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.