Simultaneous Time Interleaved MultiSlice (STIMS) for Rapid Susceptibility Weighted acquisition

Abstract

T₂* weighted 3D Gradient Echo (GRE) acquisition is the main sequence used for Susceptibility Weighted Imaging (SWI) and Quantitative Susceptibility Mapping (QSM). These applications require a long echo time (TE) to build up phase contrast, requiring a long repetition time (TR), and leading to excessively lengthy scans. The long TE acquisition creates a significant amount of unused time within each TR, which can be utilized for either multi-echo sampling or additional image encoding with the echo-shift technique. The latter leads to significant saving in acquisition time while retaining the desired phase and T₂* contrast. In this work, we introduce the Simultaneous Time Interleaved MultiSlice (STIMS) echo-shift technique, which mitigates slab boundary artifacts by interleaving comb-shaped slice groups with Simultaneous MultiSlice (SMS) excitation. This enjoys the same SNR benefit of 3D signal averaging as previously introduced multi-slab version, where each slab group is sub-resolved with kz phase encoding. Further, we combine SMS echo-shift with Compressed Sensing (CS) Wave acceleration, which enhances Wave-CAIPI acquisition/reconstruction with random undersampling and sparsity prior. STIMS and CS-Wave combination thus yields up to 45-fold acceleration over conventional full encoding, allowing a 15sec full-brain acquisition with 1.5 mm isotropic resolution at long TE of 39 ms at 3T. In addition to utilizing empty sequence time due to long TE, STIMS is a general concept that could exploit gaps due to e.g. inversion modules in magnetization-prepared rapid gradient-echo (MPRAGE) and fluid attenuated inversion recovery (FLAIR) sequences.

Keywords: CS-Wave; Echo-Shift; QSM; SMS; STI; SWI; Wave-CAIPI.

Fig1

STIMS pulse sequence diagram for 2-fold echo-shift acceleration. The first MultiBand pulse excites odd slices, and the readout is shifted to the second TR. The second RF excites the even slices, with signal readout at the third TR. This allows effective TE to be long, thereby providing increased phase and T₂* contrast. STIMS allows half of the imaging volume to be excited simultaneously, which provides the SNR benefit of volumetric signal averaging. Because the slice thickness that needs to be resolved is doubled for each slice group, the number of kz phase encodes is reduced by half, providing 2-fold acceleration.

Fig2

Prospectively accelerated acquisition at 3T using 2-fold SMS echo-shift and 9-fold Wave-CAIPI acceleration. This allowed a 36 sec acquisition with whole-brain coverage and 1.5 mm isotropic resolution. Tissue phase and QSM reconstructions were obtained from this rapid acquisition.

Fig3

Prospective acceleration at 7T using 2-fold SMS echo-shift and 12-fold Wave-CAIPI. The acquisition time was 22 sec at 1.5 mm isotropic resolution. Tissue phase and QSM contrasts were derived from the raw phase of this reconstruction.

Fig4

Retrospective acceleration at 3T using 2-fold SMS echo-shift. 12-fold Wave-CAIPI acceleration had similar quality as 15-fold CS-Wave reconstruction. Combination of STIMS and CS-Wave led to a 24 sec acquisition, which yielded the QSM images on the 3^rd row. Merging phase data from 2 additional head orientations further improved the gray/white contrast in QSM, corresponding to a 72 sec acquisition.

Fig5

Retrospective acceleration at 3T with 3-fold SMS echo-shift. 15-fold CS-Wave acceleration had slightly better RMSE performance than 12-fold Wave-CAIPI reconstruction. The STIMS CS-Wave at 45-fold acceleration yielded the phase and QSM contrasts on the last two rows from a 15 sec acquisition at 1.5 mm isotropic resolution.

Fig6

SNR comparison between the full-sampled STIMS and conventional 3D-GRE acquisitions revealed a factor of 1.43 difference, close to the theoretical √2 difference. The white boxes indicate the ROIs of size 24×24 voxels for noise measurement.

Fig7

In vivo SNR comparison between the full-sampled STIMS and conventional 3DGRE acquisitions revealed a factor of 1.41 difference. The four white boxes indicate the ROIs of size 12×12 voxels for noise measurement.

Fig8

Magnitude, phase and QSM reconstructions from the fully-sampled STIMS acquisition with 2-fold echo-shift speed-up.