Quantitative magnetic susceptibility of the developing mouse brain reveals microstructural changes in the white matter.

Cerebral development involves a complex cascade of events which are difficult to visualize and quantify in vivo. In this study we combine information from Diffusion Tensor Imaging (DTI) and Quantitative Susceptibility Mapping (QSM) to analyze developing mouse brains at five stages up to 56days postnatal. Susceptibility maps were calculated using frequency shifts in gradient echo MR images acquired at 9.4T. The mean apparent magnetic susceptibility and magnetic susceptibility anisotropy of major white matter tracts were evaluated as a function of age. During the first two weeks, susceptibility of white matter appeared paramagnetic relative to surrounding gray matter; it then gradually became more diamagnetic. While diffusion anisotropy was already apparent and high at postnatal day 2, susceptibility anisotropy only became significant during the third week. This mismatch indicated different microstructural underpinnings for diffusion anisotropy and susceptibility anisotropy. Histological exams were also performed to evaluate myelin and iron content. It is confirmed that the main source of susceptibility contrast in WM is the myelin content. The ability to quantify the magnetic properties of white matter will provide valuable information on the architecture of the brain during development and potentially a more specific indicator for myelin degenerative diseases.