Karl-Heinz Donnerhacke

Imaging spectroscopy of the human ocular fundus in vivo

Spectroscopic measurement of light that is reflected from biological tissue in vivo is being investigated for various clinical applications. One special object of investigation using optical methods is the human ocular fundus. A fundus reflectometer that enables the simultaneous acquisition of up to 192 spectra arranged in a horizontal line across the fundus is described. The underlying optical principle of the device is the confocal imaging of an illuminated narrow, slitlike field at the fundus to the entrance slit of a spectrograph. This is imaged by the grating of the spectrograph onto a two-dimensional CCD chip that records the local distribution of ocular fundus reflectance spectra within a wavelength range of 400 up to 710 nm with a resolution better than 2 nm and a local resolution of 23 mm in a field dimension of 1.5 mm. The performance of the device was investigated, the effects of confocal and nonconfocal imaging are discussed, and some representative measurements are presented.

Application of diffractive optics for axial eye-length measurement using partial coherence interferometry

In the past, new techniques to measure axial eye length using partial coherent light or wavelength-tuned light have ben developed. They are based on the interference of the light reflected at the cornea and the retina of the eye. This method has then advantage of measuring independently of axial eye movements. However, the light is reflected from the retina and the cornea with different divergencies. These divergencies have to be matched to collect as much light as possible in order to achieve a sufficient interference signal. We have employed a diffractive optical element which focuses one part of the light onto the cornea while the other part of the light remains uninfluenced and is focused onto the retina by the optical system of the eye. As shown by measurements on model and on living eyes the signal-to- noise-ratio and the sensitivity of the dual beam partial coherence interferometry was improved.

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.