Rapid SPECT simulation of downscatter in non-uniform media

A rotation-based Monte Carlo (MC) simulation method (RMC) has been develope d, designed for rapid calculation of downscatter through non-uniform media in SPECT. A possible application is downscatter correction in dual isotope SPECT. With RMC, only a fraction of all projections of a SPECT study have t o be MC simulated in a standard manner. The other projections can be estima ted rapidly using the results of these standard MC calculations. For effici ency, approximations have to be made in RMC with regard to the final scatte r angle of the detected photons. Further speed-up is obtained by combining RMC with convolution-based forced detection (CFD) instead of forced detecti on (FD), which is a more common variance reduction technique for MC. The RMC method was compared with standard MC for Tc-99m downscatter 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 per cent of t he largest value ill the projection), but RMC with CFD is about three order s in magnitude faster than standard MC with FD and up to 25 times faster th an standard MC with CFD. Using RMC combined with CFD, the generation of 64 almost noise-free downscatter projections (64 x 64) takes only a couple of minutes on a 500 MHz Pentium processor. Therefore, rotation-based Monte Car lo could serve as a practical tool for downscatter correction schemes in du al isotope SPECT.