High-resolution in vivo diffusion imaging of the human brain with generalized slice dithered enhanced resolution: Simultaneous multislice (gSlider-SMS)

Abstract

Purpose: To develop an efficient acquisition for high-resolution diffusion imaging and allow in vivo whole-brain acquisitions at 600- to 700-μm isotropic resolution.

Methods: We combine blipped-controlled aliasing in parallel imaging simultaneous multislice (SMS) with a novel slab radiofrequency (RF) encoding gSlider (generalized slice-dithered enhanced resolution) to form a signal-to-noise ratio-efficient volumetric simultaneous multislab acquisition. Here, multiple thin slabs are acquired simultaneously with controlled aliasing, and unaliased with parallel imaging. To achieve high resolution in the slice direction, the slab is volumetrically encoded using RF encoding with a scheme similar to Hadamard encoding. However, with gSlider, the RF-encoding bases are specifically designed to be highly independent and provide high image signal-to-noise ratio in each slab acquisition to enable self-navigation of the diffusion's phase corruption. Finally, the method is combined with zoomed imaging (while retaining whole-brain coverage) to facilitate low-distortion single-shot in-plane encoding with echo-planar imaging at high resolution.

Results: A 10-slices-per-shot gSlider-SMS acquisition was used to acquire whole-brain data at 660 and 760 μm isotropic resolution with b-values of 1500 and 1800 s/mm² , respectively. Data were acquired on the Connectome 3 Tesla scanner with 64-channel head coil. High-quality data with excellent contrast were achieved at these resolutions, which enable the visualization of fine-scale structures.

Conclusions: The gSlider-SMS approach provides a new, efficient way to acquire high-resolution diffusion data. Magn Reson Med 79:141-151, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Keywords: diffusion imaging; simultaneous multislab; simultaneous multislice; slice encoding.

Figure 1

5×-gSlider with ‘slice-phase dither’ encoding to provide highly independent basis, while maintaining high image-SNR in each individual slab acquisition. The DIST RF pulses are shown in the left column, with real and imaginary parts shown in black and green respectively. The corresponding slab profiles are shown in the middle column, each with a π phase dithering applied to a different sub-slice. The impulse response of the thin-slice high-resolution reconstruction with Tikhnonov regularization is shown in the right column.

Figure 2

A 10 simultaneous slice gSlider-SMS acquisition where two sagittal thin-slabs are acquired simultaneously, each at 5× the thickness of the final slice resolution. Here, zoom imaging with inferior saturation pulse in a sagittal acquisition is also used to achieve high in-plane acceleration while retaining whole-brain coverage. Five thin-slab volumes with different slice-phase dither encoding are acquired in total sequentially (middle row). These volumes are combined to create the final high-resolution image (bottom row). Also shown on the bottom row are the zoom-ins of the thin-slab axial image and the final high-resolution image, where the resolution gain can be clearly observed.

Figure 3

Diffusion results of the 660μm isotropic data, with color-FA maps shown on the left and the zoom-ins of the tensor results shown on the right. Results are displayed in three orthogonal planes to highlight the ability of this high-resolution isotropic data in enabling the visualization of fine-scale structures in all spatial orientations.

Figure 4

SNR and effect of averaging of the 660μm isotropic data. FA maps and zoom-in tensor results are shown, where each data repetition took 25 minutes to acquire. With two repetitions and 50-minute scan time, reasonable FA and tensor results can be achieved. These results improve with further data averaging to provide robust results with 4 data averaging.

Figure 5

Diffusion results of the 760μm isotropic data at b = 1800s/mm². Color-FA maps are shown on the left and the zoom-in of the tensor results is shown on the right.

Figure 6

Average DWI of the 760μm data obtained by averaging across the 128 diffusion direction dataset.

Figure 7

b0 images at 760μm isotropic resolution obtained from averaging across 13 intersperse b0 data.

Figure 8

Sensitivity analysis of gSlider encoding and reconstruction to B₁ and B₀ inhomogeneity. The slab-encoding profiles of one of the encoding basis (RF-5) along with the reconstruction’s impulse responses for all of the sub-slices are shown for three different B₁ and B₀ cases. A relatively small amount of degradation in the impulse responses is observed in the non-ideal cases.

Figure 9

Simulation of gSlider reconstruction on an anthropomorphic head phantom in the present of motion between the 5×-gSlider slab-encoded acquisitions. Three different amount of motion were simulated, with more blurring and edge artifacts being observed in the reconstruction as the motion increases, but without catastrophic reconstruction failure.