Rapid submillimeter QSM and R2*mapping using interleaved multi-shot 3D-EPI at 7 and 3 Tesla

Purpose To explore the high signal-to-noise ratio (SNR) efficiency of interleaved multi-shot 3D-EPI for fast and robust high-resolution whole-brain quantitative susceptibility (QSM) and mapping at 7T and 3T. Methods Single- and multi-TE segmented 3D-EPI is combined with conventional CAIPIRINHA undersampling for up to 72-fold effective gradient echo (GRE) imaging acceleration. Across multiple averages, scan parameters are varied (e.g. dual-polarity frequency-encoding) to additionally correct for B 0 -induced artifacts, geometric distortions and motion retrospectively. A comparison to established GRE protocols is made. Resolutions range from 1.4mm isotropic (1 multi-TE average in 36s) up to 0.4mm isotropic (2 single-TE averages in approximately 6 minutes) with whole-head coverage. Results Only 1-4 averages are needed for sufficient SNR with 3D-EPI, depending on resolution and field strength. Fast scanning and small voxels together with retrospective corrections result in substantially reduced image artifacts, which improves susceptibility and mapping. Additionally, much finer details are obtained in susceptibility-weighted image projections through significantly reduced partial voluming. Conclusion Using interleaved multi-shot 3D-EPI, single-TE and multi-TE data can readily be acquired 10 times faster than with conventional, accelerated GRE imaging. Even 0.4mm isotropic whole-head QSM within 6 minutes becomes feasible at 7T. At 3T, motion-robust and distortion-free 0.8mm isotropic whole-brain QSM and mapping in less than 7 minutes becomes clinically feasible. Stronger gradient systems may allow for even higher effective acceleration rates through larger EPI factors while maintaining optimal contrast.