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

Abstract

A dual-isotope simultaneous acquisition (DISA) of ^99mTc and ¹⁸F affects the image quality of ^99mTc by crosstalk and spill-over from ¹⁸F. We demonstrated the influence of spill-over and crosstalk on image quality and its correction effect for DISA SPECT with ^99mTc and ¹⁸F. A fillable cylindrical chamber of 30 mm with NEMA-NU4 image quality phantom was filled with ^99mTc only or a mixed ^99mTc and ¹⁸F solution (C₁₀₀). Two small-region chambers were filled with ^99mTc only or a mixed ^99mTc and ¹⁸F solution made at half the radioactivity concentration of C₁₀₀ (C₅₀) and non-radioactive water (C₀). The ¹⁸F/^99mTc ratio for DISA was set at approximately 0.4-12. Two types of ^99mTc transverse images with and without scatter correction (SC and nonSC) were created. The ^99mTc images of single-isotope acquisition (SIA) were created as a reference. The DISA/SIA ratio and contrast of ^99mTc were compared between SIA and DISA. Although the DISA/SIA ratios with nonSC of C₁₀₀, C₅₀ and C₀ gradually increased with increasing ¹⁸F^/99mTc 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 ¹⁸F/^99mTc 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.

Keywords: DISA; Mixed 99mTc and 18F solution; Scatter correction; Small-animal; Small-animal SPECT/PETCT; Spill-over.