The impact of head orientation with respect to B₀ on diffusion tensor MRI measures

Diffusion tensor MRI (DT-MRI) remains the most commonly used approach to characterise white matter (WM) anisotropy. However, DT estimates may be affected by tissue orientation w.r.t. ${\overrightarrow{B}}_0$ due to local gradients and intrinsic $T_2$ orientation dependence induced by the microstructure. This work aimed to investigate whether and how diffusion tensor MRI-derived measures depend on the orientation of the head with respect to the static magnetic field, ${\overrightarrow{B}}_0$. By simulating WM as two compartments, we demonstrated that compartmental $T_2$ anisotropy can induce the dependence of diffusion tensor measures on the angle between WM fibres and the magnetic field. In in vivo experiments, reduced radial diffusivity and increased axial diffusivity were observed in white matter fibres perpendicular to ${\overrightarrow{B}}_0$ compared to those parallel to ${\overrightarrow{B}}_0$. Fractional anisotropy varied by up to $20\text{\%}$ as a function of the angle between WM fibres and the orientation of the main magnetic field. To conclude, fibre orientation w.r.t. ${\overrightarrow{B}}_0$ is responsible for up to $7\text{\%}$ variance in diffusion tensor measures across the whole brain white matter from all subjects and head orientations. Fibre orientation w.r.t. ${\overrightarrow{B}}_0$ may introduce additional variance in clinical research studies using diffusion tensor imaging, particularly when it is difficult to control for (e.g., fetal or neonatal imaging, or when the trajectories of fibres change due to, e.g., space occupying lesions).