Improving the efficiency of emission tomography simulations using variance reduction techniques

The development for public distribution of a general-purpose code, SIMSET, for the modeling of positron-emission and single-photon-emission tomographs (PET and SPECT) is discussed. An important part of the SIMSET project is the development of methods for efficient photon simulation which can handle heterogeneous distributions of activity and attenuation. For both positron and single-photon tomographs, the solid angle of acceptance of the detector array is small, due to collimation and to the size of the detector array itself. This results in significant computational inefficiencies with conventional Monte Carlo simulation, because only a few percent of the photons generated and tracked will actually be detected. A similar kind of problem arises in radiation shielding calculations, where the flux through the shield, although significant, may correspond to only a tiny fraction of the initial emitted photons. To improve the efficiency problem of simulations, some techniques from the radiation shielding literature have been adapted to increase the yield of detected photons