The photons propagation model used in this work was developed by Haskel and colleagues^{17} and applied to preclinical small animal optical imaging by Rice and collaborators.^{18} The photon transport in a turbid medium with an absorption coefficient $\mu a$ can be described by the diffusion equation Display Formula
$D\u22072\varphi (r,t)\u2212\mu a\varphi (r,t)=1c\u2202\varphi (r,t)\u2202t\u2212S(r,t),$(1)
where $c$ is the speed of light, and $\varphi (r,t)$ and $S(r,t)$ are, respectively, the photon fluence and the source term at position $r$ and time $t$. The diffusion coefficient $D$ is equal to Display Formula$D=13(\mu s\u2032+\mu a),$(2)
where $\mu s\u2032$ is the reduced scattering coefficient.