A modular description for collimator geometry in EGS simulation tasks

EGS is a very common Monte Carlo code, used in the simulation of Nuclear Medicine devices. Simulation techniques are particularly useful, in order to optimize collimator configuration and camera design in Single Photon Emission studies. Using the EGS code, users must define the geometry where particles are transported. This can be both a very hard task and a source of inefficiency, especially in case of complex geometries as, for instance, hexagonal hole collimators or pixellated detectors. In this paper we present a modular description for such geometries. Regions are seen as a basic cell repeated in the space. Our method allows the computation of the region to which a point belongs to in a few steps; thus we are able to calculate this region in a reduced number of operations, independently from the collimator and detector dimensions. We validated the modular description, by performing the characterization of two different collimators: one with square holes and one with hexagonal holes. With modular description we can reduce the computational time up to 25%, with respect to a "traditional" geometric description. It is also possible to simulate a breast phantom for different configurations: each run (one phantom relative to a 10 min acquisition with full photons and electrons transport) would take almost 24 hours on a cluster of four PIII 800 MHz processors.