B0-Orientation Dependent Magnetic Susceptibility-Induced White Matter Contrast in the Human Brainstem at 11.7T

Purpose: To investigate B-0-field-orientation dependent white matter contrast in the human brainstem based on R-2* and frequency difference (Delta f) 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 B-0 field. The B-0-orientation dependent modulation curves of R-2* and Delta f measurements between short and long echo time regimes were used to reconstruct maps of threedimensional (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: R-2* and Delta f maps demonstrated distinct and significant contrast modulation between the corticospinal tract (CST) and transverse pontine fibers (TPF) dependent on B-0 orientation. Interleaved fiber orientations of the CST and TPF could be sensitively resolved based on field-orientation-dependent fitting of the R-2* and Delta f measurements. The fitted 3D orientation vector maps and peak-to-peak amplitude of R-2* and Delta f modulation exhibited close correspondence to primary eigenvector and anisotropy maps derived from diffusion MRI. The amplitude of B-0-orientation dependent R-2* 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 B-0-orientation dependent susceptibility-induced R-2* and Delta f contrasts to probe tract-specific orientation and microstructure in white matter. (C) 2016 Wiley Periodicals, Inc.