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

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