Suppose with the structural information of micro-CT, we know that a sphere is located in a cubic domain, the centre of the sphere is at [30, 30, 30] mm, and the radial of the sphere is 10 mm. The source is placed at position [30, 30, 0] mm with the incident direction along the -axis. The optical coefficients of the cubic domain are as follows: absorption coefficient, ; scattering coefficient, ; refractive index, ; and anisotropy, . The optical coefficients of the spherical domain are as follows: absorption coefficient, ; scattering coefficient, ; refractive index, ; and anisotropy, . The cubic domain is first discretised into voxels of . Then, voxels at the domain , which is the region of interest, are refined into voxels. A quantity of photons is simulated. The contour plots of the continuous wave (CW) fluence (with 10-db spacing) in the plane are shown in Fig. 4. Besides the three methods used in Sec. 3.1, analytical solution from the diffusion model is also added.36,37 As the boundary of a cube for MC is finite, the analytical solution for a sphere in a semi-infinite medium is only plotted near the source and inside the sphere.