential approximation, a mean lifetime of 5 ns was detected from the optic disc and large vessels whereas about 1.5 ns were detected in the parapapillary area. By evaluating the frequency of lifetimes, lipofuscin, free FAD, and collagen are probably detectable. Comparative measurements were performed in fundus specimens and on free FAD.

Collagen↗
PubMed2002

[Objective determination of optical density of xanthophyll after supplementation of lutein].

BACKGROUND: It is thought that a high optical density of xanthophyll has a protective effect against the development of aggregated macular degeneration. The aim of this study was to investigate whether an increase of the optical density of xanthophyll in the macula after a supplementation of lutein can be proven by objective methods. Most methods applied for the determination of the macular pigment require the co-operation of the proband and the ability for foveal fixation. METHOD: Imaging spectrometry and the evaluation of laser scanner images taken at 488 nm will be presented. In contrast to psychophysical methods, both methods are independent of the patients ability for foveal fixation. RESULTS: Even by evaluation of laser scanner images taken as in fluorescence angiography but without inserting the blocking filter, the 2-dimensional distribution of xanthophyll can be determined. In 10 probands taking 6 mg lutein daily over 40 days, an increase of the optical density could be determined at least in some probands. The optical density reached a plateau 30 days after starting the supplementation of lutein. CONCLUSION: The assumed protective effect of xanthophyll against age-related macular degeneration can be influenced by supplementation of lutein under objective control.

Adult↗
PubMed2001

[The optics of retinal blood vessels and their significance for scanning laser Doppler flowmetry].

Recently, the scanning laser Doppler flowmetry (SLDF) has been developed as a standard method for the quantification of retinal blood flow. However, the measured quantities flow, volume, and velocity depend on the angle between the direction of the blood flow and of the light. The underlying theory assumes isotropic illumination of the vessel. This assumption, however, is not valid for light back-scattered from retinal vessels. This paper investigates the contribution of different pathways of the light to the SLDF signal by a Monte Carlo simulation. It is shown that the light measured on a vessel having a thickness of 50 microns is predominantly back-scattered from the blood inside the vessel, whereas the measurement on vessels 20 microns in diameter or less also includes light transmitted through the vessel.

Blood Flow Velocity↗
PubMed2001

Scattering properties of the retina and the choroids determined from OCT-A-scans.

GOAL: To determine the coefficient and the anisotropy of scattering as well as the refractive indices in the retina and in the choroid noninvasively in vivo. METHODS: The power of coherent reflected light versus fundus depth is recorded in OCT-A-scans. The ratio of refractive indices is derived from the height of the reflection peaks. Provided that the absorption coefficient is known from fundus reflectometry, the scattering coefficient and anisotropy are calculated from the offset and the slope of the signal behind the reflection peaks on the basis of a single backscattering model. RESULTS: We found scattering coefficients of 12/mm (retina) and 27.5/mm (choroid) as well as anisotropy values of 0.97 (retina) and 0.90 (choroid). DISCUSSION: The OCT is usually employed for the measurement of intraocular distances. The new technique described here gives the unique opportunity to determine further interesting parameters of single ocular layers. The values given above are in good agreement with in vitro results.

Anisotropy↗
PubMed2001

A new method for the measurement of oxygen saturation at the human ocular fundus.

Measurements of the transmission and the reflectance spectra of whole blood exhibit a different dependence of the thickness of the blood layer. Whereas the transmission spectra decrease at all wavelengths, in the reflectance two ranges with different behaviour have to distinguish. In the absorption dominated range between about 500 and 600 nm, there is only a marginal influence of the thickness of the blood layer. In contrast, in the scattering dominated range higher than about 600 nm the reflectance increases if the thickness of the blood layer increases. Proofed by Monte Carlo simulation, the reflected light, which will be measured in ophthalmologic devices, consists both of light which penetrates a vessel once and will be reflected at the background and of light which is internally reflected at the blood column. Only in small vessels (diameter < 50 microm) the transmitted part dominates. With increasing diameter of the vessels the internally reflected light dominates. Especially under the conditions of a confocal laser scanner, the transmitted part of light is always small. As the measured light consists of both parts of light, a new model for the approximation of the measured reflected light is given, containing an expression for the transmitted and for the internally reflected light. The model was applied for the approximation of the reflectance of blood in a cuvette with an coloured background, delivering sufficient exact result. The oxygen saturation was determined in 256 measurements of 30 healthy persons using an ophthalmo imaging spectrometer. The mean arterial oxygen saturation was 91 +/- 5%, the mean venous oxygen saturation was 59 +/- 9% and the arterio-venous difference in the oxygen saturation was 35%, which is in good agreement with the arterio-venous difference known in the brain. First measurements of the oxygen saturation in age-related macular degeneration point to partly different pathological changes of the retinal microcirculation in dry and in wet AMD.

Diagnostic Techniques, Ophthalmological↗
PubMed2001

Basic investigations for 2-dimensional time-resolved fluorescence measurements at the fundus.

The decay time is characteristic for several natural fluorophores. The determination of the decay time is independent of the fluorescence intensity. As a consequence, a short living weak fluorescence should be detectable also if it is covered by a intensive long-living fluorescence. As the decay time is influenced by the embedding matrix, information about the cellular stage might be possible. The laser scanning technique in combination with the time correlated single photon counting technique seems to be the optimal method for the discrimination of different fluorophores at the fundus according to the decay time. Fields of an equal decay time are presented as a tau mapping. An experimental set up was developed. Until now, only basic experiments were done on structured fluorescent tests, but under the conditions of the living eye. The results are promising. For the separate detection of the most important short life-time of 120 ps of A2E (excitation at 413 nm. emission 450-600 nm) as a putative precursor of lipofuscin in age-related macular degeneration [11], a light source for pulses in the range of about 50 ps is required.

Eye Proteins↗
PubMed2001

Non-invasive measurement of the concentration of melanin, xanthophyll, and hemoglobin in single fundus layers in vivo by fundus reflectometry.

A new model of the reflection of the human ocular fundus on the basis of the adding-doubling method, an approximate solution of the radiative transport equation, is described. This model enables the calculation of the concentration of xanthophyll in the retina, of melanin in the retinal pigment epithelium and in the choroid, and of hemoglobin in the choroid from fundus reflectance spectra. The concentration values found in 12 healthy subjects are in excellent agreement with literature data. In single cases of pathologic fundus alterations possible benefits to the ophthalmologic diagnostics is demonstrated.

Choroid↗
PubMed2001

Does epidural analgesia prolong labor and increase risk of cesarean delivery? A natural experiment.

OBJECTIVE: More than 50% of pregnant women in the United States are using epidural analgesia for labor pain. However, whether epidural analgesia prolongs labor and increases the risk of cesarean delivery remains controversial. STUDY DESIGN: We examined this question in a community-based, tertiary military medical center where the rate of continuous epidural analgesia in labor increased from 1% to 84% in a 1-year period while other conditions remained unchanged-a natural experiment. We systematically selected 507 and 581 singleton, nulliparous, term pregnancies with spontaneous onset of labor and vertex presentation from the respective times before and after the times that epidural analgesia was available on request during labor. We compared duration of labor, rate of cesarean delivery, instrumental delivery, and oxytocin use between these two groups. RESULTS: Despite a rapid and dramatic increase in epidural analgesia during labor (from 1% to 84% in 1 year), rates of cesarean delivery overall and for dystocia remained the same (for overall cesarean delivery: adjusted relative risk, 0.8; 95% confidence interval, 0.6-1.2; for dystocia: adjusted relative risk, 1.0; 95% confidence interval, 0.7-1.6). Overall instrumental delivery did not increase (adjusted relative risk, 1.0; 95% confidence interval, 0.8-1.4), nor did the duration of the first stage and the active phase of labor (multivariate analysis; P >.1). However, the second stage of labor was significantly longer by about 25 minutes (P <.001). CONCLUSION: Epidural analgesia during labor does not increase the risk of cesarean delivery, nor does it necessarily increase oxytocin use or instrumental delivery caused by dystocia. The duration of the active phase of labor appears unchanged, but the second stage of labor is likely prolonged. (Am J Obstet Gynecol 2001;185:128-34).

Adult↗
PubMed2001

A scattering phase function for blood with physiological haematocrit.

Though the optics of red blood cells as well as whole blood has been studied extensively, an effective scattering phase function for whole blood is still needed. The interference of waves scattered by neighbouring cells cannot be neglected in highly concentrated suspensions such as whole blood. As a result, the phase function valid for single erythrocytes may fail to describe a single scattering process in whole blood with physiological haematocrit (Hct approximately 0.4). In this study we compared the results obtained in goniophotometric measurements of blood samples with the results of angle-resolved Monte Carlo simulations. The results show that a Henyey-Greenstein phase function with an anisotropy factor of 0.972 is an adequate approximation for the effective scattering phase function of whole blood with high haematocrit at a wavelength of 514 nm.

Blood Physiological Phenomena↗
PubMed2001

Light paths in retinal vessel oxymetry.

The oxygen utilization and, therefore, the metabolic state, of a distinctive area of the retina may be calculated from the diameter of the supplying artery and vein, the haemoglobin oxygenation, and the velocity of the blood. The first two parameters can be determined by imaging spectrometry at the patients ocular fundus. However, the reflected light emerging from a vessel followed different pathways through the ocular fundus layers and the vessel embedded in the retina. The contribution of the single pathways to the vessel reflection profile is investigated by a Monte Carlo simulation. Considering retinal vessels with diameters of 25-200 microm we found the reflection from a thin vessel to be determined by the single and double transmission of light at 560 nm. The backscattering from the blood column determines the reflectance in the case of a thick vessel. However, both components are in the same order of magnitude. This has to be considered in the calculation of the oxygen saturation of blood in retinal vessels from their reflection spectra.

Light↗
PubMed2000

kappa-(BEDT-TTF)2Cu[N(CN)(2)]Br: a fully gapped strong-coupling superconductor.

High-resolution specific-heat measurements of the organic superconductor kappa-(BEDT-TTF)(2)-Cu[N(CN)(2)]Br in the superconducting ( B = 0) and normal ( B = 14 T) states show a clearly resolvable anomaly at T(c) = 11.5 K and an electronic contribution, C(es), which can be reasonably well described by strong-coupling BCS theory. Most importantly, C(es) vanishes exponentially in the superconducting state which gives evidence for a fully gapped order parameter.

Journal Article↗