Quantitative susceptibility mapping in the human brain at 7T with phase-cycled balanced SSFP

To demonstrate the feasibility of QSM and to show the potential of simultaneous $T_1$ , $T_2$ , and proton density (PD) mapping in the human brain at 7T based on phase-cycled balanced SSFP (bSSFP) MRI.

An algorithm was developed to estimate off-resonance frequency in multi-compartment scenarios by combining elliptic phase-cycled bSSFP signal fitting with dictionary matching. Phase-cycled bSSFP-based tissue phase and susceptibility maps were compared with multi-echo gradient-echo (MEGRE)-based maps in the brains of eight healthy subjects at 7T. Additionally, $T_1$ , $T_2$ , and PD maps were obtained from the same phase-cycled bSSFP data. To demonstrate the potential of matching scan time with MEGRE, bSSFP profiles were subsampled by 50% and resulting maps compared with the reference data.

The tissue phase maps obtained from phase-cycled bSSFP data agreed well with the reference, with a mean absolute deviation of $1.47\pm 0.32$ Hz in the entire brain of all subjects. The mean absolute deviation of tissue susceptibility was $24\pm 7$ parts-per-billion (ppb). Susceptibility in the globus pallidus was overestimated by 67 ppb (p < 0.05), while no significant biases were observed in other regions: 3.2 ppb in putamen, 15.5 ppb in thalamus, and 11.9 ppb in caudate nucleus (all p > 0.05). Quantitative maps showed good contrast between different regions of the brain, aligning well with the literature. Profile subsampling did not significantly (p > 0.05) change the quantitative susceptibility maps.

The feasibility of phase-cycled bSSFP for QSM at 7T was demonstrated, with the added benefit of simultaneous $T_1$ , $T_2$ , and PD mapping, with a total scan time of ˜20 min.