Phantom evaluation of simultaneous thallium-201/technetium-99m aquisition in single-photon emission tomography

This study investigates downscatter effects in cardiac single-photon emission tomographic studies with simultaneous thallium-201/technetium-99m acquisition, and evaluated a previously proposed subtraction technique for downscatter compensation. Ten studies were carried out with different defect sizes and locations and varying activity distributions using four energy windows: 70±10% keV, 140±10% keV, 100±10% KeV, and 103±16% keV. The subtraction technique used the 100-or 103-keV data to remove scattered ^99mTc counts from the 70-keV data. The size and contrast of infarcts in the dual-isotope 70-keV image were artificially decreased compared to those in the 140-keV image, caused by scattered ^99mTe counts that were comparable to the primary ²⁰¹T1 counts in the 70-keV window. The subtraction technique produced larger defects and more heterogeneous activity in the myocardial wall in dual-isotope 70-keV images compared to the corresponding ²⁰¹T1-only images. These artifacts were caused by the markedly different spatial distributions of scattered ^99mTe counts in the 100-keV (or 103-keV) window as compared with the 70-keV window. It is concluded that scattered ^99mTc photons may cause overestimation of ischemia and myocardial viability in simultaneous dual-isotope patient studies. The proposed subtraction technique was inaccurate and produced image artifacts. Adequate downscatter compensation methods must be developed before applying simultaneous ²⁰¹T1/^99mTC acquisition in clinical practice.