Analysis of SPECT including Scatter and Attenuation Using Sophisticated Monte Carlo Modeling Methods

The effects of scatter and attenuation on single photon emission computed tomography (SPECT) images can be analyzed with the aid of sophisticated Monte Carlo simulation. Correction procedures can be evaluated by comparing corrected images with images absent of scatter and attenuation. The simulation enables control of components which govern the emission and transport of radiation through the source and attenuating medium. The basic calculation involves sampling the probability density functions (pdf) which govern the photon transport process. First, the origin of a photon is selected by sampling. Variance reduction is applied so that a detection is "forced" and weighted by the probability of an initial direction within the acceptance angle of the collimator multiplied by the probability that the photon is not attenuated. Second, the photon history is continued by sampling for a direction. The photon is forced to interact within the attenuating medium and an appropriate weight is calculated. Variance reduction is again applied with a weight determined by the product of the probability of interaction within the attenuating medium, the probability of scatter, the probability of scattering into the acceptance angle of the collimator, and the probability that the photon reaches the detector. Finally, a new direction and energy is selected. If the new energy is below the baseline energy, the history is terminated; otherwise, the second step is repeated. Presently, the collimator's geometric efficiency is considered without septal penetration.