Efficient photon cross-talk calculation in SPECT

A rotation-based Monte Carlo (MC) simulation method (RMC) has been developed, designed for rapid calculation of down-scatter through nonuniform media in SPECT. A possible application is down-scatter correction in dual isotope SPECT. With RMC, only a fraction of all projections of a SPECT study has to be MC simulated in a standard manner. The other projections can be estimated rapidly using the results of these standard MC calculations. For efficiency, in RMC approximations have to be made with regard to the final scatter angle of the detected photons. Further speed-up is obtained by combining RMC with convolution-based forced detection (CFD), instead of forced detection (FD), which is a more common variance reduction technique for MC. The RMC method was compared with standard MC for Tc-99 m down-scatter in a Tl-201 window (72 keV±10%) using a digital thorax phantom. The resulting scatter projections are in good agreement (maximum bias a few percent of largest value in the projection), but RMC with CFD is about 3 orders in magnitude faster than standard MC with FD and up to 25 times faster than standard MC with CFD. Using RMC combined with CFD, the generation of 64 almost noise-free down-scatter projections (64×64) takes only a couple of minutes on a 500 Mhz Pentium processor. Therefore, rotation-based Monte Carlo could serve as a practical tool for down-scatter correction schemes in dual isotope SPECT