Solid geometry-based object model for Monte Carlo simulated emission and transmission tomographic imaging systems

An object model based on combinations of object primitives is proposed for Monte Carlo simulated emission and transmission tomographic imaging systems. The primitives include ellipsoids, elliptic cylinders, tapered elliptic cylinders, rectangular solids, and their subsets: half, quarter, and eighth. The probability of a photon surviving interactions with the phantom medium is used as a weight for variance reduction. Calculation of the probability can be computationally intensive without properly organizing the inclusion of subregions within larger regions. A tree data structure is introduced to organize this inclusion relationship and used as the basis for two computationally efficient schemes for determining the intersection locations of a photon path with primitives and for identifying the attenuation coefficients for adjacent intersections for the survival probability computation. The approach has been validated by emission as well as transmission simulations. A thorax phantom containing overlapped ellipsoids and a heart composed of twelve overlapped quarter ellipsoids are employed to demonstrate the capability of the model.