Beyond the dipole: unveiling the hidden impact of nondipolar shifts on MRI phase contrast anisotropy in the brain

Motivation: To elucidate the contribution of nondipolar frequency shifts to the anisotropy of phase contrast in brain MRI and decode the underpinning biophysical mechanisms of phase contrast. Goal(s): To differentiate the roles of magnetic susceptibility and nondipolar frequency shifts across various head orientations in MRI scans. Approach: Utilizing DEEPOLE QUASAR, the study compares susceptibility estimates and nondipolar frequency shifts in anisotropic brain regions across multiple head orientations. Results: Nondipolar frequency shifts played a substantial role in the anisotropy of frequency shifts, with DEEPOLE QUASAR providing more stable susceptibility estimates than conventional QSM, regardless of head orientation. Impact: The study establishes the substantial influence of nondipolar frequency shifts on MRI phase contrast anisotropy, questioning established assumptions from susceptibility tensor imaging and quantitative susceptibility mapping, thereby paving the way for more accurate brain tissue characterization.