Energy Window Optimization in Simultaneous Technetium-99m TCT and Thallium-201 SPECT Data Acquisition

In simultaneous Tc-99m TCT and Tl-201 SPECT data acquisition, the emission images are degraded by not only scatter from the emission source, but also cross-contamination from the transmission photons. Also, the transmission images are degraded by scatter from the transmission source and cross-contamination from the emission photons. In this work, a criterion based on maximizing a Figure-of-Merit (FOM) was used to reduce image degradations by optimizing the position and width of the energy windows used in the data acquisition in a triple-camera fan-beam SPECT system. The FOM determines a compromise between increased detection efficiency of primary photons and reduction of scatter and cross-contamination photons. Scatter in the projection data was modeled using Monte Carlo (MC) simulation methods. Experimental studies were performed to verify the simulations and to estimate the amount of Pb x-rays and scatter in the collimator which were not modeled in the MC program. Results suggest that cross-contamination degrades the emission and transmission data. However, the contamination does not significantly alter the optimal energy window settings, which should be centered at ~77 keV with a window width of ~34% for detecting the Tl emission photons, and centered at 140 keV with a window width of ~20% for detecting the Tc transmission photons.