How Does White Matter Orientation Affect Contrast in Gradient-Echo Magnitude & Phase Images? Simulation of a Three Compartment Model

Andreas Schfer¹, Bibek Dhital¹, Christopher J. Wiggins^2,3, Robert Turner¹

The fibre bundle orientation has been shown to affect gradient-echo image contrast, both in magnitude and phase images. It has recently been suggested that anisotropic susceptibility effects are responsible for the dependence of frequency on fibre orientation. Our simulations using a microstructural WM model show that both the magnitude and frequency of the MR signal depend on the orientation of the fibres to the magnetic field, even for isotropic susceptibility of myelin. The orientation dependency of each is stronger if an anisotropic susceptibility is assumed. Furthermore, the smaller the distance between the fibres the stronger is the orientation dependency.