Three-dimensional simulation of near-infrared diffusion in tissue: boundary condition and geometry analysis for finite-element image reconstruction

Imaging of tissue with near-infrared spectral tomography is emerging as a p racticable method to map hemoglobin concentrations within tissue. However, the accurate recovery of images by using modeling methods requires a goad m atch between experiments and the model prediction of light transport in tis sue. We illustrate the potential for a match between (i) three-dimensional (3-D) frequency-domain diffusion theory, (ii) two-dimensional diffusion the ory, (iii) Monte Carlo simulations, and (iv) experimental data from tissue- simulating phantoms. Robin-type boundary conditions are imposed in the 3-D model, which can be implemented with a scalar coupling coefficient relating the flux through the surface to the diffuse fluence rate at the same locat ion. A comparison of 3-D mesh geometries for breast imaging indicates that relative measurements are sufficiently similar when calculated on either cy lindrical or female breast shapes, suggesting that accurate reconstruction may be achieved with the simpler cylindrical mesh. Simulation studies direc tly assess the effects from objects extending out of the image plane, with results suggesting that spherically shaped objects reconstruct at lower con trast when their diameters are less than 15-20 mm. The algorithm presented here illustrates that a 3-D forward diffusion model can be used with circul ar tomographic measurements to reconstruct two-dimensional images of the in terior absorption coefficient. (C) 2001 Optical Society of America.