Phase-based masking for quantitative susceptibility mapping of the human brain at 9.4T

To develop improved tissue masks for QSM.

Masks including voxels at the brain surface were automatically generated from the magnitude alone (MM) or combined with test functions from the first (PG) or second (PB) derivative of the sign of the wrapped phase. Phase images at 3T and 9.4T were simulated at different TEs and used to generate a mask, P_Itoh , with between-voxel phase differences less than π. MM, PG, and PB were compared with P_Itoh . QSM were generated from 3D multi-echo gradient-echo data acquired 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 age-based iron concentrations.

Close to air cavities, phase wraps became denser with increasing field and echo time, yielding increased values of the test functions. Compared with P_Itoh , PB had the highest Dice coefficient, while PG had the lowest and MM the highest percentage of voxels outside P_Itoh. Artifacts observed in QSM at 9.4T with MM were mitigated by stronger background filters but yielded a reduced QSM contrast. With PB, QSM contrast was greater and artifacts diminished. Similar results were obtained with challenge data, evidencing larger effects of mask close to air cavities.

Automatic, phase-based masking founded on the second derivative of the sign of the wrapped phase, including cortical voxels at the brain surface, was able to mitigate artifacts and restore QSM contrast across cortical and subcortical brain regions.