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For several years, the formation and evolution of thrombi in small arteries of rats has been quantitatively studied at the Laboratory of Physiology and Physiopathology at the V.U.B. Global size parameters can be determined by projecting the image of a small arterial segment onto photosensitive cells. The transmitted light intensity is a measure for the thrombotic phenomenon. This unique method permitted extensive in vivo study of the platelet-vessel wall interaction and local thrombosis. Now, a further development has emerged with the aim to improve the resolution of these measurements in order to get information on texture and form of the thrombotic mass at any stage of its evolution. Therefore a thorough understanding of how light propagates through non hemolized blood was essential. For this purpose, the Medical Informatics department developed a system to record and process digital images of the thrombotic phenomenon. For the processing and attempt to reconstruct the thrombi, a model describing the light transmission in a dispersive medium such as flowing blood had to be worked out. Application of results from Twersky's multiple scattering theory, combined with appropriate border conditions and parameter values was attempted. In the particular situation we studied, the dispersive properties of the flowing blood were found to be highly anisotropic. An explanation for this phenomenon could be given by considering the alignment of red blood cells in the blood flow. In order to explain the measured intensity profiles, we had to postulate alignment in the plane perpendicular to the flow as well. The theoretical predictions are in good agreement with the experimental values if we assume almost perfect alignment of the erythrocytes such that their short axes are pointing in the direction of the center of the artery. Conclusive evidence of the interaction between local flow properties and light transmission could be found by observing arteries with perturbated flow.
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