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

A solid-geometry-based approach is used to model nonuniform 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 assigned density and energy-dependent photoelectric and Compton cross sections. This approach is validated in two steps. First, only nonuniform attenuation is included. The transaxial images of a simulated thorax phantom are reconstructed using a modification of Chang´s technique for nonuniform attenuation. Second, detected Compton scattering is added to the model. Scatter fraction of a point source in a water cylinder is used to validate the scatter model for uniform scattering media. The difference between simulated and experimental results is on the order of 7%. 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.<>