Sub‐voxel quantitative susceptibility mapping for assessing whole‐brain magnetic susceptibility from ages 4 to 80

The evolution of magnetic susceptibility of the brain is mainly determined by myelin in white matter (WM) and iron deposition in deep gray matter (DGM). However, existing imaging techniques have limited abilities to simultaneously quantify the myelination and iron deposition within a voxel throughout brain development and aging. For instance, the temporal trajectories of iron in the brain WM and myelination in DGM have not been investigated during the aging process. This study aimed to map the age‐related iron and myelin changes in the whole brain, encompassing myelin in DGM and iron deposition in WM, using a novel sub‐voxel quantitative susceptibility mapping (QSM) method. To achieve this, a cohort of 494 healthy adults (18–80 years old) was studied. The sub‐voxel QSM method was employed to obtain the paramagnetic and diamagnetic susceptibility based on the approximated [Formula: see text] map from acquired [Formula: see text] map. The linear relationship between [Formula: see text] and [Formula: see text] maps was established from the regression coefficients on a small cohort data acquired with both 3D gradient recalled echo data and [Formula: see text] mapping. Large cohort sub‐voxel susceptibility maps were used to create longitudinal and age‐specific atlases via group‐wise registration. To explore the differential developmental trajectories in the DGM and WM, we employed nonlinear models including exponential and Poisson functions, along with generalized additive models. The constructed atlases reveal the iron accumulation in the posterior part of the putamen and the gradual myelination process in the globus pallidus with aging. Interestingly, the developmental trajectories show that the rate of myelination differs among various DGM regions. Furthermore, the process of myelin synthesis is paralleled by an associated pattern of iron accumulation in the primary WM fiber bundles. In summary, our study offers significant insights into the distinctive developmental trajectories of iron in the brain's WM and myelination/demyelination in the DGM in vivo. These findings highlight the potential of using sub‐voxel QSM to uncover new perspectives in neuroscience and improve our understanding of whole‐brain myelination and iron deposit processes across the lifespan.