Quasi-ballistic or "snake" photons carry useable information on the internal structure of scattering mediums such as tissues. By defining quasi-ballistic photons to be those photons that have been scattered but have not exceeded a specified radial distance threshold from their initial trajectory (equivalent to the resolving limit of the quasi-ballistic photons) and by using the Henyey-Greenstein phase function, Monte Carlo modeling has shown that the number of quasi-ballistic photons increases with depth in an isotropic scattering medium until a maximum is reached and then the quantity decreases. The quantity of quasi-ballistic photons at a specified depth can be shown to be governed by two competing processes: the decay of ballistic photons into quasi-ballistic photons and the decay of quasi-ballistic photons into scattered photons. These well-defined behaviors allow one to write a rate equation governing the growth and decay in the quantity of quasi-ballistic photons with depth. It is found that as the anisotropy factor increases with forward scattering and as the resolution limit is widened, the quantity of quasi-ballistic photons begins to exceed the quantity of ballistic photons at a specified depth and the rate of decay of quasi-ballistic photon quantity decreases. The development of a rate equation for the formation of quasi-ballistic photons allows one to analyze how efficient various detection methods are in extracting these quasi-ballistic photons, and it can be seen that there is a compromise between desired resolution and the effective scattering ratio at a detector.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.