Effects of K ₂ HPO ₄ and iodinated contrast agents in bone-equivalent solutions on the quantitative accuracy of bone single-photon emission computed tomography

Abstract Quantitative bone single-photon emission computed tomography (SPECT) using 99m Tc‑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 (K 2 HPO 4 ) 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 99m Tc solution prepared by dissolving either K 2 HPO 4 or iodinated contrast agent in 100 mL of water. Four types of inserts (with no additive or different K 2 HPO 4 or iodine concentrations) were placed in the same sectional area. Both K 2 HPO 4 and iodine exhibited strong correlations (p < 0.01) between CT values (HU) and mass densities (g/mL). No significant differences in zone map or μ‑map values were observed between K 2 HPO 4 and iodine. At equivalent CT values, percentage contrast was significantly higher with iodine than with K 2 HPO 4 . Furthermore, K 2 HPO 4 demonstrated stable percentage contrast near unity within 158.5–269.7 HU, indicating superior quantitative consistency. The zone maps and μ-maps were similar with K 2 HPO 4 and 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 K 2 HPO 4 is more suitable than iodine for creating bone equivalent solutions in quantitative bone SPECT phantom studies.