Tracking discrete off‐resonance markers with three spokes (trackDOTS) for compensation of head motion and B₀ perturbations: Accuracy and performance in anatomical imaging

Abstract

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.

Improved susceptibility‐weighted imaging for high contrast and resolution thalamic nuclei mapping at 7T

“…In parallel with motion, perturbations of the B 0 field were found averaging at 1.6‐4.6 Hz in amplitude, a range that is consistent with previous reports . The field perturbations contained both slow‐drift and faster oscillatory patterns, which have been previously observed as well .…”

“…The contributions from motion and field perturbations did vary in their relative importance across subjects, likely related to both performance and individual anatomy. Altogether, these observations demonstrate the importance of both motion and field perturbations in high‐resolution susceptibility imaging, even in realistic conditions of restricted head motion and natural breathing, thereby supporting, and complementing, previous methodological works …”

“…Additionally, susceptibility‐based contrasts (typically ‐weighted) are particularly sensitive to perturbations in B 0 over time, which can arise due to breathing . Altogether, motion and field‐related artifacts can introduce substantial blurring, ghosting and ringing artifacts, signal loss, and inhomogeneities . Numerous groups have proposed techniques to monitor and compensate for motion‐related effects, and in some cases for field perturbations as well .…”

Investigating the variability of cardiac pulse artifacts across heartbeats in simultaneous EEG-fMRI recordings: A 7T study

“…Additionally, it is known that respiration affects the local magnetic susceptibility, leading to cyclic fluctuations in the B 0 field distribution, and can also induce small cyclic movements of the head (van Gelderen et al, 2007). While the former effect is likely too small to play a relevant role (B 0 variations of only a few parts-per-million (Jorge et al, 2018)), the induced cyclic changes in head position/orientation could potentially have a measurable influence on the ongoing PAs (Gretsch et al, 2018).…”

Motion detection with NMR markers using real‐time field tracking in the laboratory frame

“…Tracking of head-mounted NMR markers relies on observing the MRI system's own gradient fields and thus intrinsically yields positions in the scanner coordinate system. [20][21][22][23] It does not require a line of sight because the gradient fields permeate the radiofrequency (RF) setup and the body virtually unhindered. One drawback, shared with navigators, has been the need to interleave dedicated tracking modules with each host sequence.…”