T-matrix computations of light scattering by red blood cells

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 theT-matrix theory. This method is a powerful tool thatmakes it possible to study the influence of cell shape on the angulardistribution of scattered light. General observations were that thethree-dimensional shape, as well as the optical thickness apparent tothe incident field, affects the forward scattering. Thebackscattering was influenced by the shape of the surface facing theincident beam. Furthermore sphering as well as elongation of anoblate RBC into a volume-equivalent sphere or a prolate spheroid, respectively, was theoretically modeled to imitate physiologicalphenomena caused, e.g., by heat or the increased shear stress offlowing blood. Both sphering and elongation were shown to decreasethe intensity of the forward-directed scattering, thus yielding lowerg factors. The sphering made the scattering patternindependent of azimuthal scattering angle phi(s), whereas the elongation induced more apparent phi(s)-dependent patterns. The lightscattering by a RBC volume-equivalent spheroid was thus found to behighly influenced by the shape of the scattering object. Anear-field radius r(nf) was evaluated as thedistance to which the maximum intensity of the total near field haddecreased to 2.5 times that of the incident field. It was estimatedto 2-24.5 times the maximum radius of the scattering spheroid, corresponding to 12-69 mum. Because the near-field radiuswas shown to be larger than a simple estimation of the distance betweenthe RBC's in whole blood, the assumption of independent scattering, frequently employed in optical measurements on whole blood, seemsinappropriate. This also indicates that one cannot extrapolate theresults obtained from diluted blood to whole blood by multiplying witha simple concentration factor.