Quantitative Magnetic Susceptibility of the Developing Mouse Brain

Abstract

Cerebral development involves a complex cascade of events which are difficult to visualize in vivo. In this study we combine information from Diffusion Tensor Imaging (DTI) and Quantitative Susceptibility Mapping (QSM) on developing mouse brains at five stages, for three central white matter (WM) regions. QSM can be calculated using frequency shift Gradient Echo MR images acquired at high field. Extracted mean values from small white matter regions of QSM brain maps depend on the orientation of the neuronal fibers of each voxel to the main magnetic field B0. Using fiber tracking information from DTI a correlation of the myelin content of regions of interest (ROI) to the orientation of those fibers to B0 can be made. Plots of the myelin anisotropy, as it increases with age, were generated with this method, suggesting that the neuronal axon is paramagnetic while the myelin surrounding the axon is diamagnetic. In addition the fractional anisotropy (FA) and the mean Apparent Diffusion Coefficient (ADC) of the same ROI were plotted against age. Histological exams were also performed to evaluatemyelin and iron content. It is confirmed that the main source of magnetic susceptibility in WM is the myelin content. The interpretation of all this brain data will provide valuable information on the architecture of the brain during development and a more accuratediagnosis in the case of a myelin degenerative disease.