Improved importance sampling for cone-beam simulations using SimSET

Abstract

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.