SPECT camera imaging of myocardium perfusion is dependent upon maintaining usable geometry between the sodium-iodide (NaI) detector and the view of the patient through the use of an attached lead (Pb) collimator. Both Tl-201 and Tc-99m radiopharmaceuticals can indicate the perfusion characteristics of the myocardium. Of the two, Tl-201 has the capability to differentiate between viable or scarred myocardium. Currently SPECT myocardial imaging protocols require two scans, a resting scan and a post-stress scan. Simultaneous imaging of Tl-201 rest and Tc-99m stress would have the benefits of eliminating potential errors caused by position misalignment between rest/stress scans, significantly reducing study time. In addition, simultaneous Tl-201/Tc-99m would provide optimal perfusion imaging and tissue viability signaling. This would further enhance the diagnostic ability of the modality, especially for those patients contraindicated for other functional imaging such as PET.

However, the standard Pb collimator interacts with the Tc-99m 140 keV photon to create Compton down-scatter components and k-shell x-rays which interfere with imaging the ~70-80 keV Tl-201 photons. This down-scatter reduces image resolution with the resolution poor NaI detector, and obscures Tl-201 defects, falsely indicating viable myocardium. Replacing the Pb collimator with one of differing density and lower energy K-shell x-ray cross-section would potentially reduce the Tc-99m down-scatter photons in the Tl-201 photopeak range by shifting the k-shell x-ray out of the Tl-201 photopeak. Using a semiconductor detector with superior resolution characteristics should allow isolation of both Tl-201 and Tc-99m photopeaks once the scatter components have been eliminated. The aim of the project was to test the ability of a semiconductor detection system with a tungsten (W) collimator in reducing the detrimental effects of Pb generated down-scatter during simultaneous dual-isotope ²⁰¹Tl/^99mTc imaging. Outcomes indicate a significant reduction in down-scatter and increased resolution using semiconductor detection system with W attenuators compared to NaI detection systems with Pb attenuators.