T-matrix computations of light scattering by red blood cells

The electromagnetic far field, as well as near field, originating from light
interaction with a red blood cell (RBC) volume equivalent spheroid, were
analyzed utilizing T-matrix theory. This method is a powerful tool which enables
the influence of cell shape on the angular distribution of scattered light
to be studied. General observations were that the three-dimensional shape,
as well as optical thickness apparent to the incident field, affect the forward
scattering. The back scattering was influenced by the shape of the surface
facing the incident beam. Furthermore, sphering as well as elongation of an
oblate shaped RBC into a volume equivalent sphere or prolate shaped spheroid,
respectively, were theoretically modeled in order to imitate physiological
phenomena caused, e.g., by sphering agents, heat or 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 the azimuthal scattering
angle φs, while the elongation induced more apparent φs-dependent patterns.
The light scattering by an RBC volume equivalent spheroid, was thus found
to be highly influenced by the shape of the scattering object. A near-field
radius, rnf, 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. When the absorption properties of a red
blood cell were incorporated in the computations, the near-field radius was
only slightly reduced by 0.2-0.6 times the maximum radius. As the near-field
radius was shown to be larger than a simple estimation of the distance between
the RBCs in whole blood, the assumption of independent scattering,
frequently employed in optical measurements on whole blood, seems inappropriate.
This also indicates that results obtained from diluted blood, cannot
be extrapolated to whole blood, by multiplying with a simple concentration
factor.