Improved importance sampling for cone-beam simulations using SimSET

The SimSET (Simulation System for Emission Tomography) software package uses importance sampling (IS) techniques to improve its efficiency. The original IS techniques work well for parallel-hole and fan-beam tomography, but are not well suited to cone-beam tomography. In the authors' original IS algorithms, photons are forced to hit the inside face of the collimator within a user specified axial acceptance angle. Small acceptance angles result in the largest efficiency gains. For parallel-hole collimators, the angle is usually chosen to be the maximum deviation from 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 efficiency. The authors 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 the authors' IS algorithms and look-up tables. The new algorithm causes no bias in simulations using forced detection. For cone-beam simulations it leads to a 2-4/spl times/ efficiency gain over the authors' original IS algorithm.