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