A finite element approach for modeling Diffusion equation Subha Srinivasan 10/30/09

Forward Problem Definition Given a distribution of light sources on the boundary of an object and a distribution of tissue parameters within, to find the resulting measurement set on

Light Propagation in a 3-D Breast Model using BEM

Inverse Problem Definition Given a distribution of light sources and a distribution of measurements on the boundary, to derive the distribution of tissue parameters within

Diffusion equation in frequency domain is the isotropic fluence, is the Diffusion coefficient, is the absorption coefficient and is the isotropic source is the reduced scattering coefficient

Solutions to Diffusion equation Analytical solutions exist in simple geometries Finite difference methods (FDM) use approximations for differentiation and integration. Works well for 2D problems with regular boundaries parallel to coordinate axis, cumbersome for regions with curved or irregular boundaries Finite element methods (FEM) can be easily applied to complicated and inhomogeneous domains and boundaries. Versatile and computationally feasible (compared to Monte Carlo methods)

Using FEM for Modeling Main concept: divide a volume/area into elements and build behavior in entire area by characterizing each element (Mosaic) Uses integral formulation to generate a set of equations Uses continuous piecewise smooth functions for approximating unknown quantities

Basis Functions x1=0x1=0 x 2 =L/2 x 3 =L φ1φ1 φ3φ3 For a set of basis functions, we can choose anything. For simplicity here, shown are piecewise linear “hat functions”. Our solution will be a linear combination of these functions. φ2φ2

Derivation of FEM formulation for Diffusion Equation The approximate solution is: And for flux: Galerkin formulation gives the weighted residual to equal zero: Galerkin weak form: Green’s identity: Substituting:

Matrix form of FEM Model Discretizing parameters: Overall: Matrix form: For A,B detailed, refer to Paulsen et al, Med Phys, 1995 Need to apply BCs

Boundary Conditions Type III BC, Robin type α incorporates reflection at the boundary due to refractive index change

Source Modeling Point source: contribution of source to element in which it falls Gaussian source: modeled with known FWHM Distributed source model: Hybrid monte-carlo model: Monte carlo model close to source & diffusion model away from source Paulsen et al, Med Phys, 1995

Forward Model:

Forward Model for Homogeneous Domain: Multiple Sources

Forward Model with Inclusion

Boundary Measurements

Hybrid Source Model Ashley Laughney summer project, 2007

Plots near the source Ashley Laughney summer project, 2007

References Arridge et al, Med Phys, 20(2), 1993 Schweiger et al, Med Phys, 22(11), 1995 Paulsen et al, Med Phys, 22(6), 1995 Wang et al, JOSA(A), 10(8), 1993