Influence of spill-over for 99mTc images and the effect of scatter correction for dual-isotope simultaneous acquisition with 99mTc and 18F using small-animal SPECT-PET/CT system

A dual-isotope simultaneous acquisition (DISA) of Tc-99m and F-18 affects the image quality of Tc-99m by crosstalk and spill-over from F-18. We demonstrated the influence of spill-over and crosstalk on image quality and its correction effect for DISA SPECT with Tc-99m and F-18. A fillable cylindrical chamber of 30 mm with NEMA-NU4 image quality phantom was filled with Tc-99m only or a mixed Tc-99m and F-18 solution (C-100). Two small-region chambers were filled with Tc-99m only or a mixed Tc-99m and F-18 solution made at half the radioactivity concentration of C-100 (C-50) and non-radioactive water (C-0). The F-18/Tc-99m ratio for DISA was set at approximately 0.4-12. Two types of Tc-99m transverse images with and without scatter correction (SC and nonSC) were created. The Tc-99m images of single-isotope acquisition (SIA) were created as a reference. The DISA/SIA ratio and contrast of Tc-99m were compared between SIA and DISA. Although the DISA/SIA ratios with nonSC of C-100, C-50 and C-0 gradually increased with increasing (FTc)-F-18-Tc-/99m ratio, it was nearly constant by SC. The contrasts of C-100 and C-50 were similar to a reference value for both nonSC and SC. In conclusion, DISA images showed lower image quality as the F-18/Tc-99m ratio increased. The image quality in hot-spot regions such as C-100 and C-50 was improved by SC, whereas cold-spot regions such as C-0 could not completely remove the influence of spill-over even with SC.