Source-separated susceptibility tensor MRI reveals paramagnetic and diamagnetic anisotropy in a postmortem chimpanzee brain

Magnetic susceptibility in brain white matter is anisotropic because of the ordered arrangement of myelin lipids, whereas paramagnetic iron is generally not considered a source of susceptibility anisotropy. Whether spatially organized paramagnetic sources also contribute to tissue-level susceptibility anisotropy remains unclear. Here we analyze multi-orientation gradient-echo MRI and diffusion MRI data from a paraformaldehyde-fixed postmortem adult male western chimpanzee ( Pan troglodytes verus ) brain and use DECOMPOSE-QSM to separate paramagnetic and diamagnetic susceptibility components before tensor reconstruction. The diamagnetic component shows the expected anisotropy in white matter and aligns with major diffusion-based fiber orientations. The paramagnetic component also shows coherent anisotropy, most prominently in deep gray matter and in parts of white matter. Numerical modeling further supports that an ordered spatial arrangement of otherwise isotropic paramagnetic sources can produce orientation-dependent susceptibility. These results suggest that source-separated susceptibility tensor imaging provides complementary information about tissue-specific susceptibility organization beyond conventional susceptibility tensor imaging.