Influence of downscatter in simultaneously acquired thallium-201/technetium-99m-PYP SPECT.

Simultaneously acquired dual-isotope imaging is a unique and useful approach in SPECT. Photon spillover, however, is a potential limitation of this technique. Methods : To investigate the degree of 99m Tc downscatter into the 201 Tl window in patients, simultaneously acquired dual-isotope 201 Tl/ 99m Tc-pyrophosphate imaging was performed in 17 patients with acute myocardial infarction (MI). Thallium-201 SPECT imaging was performed first, with a 201 Tl photopeak window after the 201 Tl injection (early 201 Tl images), followed by 99m Tc injection and SPECT acquisition using dual-isotope windows (dual 201 Tl images). Twenty-four hours after the 99m Tc injection, a third set of 201 Tl images was obtained (24-hr 201 Tl images). Thallium defect size (extent score) and defect severity (severity score) were calculated from these three sets of 201 Tl images to quantify the MI. Results : Technetium-99m accumulation of varying intensity was recognized in all patients. Extent scores and severity scores were identical in early 201 Tl images and 24-hr 201 Tl images. Both scores, however, in the dual 201 Tl images were decreased by 36% and 53%, respectively. Conclusion : There is a considerable 99m Tc downscatter into the 201 Tl window, which prevents precise quantification of Ml in simultaneously acquired dual-isotope 201 Tl/ 99m Tc-pyrophosphate imaging.