“…Our earlier studies to map the validity of using an FE 3-D diffusion model as an engineering tool for the predictive assessment of imageability through a systematic comparison of depth-resolved fluence calculated by diffusion equation and the gold standard Monte Carlo model had shown that although, in theory, the diffusion model applies when s Јӷ a , in practice, its performance can vary sharply for the same s Ј/ a ratio. 37,38 We summarized the rules of thumb for applicability of the FE diffusion equation for excitation fluence as:…”