Realistic Monte Carlo simulation of Ga-67SPECT imaging

We describe a comprehensive Monte Carlo program tailored for efficient simu lation of realistic Ga-67 SPECT imaging through the entire range of photon emission energies. Our approach incorporates several new features developed by us and by others. It is now being used to optimize and evaluate the per formance of various methods of compensating for photon scatter, attenuation , and nonstationary distance- and energy-dependent detector resolution. Imp rovements include 1) the use of a numerical torso phantom with accurate org an source and attenuation maps obtained by segmenting CT images of a Radiol ogy Support Devices anthropormorphic heart/thorax phantom, modified to incl ude eight axillary lymph nodes; 2) accelerated photon propagation through t he attenuator using a variant of the maximum rectangular region algorithm o f Suganuma and Ogawa; and 3) improved variance reduction using modified spa tial sampling for simulation of large-angle collimator penetration, scatter , and lead X-rays. Very-high-count projections were simulated in 55 energy windows spaced irregularly in the range 60-370 keV, these essentially noise -free images were used as a basis for generating Poisson noise realizations characteristic of 72-h postinjection Ga-67 studies. Comparisons of spatial and energy distributions demonstrated good agreement between data experime ntally measured from the RSD phantom and those simulated from the mathemati cal segmentation of the same phantom.