Scattered light measured from tissue can be uniquely correlated to tissue substructure, function and progression of disease. The ultrastructural information provided by scatter may render optical techniques valuable to diagnosis.
Many recent studies have demonstrated that scattered light measured from tissue can be uniquely correlated to tissue substructure, function and progression of disease, if the wavelength dependence of the light is obtained at each pixel. This is because the morphologic changes associated with cancer progression cause organelle and structural matrix alteration, which can be observed macroscopically as local fluctuations in the optical refractive index (RI). These changes include hyper-proliferation of epithelium, nuclear crowding and enlargement, as well as intracellular organelle changes and sub-cellular stromal matrix alteration. Therefore, it is quite reasonable to assume that the ultrastructural information provided by scatter may render optical techniques valuable to diagnosis; the limiting factor being our lack of knowledge about how light scatters through heterogeneous tissues.
Extracting information from scatter spectra requires an ability to reasonably model the behavior of light as it passes through a tissue. This is a rather convoluted problem because it is difficult to separate light that has weakly scattered from that which has multiply scattered, in addition the effects of absorption and scatter are intermingled. To circumvent this complication, optical constraints are applied to limit detected photons to those primarily experiencing a single elastic collision. A raster scanning reflectance spectroscopy imaging system is used to characterize fresh, excised tumors and normal specimens with 100 micron spatial resolution (approximately one mean free scattering path length in tissue). This system was designed to sample the scatter directly, allowing empirical separation of the absorption and scattering effects. Scatter measures are then elucidated with pathology so that diagnostic categories of breast tissue may be optically characterized for a classification algorithm.
To enhance the diagnostic utility of our system, we are also using electron microscopy to visualize individual, sub-wavelength scatterers to better understand how the distribution of small scatterers in the extra-cellular matrix influences optical signals. The focus of this analysis is on collagen fibers because scattering from epithelial cells is well approximated by Mie theory and little is known about collagen, a dominant, non-spherical scatterer. Understanding light-tissue interactions at the microscopic level will improve models of light propagation through breast tissue and consequently data parameterization.