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Georgia Journal of Science

Article Title

APPLICATION OF TUNGSTEN (W) ATTENUATORS FOR THE REDUCTION OF DOWNSCATTER PHOTONS IN DUAL-ISOTOPE SIMULTANEOUS-ACQUISITION NUCLEAR CARDIOLOGY IMAGING

Abstract

Both Tl-201 and Tc-99m radiopharmaceuticals can indicate the perfusion characteristics of the myocardium. Tl-201 has the additional capability to differentiate between viable vs non-viable myocardium. Currently both SPECT single and dual-isotope myocardial imaging protocols require two scans. Simultaneous imaging of Tl-201 and Tc-99m would have the benefits of eliminating potential errors caused by position misalignment between scans, significantly reducing study time. Additionally, simultaneous Tl-201/Tc-99m would provide optimal perfusion imaging and tissue viability signaling. This would further enhance the diagnostic ability of the modality for those patients contraindicated for other functional imaging such as PET. A problem occurs when the Pb collimator used in SPECT imaging interacts with the Tc-99m 140 keV photon to create k-shell x-rays which interfere with imaging the ~70-80 keV Tl-201 photons. This down-scatter reduces image resolution and fills in Tl-201 defects, falsely indicating viable myocardium. The aim of the project was to test the ability of a tungsten (W) pinhole attenuator in reducing the detrimental effects of Pb generated down-scatter during simultaneous dual-isotope Tl-201/Tc-99m imaging through increased absorption and shifting the k-shell x-ray out of the Tl-201 photopeak. Multi-channel analyzers were used to measure Tc-99m and Tl-201 spectra under conditions of good geometry using 0.25 mm thick W and Pb attenuators, each with a 1 mm diameter pinhole. Transmission and attenuation in the Tc-99m and Tl-201 windows were calculated and compared statistically. Measurements show Pb increased Tc-99m downscatter counts in Tl-201 window by 55.3%, and W decreased Tc-99m downscatter counts in Tl-201 window by 17.2%. Replacing the Pb collimator with W significantly reduced the Tc-99m down-scatter photons in the Tl-201 photopeak range, a condition required for dual isotope single acquisition cardiac imaging. Outcomes indicate the significant reduction in down-scatter using W could facilitate dual isotope single acquisition cardiac protocols.

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