Perturbation theory for diffuse light transport in complex biological tissues

A perturbation theory for the forward problem of optical transport in turbid media is developed. It is applicable to media with scattering and absorption inhomogeneities and steady-state and modulated light. Absorbing perturbations can be described by a volume distribution of virtual sources that primarily causes a monopole perturbation light field. Scattering objects have an additional contribution that, in the limiting case of sharply bounded objects, is represented by a surface distribution of virtual sources and causes a dipolelike perturbation pattern. Using the concept of virtual sources, we discuss a possible ambiguity between the perturbations from scattering and absorbing inhomogeneities and the implications for the source-detector placement in inverse problems. We show that the surface effects due to sharp boundaries of scattering objects pose both a numerical problem and a chance to improve the resolution of inverse algorithms.