Quantitative susceptibility mapping of the human brain at 9.4T
Quantitative Susceptibility Mapping (QSM) can be used to investigate local tissue structure, but is hampered by several unwanted effects, like background magnetic fields. Spatial characteristics of the magnetic background fields with impact on the gradient echo phase signal, were simulated at different echo times at 3T and 9.4T. Voxels with under sampled phase, which violate the Itoh condition, were identified and excluded from the ideal mask, PItoh, covering voxels with a phase differing by less than π from neighboring voxels. Automatically generated brain tissue masks that included voxels at the brain surface were obrained from the magnitude image alone (MM) or in combination with test functions from the first (PG) or second (PB) derivative of the sign of the wrapped phase. These masks were compared with PItoh. QSM were generated from 3D multi-echo gradient-echo data acquired from healthy volunteers at 9.4T (21 subjects aged: 20-56y), and from the QSM2016 challenge 3T data using different masks, unwrapping, background removal and dipole inversion algorithms. QSM contrast was quantified using brain region-specific, age-based iron concentrations.
Close to air cavities, phase wraps became denser with increasing field and echo time, yielding increased values of the PG and PB test functions. PB was most similar to the ideal PItoh mask (highest Dice coefficient), followed by MM and PG, however MM had the highest percentage of voxels outside PItoh.. Artifacts observed in QSM at 9.4T with MM were mitigated by stronger background filters but this approach yielded a reduced QSM contrast. With PB, QSM contrast was greater, and artifacts diminished (Figure). Similar results were obtained with challenge data, evidencing larger effects of different masking approaches close to air cavities.