Fan beam forced detection in Gate

Fan beam collimators can obtain a higher sensitivity without loss in resolution at the cost of a reduced field of view. The geometric response has been studied both analytically and with numerical and Monte Carlo simulations, but a fast and accurate Monte Carlo simulator for fan beam geometry is not available. The goal of this work is therefore to accelerate ^99mTc fan beam simulations in Gate, with full MC modeling of the collimator and detector in order to retain the characteristic hexagonal hole pattern of the collimator. To this end, two problems need to be solved: the long calculation time of particle transport in fan beam collimator geometry and the lack of dedicated variance reduction techniques. The first problem is solved by a dedicated tracking algorithm for fan beam collimator. The second problem is solved by the introduction of fan beam forced detection with variable solid angles. Our methods were validated with both analog Gate simulations and measurements. A good agreement was found for the hexagonal hole pattern, energy spectra, spatial resolution and sensitivity.