Eike Thamm

Single scattering by red blood cells

A highly diluted suspension of red blood cells (hematocrit 0.01) was illuminated with an Ar or a dye laser in the wavelength range of 458-660 nm. The extinction and the angle-resolved intensity of scattered light were measured and compared with the predictions of Mie theory, the Rayleigh-Gans approximation, and the anomalous diffraction approximation. Furthermore, empirical phase functions were fitted to the measurements. The measurements were in satisfactory agreement with the predictions of Mie theory. However, better agreement was found with the anomalous diffraction model. In the Rayleigh-Gans approximation, only small-angle scattering is described appropriately. The scattering phase function of erythrocytes may be represented by the Gegenbauer kernel phase function.

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

The investigation of changes in the metabolism as a functional diagnosis is a promising way for the detection of early reversible pathological alterations before they get manifest. Such a functional parameter of the microcirculation is the oxygen saturation of blood. The oxygen saturation in an artery describes in the connection with the blood flow the supply of oxygen in a certain region of the fundus. The consumption of oxygen is proportional to the product of the arterio-venous difference in the oxygen saturation and the blood flow.

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

Fluorescence measurements are considered as a tool in the discovering of the pathomechanism of ocular diseases especially in age-related macular degeneration. The presentation of fluorescence images is a kind of 2-dimensional functional diagnosis. For an interpretation of fluorescence images, it is necessary to know, which substances are detected at different fundus sites. For that reason, it was the goal of the investigation is to find out a method, which allows both the discrimination of different fluorophores and the 2-dimensional distribution of autofluorescent substances at the fundus. The main problem in fluorescence measurements at the fundus is the very weak detectable signal, which cannot be increased because of the limitation of the excitation by the maximal permissible exposure. As the eye is a moving object, an averaging of a number of fluorescence images to improve the signal to noise ratio is possible up to a certain degree, but it requires additional expenditures. A further limitation for fluorescence measurements is given by the spectral transmission of the ocular medida, which allows spectral investigations only in a wavelength-range between about 400 nm and 900 nm.

Optical properties of ocular fundus tissues determined by optical coherence tomography

Abstract Goal: To determine the extinction coefficient and the anisotropy of scattering as well as the refractive indices in the retina and in the choroid non-invasively in vivo. Method: The power of coherent reflected light versus fundus depth is recorded by optical coherence tomography (OCT). The ratio of the refractive indices is derived from the height of the reflection peaks. The extinction coefficient and anisotropy of scattering are calculated from the offset and the slope of the signal behind the reflection peaks. Results: Values similar to those known from in vitro measurements were found for the extinction coefficient and scattering anisotropy of the retina and the choroid. Conclusions: The OCT, usually employed for the measurement of intraocular distances and tomographic imaging, is capable to determine further interesting parameters of single ocular layers.

Objektive Bestimmung der optischen Dichte von Xanthophyll nach Supplementation von Lutein

ZusammenfassungHintergrund. Es wird vermutet, dass eine hohe optische Dichte von Xanthophyll protektiv der Entwicklung der altersbezogenen Makuladegeneration entgegenwirkt. Es war zu untersuchen, ob eine Erhöhung der optischen Dichte von Xanthophyll in der Makula nach Supplementation von Lutein mit objektiven Methoden nachweisbar ist. Die meistens angewandten psychophysikalischen Methoden zur Bestimmung des Makulapigments setzen die Mitarbeit des Probanden und dessen foveale Fixationsfähigkeit voraus. Methode. Es werden die Imaging-Spektrometrie und die Auswertung von Laserscannerbildern, aufgenommen nur bei 488 nm, zur objektiven Bestimmung der optischen Dichte von Xanthophyll vorgestellt. Beide Methoden erfordern im Gegensatz zu psychophysikalischen Methoden keine foveale Fixationsfähigkeit des Probanden. Ergebnisse. Durch Auswerten von Laserscannerbildern, die wie Leeraufnahmen vor einer Fluoreszenzangiographie, aber ohne eingeschwenkten Sperrfilter aufgenommen werden, kann die zweidimensionale Verteilung des Xanthophylls bestimmt werden. An 10 Probanden, die während 40 Tagen täglich 6 mg Lutein zu sich nahmen, konnte eine Erhöhung der optischen Dichte des Makulapigments Xanthophyll zumindest partiell nachgewiesen werden. Diese erreichte nach ca. 30 Tagen ein Plateau. Schlussfolgerung. Die vermutete protektive Wirkung von Xanthophyll gegenüber Prädisposition zur Erkrankung an altersbezogener Makuladegeneration kann durch Supplementation von Lutein – objektiv kontrolliert – positiv beeinflusst werden.AbstractBackground. It is thought that a high optical density of xanthophyll has a protective effect against the development of agerelated 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.

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. Providedthat 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 scatteringcoefficients 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 intra-ocular 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.

Die altersabhägige Makulopathie Vergleichende Untersuchungen zwischen Patienten, deren Kindern und Augengesunden

ZusammenfassungHintergrund: Es wurde untersucht, ob zwischen Patienten mit früher und später altersabhängiger Makulopathie (ARM) und deren Kindern (F1-Generation) sowie Normalpersonen objektiv meßbare Unterschiede in spektrometrischen Parametern des Fundus bestehen. Methode: Mit dem Jenaer Imaging-Spektrometer wurden retinale Sauerstoffsättigung, Xanthophyll und natürliche Fluorophore und mit dem Rodenstock SLO-101 die örtliche Verteilung des Xanthophylls gemessen. Ergebnisse: Xanthophyll ist bei der späten ARM gegenüber der F1-Generation und der Kontrollgruppe signifikant reduziert (α<0.01). Die bei kurz- und langwelliger Anregung gemessenen unterschiedlichen Autofluoreszenzspektren sind ein Hinweis darauf, daß mehr als ein Fluorophor detektierbar ist und daß sich bei ARM-Patienten gegenüber deren F1-Generation die Anteile der Fluorophore ändern. Die langwellige Autofluoreszenz ist nur bei dem Spätstadium der ARM altersabhängig (R2=0.81). Zudem ist die Sauerstoffsättigung in retinalen Gefäßen bei ARM verändert. Schlußfolgerung: Xanthophyll ist nur bei der späten ARM reduziert. Autofluoreszenz und Sauerstoffsättigung unterscheiden sich zwischen Patienten mit ARM und Kindern von ARM-Patienten und Augengesunden, jedoch wurde kein Hinweis auf eine genetische Prädisposition bei Kindern von ARM-Patienten auf der Grundlage dieser Parameter gefunden.SummaryBackground: The aim of this study was to evaluate prospectively whether there are differences in spectrometrically measurable parameters of the fundus between patients with early and late age-related maculopathy (ARM), the children of the ARM patients (F1 generation) and normals. Method: Using the “Jenaer Imaging Spectrometer”, retinal oxygen saturation, xanthophyll, and intrinsic fluorescence were measured; the spatial distribution of xanthophyll was determined using the Rodenstock SLO 101 model. Results: Xanthophyll is reduced in late ARM as compared to the F1 generation and the control group (a<0.01). The different fluorescence spectra, measured at shortwave and at longwave excitation, suggest the presence of more than one fluorophore. Furthermore, the components of the fluorophores seem to be different between patients with ARM and their F1 generation. The longwave autofluorescence is age-dependent only in late ARM (r2=0.81). For the first time, we found an alteration in oxygen saturation in retinal vessels in patients with ARM. Conclusions: Xanthophyll is reduced only in late ARM. Autofluorescene and oxygen saturation are different between ARM patients, the F1 generation and normals, however, we were not able to identify a genetically based predisposition concerning the parameters studied.

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 theretinal 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 possiblebenefits to the ophthalmologic diagnostics is demonstrated.

Monte-Carlo simulation of retinal vessel profiles for the interpretation of in-vivo oxymetric measurements by imaging fundus reflectometry

Background: 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. This technique enables the simultaneous measurement of reflectance spectra of neighboring locations at the fundus, e.g. across retinal vessels. However, the reflected light emerging from a vessel is determined by different mechanisms of reflection and backscattering. The following most significant light fractions, contributing to the measuring signal, are considered: Light which is backscattered from deeper fundus layers and transmitted once or twice through the vessel, light which is backscattered from the blood column inside the vessel, and light which is specular reflected at the vessel wall. Goals are the investigation of the contribution of the single fractions to the vessel profile and its approximation by an analytical function which can be used to compensate specular reflection at in vivo measured profiles. Method: To evaluate the contribution of the different pathways we set up a Monte Carlo model of radiative transport inside the ocular fundus as a layered structure containing a vessel with circular cross section. The developed software is able to distinguish photons contributing to the simulated vessel profile which penetrated the blood column once, twice, or never. Experimentally determined absorption and scattering parameters of the fundus tissues were used in the simulation. Results: Considering retinal vessels with diameters of 25 micrometers to 200 micrometers we found the reflection from a thin vessel to be determined by the single and double transmission of light at 559 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. Discussion: The spectra measured even at the center of retinal branch vessels are composed from light which is backscattered from the blood column inside the vessel and from transmitted light fractions which were reflected behind the vessel. Since the latter is influenced also by the absorption of melanin and haemoglobin in the choroid, care must be taken in the calculation of the oxygen saturation of blood in retinal vessels from this spectra. The reconstruction of an in vivo measured vessel profile which is distorted by specular reflexes is possible by the use of a polynomial of sixth degree.

Investigations on the measurement of oxygen saturation in retinal vessels using polarized light

SummaryThe accuracy of the spectrometric measurement of the oxygen saturation in retinal vessels is limited by its signal-to-noise ratio. The aim of this study was to investigate the possibility of enhancement of the reflection signal by the use of polarized light. Materials and method: The Jena ophthalmospectrometer was equipped with two polarizing filters: one in the illumination and the other in front of the detector. Reflection spectra of erythrocytes streaming through a cuvette in the focus of an artificial eye were recorded. The influence of the polarization on the reflection spectra was investigated by rotating the polarizer in front of the detector. Furthermore, the degree of polarization of the light reflected from retinal vessels in vivo was determined. Results: The degree of polarization of the light reflected from the erythrocytes was 0.6–0.8, whereas the polarization of light reflected by a standard white reflectance target was virtually zero. Conclusion: Polarized light can be used for the reduction of error in retinal vessel oximetry.ZusammenfassungHintergrund: Die Genauigkeit der spektrometrischen Messung der Sauerstoffsättigung in retinalen Gefäßen ist begrenzt durch das Signal-Rausch-Verhältnis. Eine Erhöhung der Reflexion durch die Verwendung polarisierten Lichtes könnte die Genauigkeit verbessern. Material und Methode: In die Beleuchtungs- und in die Beobachtungsoptik des Jenaer Ophthalmospektrometers wurde je ein Polarisationsfilter eingefügt. So wurde eine mit einer Erythrozytensuspension durchströmte Küvette im Fokus eines Modellauges mit linear polarisiertem Licht beleuchtet. Die Reflexionsspektren der Erythrozyten wurden zweimal aufgenommen. Dabei waren die Polarisationsrichtungen der beiden Filter bei der ersten Messung parallel zueinander ausgerichtet, bei der zweiten Messung senkrecht. Ferner wurde der Polarisationsgrad des von retinalen Gefäßen in vivo reflektierten Lichtes bestimmt. Ergebnisse: Das von Erythrozyten reflektierte Licht weist einen Polarisationsgrad von 0,6–0,8 auf. Die Polarisation des von einem als Referenz verwendeten Weißstandard reflektierten Lichtes ist dagegen auch bei linear polarisierter Beleuchtung isotrop. Schlußfolgerung: Die Verwendung polarisierten Lichtes kann zu einer sichereren Messung der Sauerstoffsättigung in retinalen Gefäßen führen.