Efficient simulation of SPECT down-scatter including photon interactions with crystal and lead

A major image degrading factor in simultaneous dual isotope (DI) SPECT or simultaneous emission-transmission (ECT-TCT) imaging, is the detection of photons emitted by the higher energy isotope in the energy window used for imaging the lower energy isotope. In Tc-99m/Tl-201 DI SPECT typically tens of percents of the total down-scatter is caused by lead X-rays. In Tc-99m/Gd-153 ECT TCT, a comparable fraction of the down-scatter originates from Tc-99m photons which only partly deposit their energy in the detector crystal. When the spatial distribution of the isotope causing down-scatter is known, projections can be estimated using photon transport calculations. Such projections can be used for down-scatter correction. In this paper we extend a previously proposed efficient down-scatter simulation method, by incorporating into the scatter model the interactions of photons with the detector crystal and collimator lead. To this end, point spread function tables including crystal and lead interactions are simulated. Subsequently, photons are traced through the patient body until their last scatter position, and the pre-calculated responses are used to project the photons onto the detector plane, taking photon attenuation into account. The approach is evaluated by comparing calculated Tc-99m down-scatter projections with measured projections. The inclusion of the crystal and lead interactions tremendously increases accuracy of the simulations. Calculating 60 down-scatter projections of an extended distribution on a 64 × 64 × 64 grid takes about 3 minutes on a PC with two 1.2 GHz processors. We conclude that accurate simulation of down-scatter is now possible including all relevant effects of non-uniform density and photon interactions with the crystal and collimator lead.