IMPROVED IMPORTANCE SAMPLING FOR CONE-BEAM SIMULATIONS USING SIMSET

The SimSET (Simulation System for Emission Tomography) software packag e uses importance sampling (IS) techniques to improve its efficiency. The original IS techniques work well for parallel-hole and fan-beam to mography, but are not well suited to cone-beam tomography. In our orig inal IS algorithms, photons are forced to hit the inside face of the c ollimator within a user-specified axial acceptance angle. Small accept ance angles result in the largest efficiency gains. For parallel-hole collimators, the angle is usually chosen to be the maximum deviation f rom perpendicular at which a photon can pass through a collimator hole . For cone-beam, the acceptance angle must be set using the holes with the greatest deviation from perpendicular, resulting in poor efficien cy. We have designed a new IS algorithm for cone-beam which bases the acceptance angle on the axial deviation from the central angle of the cone-beam holes. This results in substantial changes to our IS algorit hms and look-up tables. The new algorithm causes no bias in simulation s using forced detection. For cone-beam simulations it leads to a 2-4x efficiency gain over our original IS algorithm.