Origin of B0 Orientation Dependent R2* (=1/T2*) in White Matter: Magic Angle Effect Vs. Magnetic Susceptibility

Se-Hong Oh¹, Jongho Lee¹

Recent MRI studies have demonstrated that the relative orientation of white matter fibers to the B0 field significantly affects R2* measurement. In this work, the origin of this effect was investigated by measuring R2 and R2* in multiple orientations and fitting the results to a magnetic susceptibility-based model and a magic angle-based model. To further explore the source of magnetic susceptibility effect, the contribution of tissue iron to the orientation dependent R2* contrast was investigated. Additionally, R2* was measured in basal ganglia area to confirm there is no orientation dependence in deep gray matter. Our results suggest that myelin is a primary source for R2* contrast because of its highly oriented structure and large susceptibility value.