The tensorial nature of magnetic susceptibility affects frequency-shift images and quantitative maps (QSM), whose reliability should be questioned. Three healthy volunteers underwent 7T MRI exams including diffusion tensor imaging (DTI) and three QSM acquisitions, each with different orientation of the head. In order to assess the effect of susceptibility anisotropy, we sorted brain voxels depending on their fractional anisotropy (FA) and, by plotting their QSM values against the angle between the static field and the eigenvector corresponding to the largest eigenvalue in DTI, we observed an empirical threshold of FA that reflects the reliability of susceptibility measures.
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