Molecular Underpinnings of Magnetic Susceptibility Anisotropy in the Brain White Matter

Wei Li¹, Bing Wu¹, Alexandru V. Avram¹, Chunlei Liu¹, ²

Frequency shift and susceptibility from gradient-echo MRI shows excellent gray and white matter contrast, both of which depend on white matter fiber orientation. In this work, we explored the molecular underpinnings of orientation dependence of susceptibility in brain white matter. A biophysical model is developed to link the molecular susceptibility anisotropy of myelin components to the MRI-determined bulk anisotropy. This model provides a consistent interpretation of the orientation dependence of macroscopic magnetic susceptibility and the microscopic origin of anisotropic susceptibility. The results suggested that the cylindrically aligned lipid molecules in myelin are the main source of bulk susceptibility anisotropy.