T-matrix computations of light scattering by red blood cells

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Abstract
The electromagnetic far field, as well as the near field, originating from light interaction with a red blood cell (RBC) volume-equivalent spheroid, was analyzed by utilizing the T-matrix theory. This method is a powerful tool that makes it possible to study the influence of cell shape on the angular distribution of scattered light. General observations were that the three-dimensional shape, as well as the optical thickness apparent to the incident field, affects the forward scattering. The backscattering was influenced by the shape of the surface facing the incident beam. Furthermore sphering as well as elongation of an oblate RBC into a volume-equivalent sphere or a prolate spheroid, respectively, was theoretically modeled to imitate physiological phenomena caused, e.g., by heat or the increased shear stress of flowing blood. Both sphering and elongation were shown to decrease the intensity of the forward-directed scattering, thus yielding lower g factors. The sphering made the scattering pattern independent of azimuthal scattering angle ϕs, whereas the elongation induced more apparent ϕs-dependent patterns. The light scattering by a RBC volume-equivalent spheroid was thus found to be highly influenced by the shape of the scattering object. A near-field radius r nf was evaluated as the distance to which the maximum intensity of the total near field had decreased to 2.5 times that of the incident field. It was estimated to 2–24.5 times the maximum radius of the scattering spheroid, corresponding to 12–69 μm. Because the near-field radius was shown to be larger than a simple estimation of the distance between the RBC’s in whole blood, the assumption of independent scattering, frequently employed in optical measurements on whole blood, seems inappropriate. This also indicates that one cannot extrapolate the results obtained from diluted blood to whole blood by multiplying with a simple concentration factor.