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 (C100 ). 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 C100 (C50 ) and non-radioactive water (C0 ). 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 C100 , C50 and C0 gradually increased with increasing 18 F/99m Tc ratio, it was nearly constant by SC. The contrasts of C100 and C50 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 C100 and C50 was improved by SC, whereas cold-spot regions such as C0 could not completely remove the influence of spill-over even with SC.