Dixon-based B₀ self-navigation in radial stack-of-stars multi-echo gradient echo imaging

To develop a Dixon-based $B_0$ self-navigation approach to estimate and correct temporal $B_0$ variations in radial stack-of-stars gradient echo imaging for quantitative body MRI.

The proposed method estimates temporal $B_0$ variations using a $B_0$ self-navigator estimated by a graph-cut-based water-fat separation algorithm on the oversampled k-space center. The $B_0$ self-navigator was employed to correct for phase differences between radial spokes (one-dimensional [1D] correction) and to perform a motion-resolved reconstruction to correct spatiotemporal pseudo-periodic $B_0$ variations (three-dimensional [3D] correction). Numerical simulations, phantom experiments and in vivo neck scans were performed to evaluate the effects of temporal $B_0$ variations on the field-map, proton density fat fraction (PDFF) and $T_2^{\ast }$ map, and to validate the proposed method.

Temporal $B_0$ variations were found to cause signal loss and phase shifts on the multi-echo images that lead to an underestimation of $T_2^{\ast }$ , while PDFF mapping was less affected. The $B_0$ self-navigator captured slowly varying temporal $B_0$ drifts and temporal variations caused by respiratory motion. While the 1D correction effectively corrected $B_0$ drifts in phantom studies, it was insufficient in vivo due to 3D spatially varying temporal $B_0$ variations with amplitudes of up to 25 Hz at 3 T near the lungs. The proposed 3D correction locally improved the correction of field-map and $T_2^{\ast }$ and reduced image artifacts.

Temporal $B_0$ variations particularly affect $T_2^{\ast }$ mapping in radial stack-of-stars imaging. The self-navigation approach can be applied without modifying the MR acquisition to correct for $B_0$ drift and physiological motion-induced $B_0$ variations, especially in the presence of fat.