Solid geometrv based modelling of non-uniform attenuation and compton scattering in objects for SPECT imaging systems

A solid geometry based approach is used to model non-uniform attenuation and Compton scattering in irregularly shaped objects for Monte Carlo simulated single photon emission computed tomography (SPECT) imaging systems. The shape of an object is described by a combination of object primitives: ellipsoids, elliptical cylinders, tapered elliptical cylinders, rectangular solids, and their halves, quarters, and eighths, based on set union, intersection, and difference. The primitives are then assigned density and energy-dependent photoelectric and Compton cross sections. This approach is validated in two steps. Firstly, only non-uniform attenuation is included. The transaxial images of a simulated thorax phantom are reconstructed using a modified Chang's technique for nonuniform attenuation. Secondly, detected Compton scattering is added to the model. Scatter fraction of a point source in a water cylinder is used to validate our scatter model for uniform scattering media. The difference between simulated and experimental results is on the order of 10%. For the model of nonuniform attenuation and detected Compton scattering, a thorax phantom is used for validation. Normalized profiles across the center of the sphere projections for simulation and experiment are observed to have insignificant difference. These results validate our object modelling approach for nonuniform attenuation and Compton scattering. The performance of the solid model is demonstrated to be faster and require substantially less parameter storage than a voxel based model.