Electron Density and Effective Atomic Number of Normal-Appearing Adult Brain Tissues: Age-Related Changes and Correlation with Myelin Content

<b>Objectives:</b> Few studies have reported in vivo measurements of electron density (ED) and effective atomic number (Z_eff) in normal brain tissue. To address this gap, dual-energy computed tomography (DECT)-derived ED and Z_eff maps were used to characterize normal-appearing adult brain tissues, evaluate age-related changes, and investigate correlations with myelin partial volume (V_my) from synthetic magnetic resonance imaging (MRI). <b>Materials and Methods:</b> Thirty patients were retrospectively analyzed. The conventional computed tomography (CT) value (CT_conv), ED, Z_eff, and V_my were measured in the normal-appearing gray matter (GM) and white matter (WM) regions of interest. V_my and DECT-derived parameters were compared between WM and GM. Correlations between V_my and DECT parameters and between age and DECT parameters were analyzed. <b>Results:</b> V_my was significantly greater in WM than in GM, whereas CT_conv, ED, and Z_eff were significantly lower in WM than in GM (all <i>p</i> < 0.001). Z_eff exhibited a stronger negative correlation with V_my (ρ = -0.756) than CT_conv (ρ = -0.705) or ED (ρ = -0.491). ED exhibited weak to moderate negative correlations with age in nine of the 14 regions. In contrast, Z_eff exhibited weak to moderate positive correlations with age in nine of the 14 regions. CT_conv exhibited negligible to insignificant correlations with age: <b>Conclusions:</b> This study revealed distinct GM-WM differences in ED and Z_eff along with opposing age-related changes in these quantities. Therefore, myelin may have substantially contributed to the lower Z_eff observed in WM, which underlies the GM-WM contrast observed on non-contrast-enhanced CT.