Rapid and accurate navigators for motion and B₀ tracking using QUEEN: Quantitatively enhanced parameter estimation from navigators

Purpose: To develop a framework that jointly estimates rigid motion and polarizing magnetic field (B₀ ) perturbations ( $\delta B_0$ ) for brain MRI using a single navigator of a few milliseconds in duration, and to additionally allow for navigator acquisition at arbitrary timings within any type of sequence to obtain high-temporal resolution estimates.

Theory and methods: Methods exist that match navigator data to a low-resolution single-contrast image (scout) to estimate either motion or $\delta B_0$ . In this work, called QUEEN (QUantitatively Enhanced parameter Estimation from Navigators), we propose combined motion and $\delta B_0$ estimation from a fast, tailored trajectory with arbitrary-contrast navigator data. To this end, the concept of a quantitative scout (Q-Scout) acquisition is proposed from which contrast-matched scout data is predicted for each navigator. Finally, navigator trajectories, contrast-matched scout, and $\delta B_0$ are integrated into a motion-informed parallel-imaging framework.

Results: Simulations and in vivo experiments show the need to model $\delta B_0$ to obtain accurate motion parameters estimated in the presence of strong $\delta B_0$ . Simulations confirm that tailored navigator trajectories are needed to robustly estimate both motion and $\delta B_0$ . Furthermore, experiments show that a contrast-matched scout is needed for parameter estimation from multicontrast navigator data. A retrospective, in vivo reconstruction experiment shows improved image quality when using the proposed Q-Scout and QUEEN estimation.

Conclusions: We developed a framework to jointly estimate rigid motion parameters and $\delta B_0$ from navigators. Combing a contrast-matched scout with the proposed trajectory allows for navigator deployment in almost any sequence and/or timing, which allows for higher temporal-resolution motion and $\delta B_0$ estimates.