Effects of K₂HPO₄and iodinated contrast agents in bone-equivalent solutions on the quantitative accuracy of bone...

Quantitative bone single-photon emission computed tomography (SPECT) using^99mTc-labeled phosphates has become increasingly feasible due to advances in SPECT/CT technology and reconstruction algorithms. The xSPECT bone algorithm further improves local anatomical fidelity by segmenting CT images into six tissue classes to generate a zone map for context-specific enhancement of spatial resolution. Therefore, when evaluating xSPECT Bone using phantom studies, bone-equivalent solutions must be prepared to ensure that CT values fall within the bone segmentation range. This study aims to clarify how two commonly used substances (dipotassium hydrogen phosphate (K₂HPO₄) and iodinated contrast agent) affect CT values, zone maps, attenuation coefficients (μ-map), and quantitative accuracy of xSPECT bone. The inner container of the JS 10 SPECT performance phantom was filled with a^99mTc solution prepared by dissolving either K₂HPO₄or iodinated contrast agent in 100 ml of water. Four types of inserts (with no additive or different K₂HPO₄or iodine concentrations) were placed in the same sectional area. Both K₂HPO₄and iodine exhibited strong correlations (p< 0.01) between CT values (HU) and mass densities (g ml^-1). No significant differences in zone map orμ-map values were observed between K₂HPO₄and iodine. At equivalent CT values, percentage contrast was significantly higher with iodine than with K₂HPO₄. Furthermore, K₂HPO₄demonstrated stable percentage contrast near unity within 158.5-269.7 HU, indicating superior quantitative consistency. The zone maps andμ-maps were similar with K₂HPO₄and iodine; however, differences in attenuation during SPECT acquisition due to differences in mass density were not considered. This suggests that attenuation correction becomes overcorrected, thereby decreasing quantitative accuracy. These findings indicate that K₂HPO₄is more suitable than iodine for creating bone equivalent solutions in quantitative bone SPECT phantom studies.