B₀ navigator enables respiratory motion navigation in radial stack-of-stars liver Look-Locker T₁ mapping

To develop a $B_0$ self-navigation approach to estimate respiratory motion for motion-corrected liver $T_1$ mapping using a Look-Locker acquisition with radial stack-of-stars trajectory.

The proposed method derives 1D field-map profiles from the oversampled k-space center to estimate a normalized breathing curve and the $B_0$ variation amplitude for each slice and coil. $B_0$ drift and contrast variations, inherent to the Look-Locker acquisition, were modeled and corrected by fitting and demodulating drift and offset terms. The breathing curve was employed to bin data into motion states for motion-resolved reconstruction, followed by water-specific $T_1$ mapping. Simulations with an anatomical body model and in vivo experiments with a Look-Locker multi-echo gradient echo sequence were performed to validate the technique. The estimated normalized breathing curve was compared with magnitude- and phase-based self-navigation approaches using principal component analysis.

The proposed $B_0$ self-navigation reliably estimated the normalized breathing curve and the $B_0$ variation amplitude in simulations and in vivo. $B_0$ variation amplitudes increased with greater tissue displacement, with median values across slices and coils ranging from 4 to 15 Hz at 3 T in volunteers. Motion-resolved reconstruction using the estimated breathing curve reduced motion artifacts and improved image and $T_1$ mapping quality compared to motion-averaged reconstruction.

$B_0$ self-navigation allows estimation of respiratory motion in acquisitions with varying contrast and quantifies the $B_0$ variation amplitude, providing a possible surrogate signal for tissue displacement and enabling self-gated liver $T_1$ mapping using a Look-Locker approach.