Path integral description of light transport in tissue

The early photons that arrive at a collector through a large thickness of tissue have potential for imaging internal organ structure, function, and status with improved image resolution relative to late arriving photons which have been diffusely scattered. Imaging algorithms require a theory to calculate early photon arrival for comparison with experimental data. The path integral description of light transport describes the movement of photons as particles undergoing collisions in a scattering medium based on the Brownian motion formalism of Feynman and Hibbs (unconstrained path) which applies the principle of least action, Including the additional constraint that photons have a constant velocity of c yields paths that conserve the speed of light (constrained path). This paper outlines the basic derivation of the path integral method and compares the constrained and unconstrained paths.