A system and method for determining or estimating the presence and effect of light-attenuating inhomogeneities in tissue when performing medical treatment or diagnostics using optical members in tissue. The system and method comprising inserting a plurality of optical members into the tissue, and then measuring by emitting light through a sub-set of optical members at a time, while measuring the light collected by another sub-set of optical members. Combining said measurement data with a model for light propagation in tissue, and setting up a system of equations. Solving the unknown attenuation values from said system of equations.
