Elastographic magnetization prepared imaging with rapid encoding.

Abstract

Purpose: To introduce a novel sequence for achieving fast, whole-brain MR elastography data through the introduction of a magnetization preparation block for motion encoding along with rapid imaging readouts.

Theory and methods: We implemented MRE motion encoding in a magnetization preparation pulse sequence block, where spins are excited, motion encoded, and then stored longitudinally. This magnetization is accessed through a train of rapid gradient echoes and encoded with a 3D stack-of-spirals trajectory. Spoilers are included to crush unprepared magnetization and avoid image artifacts. We demonstrate the feasibility of the proposed method in capturing MRE displacement data for estimating mechanical properties and accelerating scan times.

Results: We measured stability of phase across gradient echo readouts in the readout train after magnetization preparation. Additionally, we obtained displacement fields with high OSS-SNR with retrospective sampling and differences in average stiffness properties (NRMSE) of 3.6% ( $R_{xy}=2$ ) and 7.7% ( $R_{xy}=4$ ) between retrospectively undersampled and fully sampled data. Prospective undersampling showed highly similar multiscale similarity measures and global property differences between 1.0% to 3.5% with one outlier of 8.1% between the proposed method and reference EPI scans.

Conclusion: Magnetization preparation for MRE is feasible and can accelerate brain MRE scans, producing high-quality mechanical property maps at 2.5 mm isotropic resolution in 1 min 20 s.