No abstract

The FatNavs were shown to be an excellent candidate for accurate, fast, and precise estimation of global field variations for the tested patterns of respiration. Future work will investigate ways to increase the temporal sampling to increase robustness to variations in breathing patterns. Magn Reson Med 80:585-597, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Purpose To compare motion tracking by two modern methods (fat navigators [FatNavs] and Moiré phase tracking [MPT]) as well as their performance for retrospective correction of very high resolution acquisitions. Methods A direct comparison of FatNavs and MPT motion parameters was performed for several deliberate motion patterns to estimate the agreement between methods. In addition, two different navigator resolution were applied. 0.5 mm isotropic MP2RAGE images with simultaneous MPT and FatNavs tracking were acquired in 9 cooperative subjects with no intentional motion. Retrospective motion corrections based on both tracking modalities were compared qualitatively and quantitatively. The FatNavs impact on quantitative T1 maps was also investigated. Results Both methods showed good agreement within a 0.3 mm/° margin in subjects that moved very little. Higher resolution FatNavs (2 mm) showed overall better agreement with MPT than 4 mm resolution ones, except for fast and large motion. The retrospective motion corrections based on MPT or FatNavs were at par in 33 cases out of 36, and visibly improved image quality compared to the uncorrected images. In separate fringe cases, both methods suffered from their respective potential shortcomings: unreliable marker attachment for MPT and poor temporal resolution for FatNavs. The magnetization transfer induced by the navigator RF pulses had a visible impact on the T1 values distribution, with a shift of the gray and white matter peaks of 12 ms at most. Conclusion This work confirms both FatNavs and MPT as excellent retrospective motion correction methods for very high resolution imaging of cooperative subjects.

Purpose: To develop a novel approach for head motion and B 0 field monitoring based on tracking discrete off-resonance markers with three spokes (trackDOTS). Methods: Small markers filled with acetic acid were built and attached to a head cap. Marker positions and phase were tracked with fast MR navigators (DotNavs) consisting of three offresonance, double-echo, orthogonal one-dimensional projections. Individual marker signals were extracted using optimized coil combinations, and used to estimate head motion and field perturbations. To evaluate the approach, DotNavs were integrated in submillimeter MP2RAGE and long-echo time gradient-echo sequences at 7 Tesla, and tested on six healthy volunteers. Results: The DotNav-based motion estimates differed by less than 0.11 6 0.09 mm and 0.19 6 0.17 from reference estimates obtained with an existing navigator approach (FatNavs). Retrospective motion correction brought clear improvements to MP2RAGE image quality, even in cases with submillimeter involuntary motion. The DotNav-based field estimates could track deep breathing-induced oscillations, and in cases with small head motion, field correction visibly improved the gradient-echo data quality. Conversely, field estimates were less robust when strong motion was present. Conclusions: The trackDOTS approach is suitable for headmotion tracking and correction, with significant benefits for highspatial-resolution MRI. With small head motion, DotNav-based field estimates also allow correcting for deep-breathing artifacts in T 2 *-weighted acquisitions.