Light scattering by complex particles

Elastic light scattering is the most fundamental process underlying light-tissue interaction. It is determined by the spatial distribution of intra and extracellular density in tissue. Biological tissue is comprised of structures ranging from a few nanometers to tens of microns and all of these length scales may potential affect elastic scattering. Due to this extreme complexity of biological tissue, light scattering is poorly understood. Frequently, due to the lack of proper analytical methodologies, tissue structures are treated as independent, uniform spheres when considered as light-scatterers. This over-simplification leads to significant obstacles to accurately model light scattering and, in turn, characterize tissue properties based on the light scattering data.

We believe that a more appropriate description relies upon the realization that tissue is not comprised of independent light-scattering particles but rather spatially continues refractive index variations. The objective of this project is to understand the mechanisms of light scattering in tissue. We are attacking this problem from a number of directions:

  1. We develop analytical models of light scattering by complex nonspherical and inhomogeneous structures. Our results demonstrate that light scattering by even highly complex particles can be accurately described by simple analytical expressions.
  2. We develop analytical models of light propagation in tissue that is treated as a medium with spatially continuous refractive index fluctuations.
  3. We use electron microscopy, optical microscopy and other imaging techniques to characterize cellular and tissue organization and the corresponding refractive index distribution at length scales from nanometers to tens of microns.
  4. We develop novel numerical approaches to accurately model light-tissue interaction. In particular, we develop finite-difference time-domain (FDTD) and PSTD simulations to model light-tissue interaction from subcellular to tissue level.

The ultimate goal, of course, is to enable investigators in the field of tissue optics reliably use light scattering information to characterize cell and tissue structure.

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Backman's Biophotonics Laboratory at Northwestern University

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