Real-time motion and retrospective coil sensitivity correction for CEST using volumetric navigators (vNavs) at 7T

Abstract

Purpose: To explore the impact of temporal motion-induced coil sensitivity changes on CEST-MRI at 7T and its correction using interleaved volumetric EPI navigators, which are applied for real-time motion correction. Methods: Five healthy volunteers were scanned via CEST. A 4-fold correction pipeline allowed the mitigation of (1) motion, (2) motion-induced coil sensitivity variations, (Formula presented.), (3) motion-induced static magnetic field inhomogeneities, ΔB0, and (4) spatially varying transmit RF field fluctuations, (Formula presented.). Four CEST measurements were performed per session. For the first 2, motion correction was turned OFF and then ON in absence of voluntary motion, whereas in the other 2 controlled head rotations were performed. During post-processing (Formula presented.) was removed additionally for the motion-corrected cases, resulting in a total of 6 scenarios to be compared. In all cases, retrospective ∆B0 and - (Formula presented.) corrections were performed to compute artifact-free magnetization transfer ratio maps with asymmetric analysis (MTRasym). Results: Dynamic (Formula presented.) correction successfully mitigated signal deviations caused by head motion. In 2 frontal lobe regions of volunteer 4, induced relative signal errors of 10.9% and 3.9% were reduced to 1.1% and 1.0% after correction. In the right frontal lobe, the motion-corrected MTRasym contrast deviated 0.92%, 1.21%, and 2.97% relative to the static case for Δω = 1, 2, 3 ± 0.25 ppm. The additional application of (Formula presented.) correction reduced these deviations to 0.10%, 0.14%, and 0.42%. The fully corrected MTRasym values were highly consistent between measurements with and without intended head rotations. Conclusion: Temporal (Formula presented.) cause significant CEST quantification bias. The presented correction pipeline including the proposed retrospective (Formula presented.) correction significantly reduced motion-related artifacts on CEST-MRI.