Realistic Monte Carlo simulation of Ga-67 imaging for optimization and evaluation of correction methods

Describes a comprehensive Monte Carlo program tailored for efficient simulation of realistic Ga-67 SPECT data through the entire range of photon emission energies. The program incorporates several new features developed by us and by others, and is now being used to optimize and evaluate the performance of various methods of compensating for photon scatter, attenuation, and nonstationary distance- and energy-dependent detector resolution. Improvements include (a) the use of a numerical torso phantom with accurate organ source and attenuation maps obtained by segmenting CT images of an RSD^TM anthropomorphic heart/thorax phantom, modified to include 8 axillary lymph nodes, (b) accelerated photon propagation through the attenuator using a variant of the maximum rectangular region (MRR) algorithm of Suganuma and Ogawa, and (c) improved variance reduction using modified spatial 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 are used as a basis for generating Poisson noise realizations characteristic of 72-hour post-injection Ga-67 studies. Comparisons of spatial and energy distributions demonstrate good agreement between data experimentally measured from the RSD phantom and those simulated from the mathematical segmentation of the same phantom