Importance sampling in PET collimator simulations

SimSET (a Simulation System for Emission Tomography) incorporates importance sampling (IS) to accelerate emission tomography simulations. To this point IS has been used in the object being imaged, but not in the collimator. This is an effective strategy for simulations using geometric or perfect-absorber collimators, but breaks down when more realistic collimators are simulated. We are improving the efficiency of collimator simulation with algorithms that complement the IS algorithms used in the object. Recent improvements to the IS in the object have led to many more photons passing through the collimator without intersecting the septa. However, the photons that do intersect septa have much higher weights, resulting in poor quality factors if they are detected. The collimator IS algorithms force these high-weight photons either to penetrate the septa or to scatter toward the detector and then penetrate the septa. This reduces the weights of the photons, improve detection efficiency, and raise the quality factor. The new algorithms are currently focused on collimators for position emission tomography (PET), but the underlying techniques are more broadly applicable.