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

PurposeTo 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 [Formula] mapping at 7T and 3T. MethodsSingle- 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 B0-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. ResultsOnly 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 [Formula] mapping. Additionally, much finer details are obtained in susceptibility-weighted image projections through significantly reduced partial voluming. ConclusionUsing 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 [Formula] 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.