Development of a new, fast, user friendly, ray tracing program "CSIM" for the simulation of parallelhole collimators.

We have developed a fast, user friendly, ray-tracing program, "CSIM" for low-energy gamma rays (up to ∼200keV) to simulate the performance characteristics of parallelhole collimators. We have used a ray-tracing approach to find the sensitivity and resolution of the parallelhole collimator by including the penetration of photons through the collimator due to the finite attenuation of the collimator material. "CSIM" can calculate the sensitivity of the collimator, the geometric and penetrating photon ratios, and the 1D and 2D point source response functions (PSF) with the statistical uncertainty for different hole shapes (e.g. square, hexagonal, and cylindrical). We have used "CSIM" to simulate the collimator of the YAP-(S)PETII small animal scanner. We present the analysis of the YAP-(S)PETII scanner round-hole parallel collimator designed for nuclear medicine imaging at 140keV. For this aim, different designs have been considered for a variety of source-collimator distances (b=5, 10, 15, 20cm). Resolution and sensitivity characteristics have been plotted as a function of the collimator thickness and the diameter of the hole. For each value of the source-collimator distance, and for each collimator thickness investigated, the trade-off between sensitivity and spatial resolution has been given as a series of characteristic curves. Then, we compare our simulated resolution and sensitivity results to the analytically calculated ones and found that the analytically calculated results for the YAP-(S)PETII scanner collimator are not far away the results predicted by CSIM and also with the experimentally measured resolution values.