INFLUENCE OF DOWNSCATTER IN SIMULTANEOUSLY ACQUIRED THALLIUM-201 TECHNETIUM-99M-PYP SPECT/

Simultaneously acquired dual-isotope imaging is a unique and useful ap proach in SPECT. Photon spillover, however, is a potential limitation of this technique. Methods: To investigate the degree of (TC)-T-99m do wnscatter into the Tl-201 window in patients, simultaneously acquired dual-isotope Tl-201/Tc-99m-pyrophosphate imaging was performed in 17 p atients with acute myocardial infarction (MI). Thallium-201 SPECT imag ing was performed first, with a Tl-201 photopeak window after the Tl-2 01 injection (early Tl-201 images), followed by (TC)-T-99m injection a nd SPECT acquisition using dual-isotope windows (dual Tl-201 images). Twenty-four hours after the Tc-99m injection, a third set of Tl-201 im ages was obtained (24-hr Tl-201 images). Thallium defect size (extent score) and defect severity (severity score) were calculated from these three sets of (TI)-T-201 images to quantify the MI. Results: Techneti um-99m accumulation of varying intensity was recognized in all patient s. Extent scores and severity scores were identical in early Tl-201 im ages and 24-hr Tl-201 images. Both scores, however, in the dual Tl-201 images were decreased by 36% and 53%, respectively. Conclusion: There is a considerable Tc-99m downscatter into the Tl-201 window, which pr events precise quantification of MI in simultaneously acquired dual-is otope Tl-201/Tc-99m-pyrophosphate imaging.