B₀-orientation dependent magnetic susceptibility-induced white matter contrast in the human brainstem at 11.7T

Purpose To investigate B₀-field-orientation dependent white matter contrast in the human brainstem based on R₂∗ and frequency difference (Δf) mapping from gradient echo (GRE) imaging at 11.7T. Methods Multi-echo GRE data were acquired from two fixed human brainstem specimens at multiple orientations with respect to the static B₀ field. The B₀-orientation dependent modulation curves of R₂∗ and Δf measurements between short and long echo time regimes were used to reconstruct maps of three-dimensional (3D) white matter orientation vectors. The results were compared with maps from diffusion MRI, susceptibility tensor imaging, and histological staining of the same specimens. Results R₂∗ and Δf maps demonstrated distinct and significant contrast modulation between the corticospinal tract (CST) and transverse pontine fibers (TPF) dependent on B₀ orientation. Interleaved fiber orientations of the CST and TPF could be sensitively resolved based on field-orientation-dependent fitting of the R₂∗ and Δf measurements. The fitted 3D orientation vector maps and peak-to-peak amplitude of R₂∗ and Δf modulation exhibited close correspondence to primary eigenvector and anisotropy maps derived from diffusion MRI. The amplitude of B₀-orientation dependent R₂∗ modulation was significantly (P < 0.005) higher in the CST compared with TPF, while fractional anisotropies were comparable. Conclusion The findings of this study demonstrate the potential of B₀-orientation dependent susceptibility-induced R₂∗ and Δf contrasts to probe tract-specific orientation and microstructure in white matter.