Hybrid Theory For Photon Transport in Tissue - Diffusion Theory

Diffusion Theory

The Diffusion Theory is an approximation of the radiative transfer equation (RTE), and an analytical way to simulate photon transport. As such, it has the ability to model photon propagation through tissue quickly.

As an example, one way to attain a solution for a pencil beam that is vertically incident on a semi-infinite homogeneous scattering medium is by taking three approximation steps as follows:

• The anisotropically scattering medium is converted to an isotropically scattering medium. That is, the scattering coefficient is scaled by, where is the anisotropy. The anisotropy is then set to zero;
• The unit-power pencil beam is converted into an equivalent isotropic point source at a depth that is equal to the transport mean free path, with a power equal to the transport albedo;
• The boundary effect of the scattering medium is removed by adding an image source to satisfy the boundary condition.

Advantages and Disadvantages

Diffusion Theory is more computationally efficient than MCML. However, it is also less accurate than MCML near the source and boundaries.