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

A dual-isotope simultaneous acquisition (DISA) of 99m Tc and 18 F affects the image quality of 99m Tc by crosstalk and spill-over from 18 F. We demonstrated the influence of spill-over and crosstalk on image quality and its correction effect for DISA SPECT with 99m Tc and 18 F. A fillable cylindrical chamber of 30 mm with NEMA-NU4 image quality phantom was filled with 99m Tc only or a mixed 99m Tc and 18 F solution (C 100 ). Two small-region chambers were filled with 99m Tc only or a mixed 99m Tc and 18 F solution made at half the radioactivity concentration of C 100 (C 50 ) and non-radioactive water (C 0 ). The 18 F/ 99m Tc ratio for DISA was set at approximately 0.4–12. Two types of 99m Tc transverse images with and without scatter correction (SC and nonSC) were created. The 99m Tc images of single-isotope acquisition (SIA) were created as a reference. The DISA/SIA ratio and contrast of 99m Tc 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 18 F /99m Tc 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 18 F/ 99m Tc 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.