High‐fidelity, high‐isotropic‐resolution diffusion imaging through gSlider acquisition with and T₁ corrections and integrated ΔB₀/Rx shim array

Abstract

Purpose B1+ and T1 corrections and dynamic multicoil shimming approaches were proposed to improve the fidelity of high‐isotropic‐resolution generalized slice‐dithered enhanced resolution (gSlider) diffusion imaging. Methods An extended reconstruction incorporating B1+ inhomogeneity and T1 recovery information was developed to mitigate slab‐boundary artifacts in short‐repetition time (TR) gSlider acquisitions. Slab‐by‐slab dynamic B0 shimming using a multicoil integrated ΔB0/Rx shim array and high in‐plane acceleration (Rinplane = 4) achieved with virtual‐coil GRAPPA were also incorporated into a 1‐mm isotropic resolution gSlider acquisition/reconstruction framework to achieve a significant reduction in geometric distortion compared to single‐shot echo planar imaging (EPI). Results The slab‐boundary artifacts were alleviated by the proposed B1+ and T1 corrections compared to the standard gSlider reconstruction pipeline for short‐TR acquisitions. Dynamic shimming provided >50% reduction in geometric distortion compared to conventional global second‐order shimming. One‐millimeter isotropic resolution diffusion data show that the typically problematic temporal and frontal lobes of the brain can be imaged with high geometric fidelity using dynamic shimming. Conclusions The proposed B1+ and T1 corrections and local‐field control substantially improved the fidelity of high‐isotropic‐resolution diffusion imaging, with reduced slab‐boundary artifacts and geometric distortion compared to conventional gSlider acquisition and reconstruction. This enabled high‐fidelity whole‐brain 1‐mm isotropic diffusion imaging with 64 diffusion directions in 20 min using a 3T clinical scanner.

Fast submillimeter diffusion MRI using gSlider‐SMS and SNR‐enhancing joint reconstruction

“…Finally, we should note that the datasets shown in this paper were acquired on the 3T CONNECTOM system, which has unique capabilities that may not necessarily translate directly to more common scanner hardware configurations. However, gSlider can still be implemented on standard 3T MRI hardware and the combination of gSlider acquisition and SER‐based image reconstruction can still offer substantial benefits in such settings, as we have demonstrated in recent preliminary work . While we believe that a detailed exploration of this is beyond the scope of the present paper, we show illustrative results with 860 μm isotropic resolution obtained in 10 minutes from a standard (commercially‐available) 3T Prisma scanner in Supporting Information Figure S5.…”

“…For simplicity and brevity, this paper will describe gSlider‐SMS for the simplified case in which parallel imaging and SMS reconstruction have already been performed, and it remains to reconstruct the high‐resolution spatial information along the slice dimension. Rather than exciting thin slices, conventional gSlider encoding excites slabs that are several times thicker than the desired high‐resolution slice thickness, and uses multiple RF slab‐encodings across consecutive TRs to resolve the constituent subslices that comprise the slab.…”

“…On the data‐acquisition side, some of the major advances have been driven by the use of simultaneous multi‐slice (SMS) and volumetric excitation methods, which yield substantially higher SNR efficiency relative to standard 2D spatial encoding methods. Related approaches acquire multiple sets of low‐resolution high‐SNR images with RF encoding, and computationally fuse these low‐resolution images together into a “super‐resolved” high‐resolution image . One recent approach, gSlider‐SMS, combines SMS and RF encoding ideas, and is able to achieve whole‐brain in vivo human diffusion MRI with isotropic submillimeter resolution with relatively high SNR‐efficiency.…”

“…Related approaches acquire multiple sets of low‐resolution high‐SNR images with RF encoding, and computationally fuse these low‐resolution images together into a “super‐resolved” high‐resolution image . One recent approach, gSlider‐SMS, combines SMS and RF encoding ideas, and is able to achieve whole‐brain in vivo human diffusion MRI with isotropic submillimeter resolution with relatively high SNR‐efficiency. Unfortunately, the small size of the imaging voxels still means that gSlider‐SMS often requires signal averaging.…”

Dynamic B0 shimming for multiband imaging using high order spherical harmonic shims

“…Consistent with Figure 4, the largest improvements in homogeneity for static 1st-4th+ shimming over static 1st & 2nd shimming are found in the cerebellum and temporal lobe (slices 1-13) and superior brain regions (slices 40-55). The largest improvements in homogeneity for DSU over static 1st-4th+ shims are seen in dynamic updating using three slice ROIs: the cerebellum and inferior temporal lobe (slices 1-15) and inferior frontal lobe (slices [22][23][24][25][26][27][28]. Improvements over the superior brain slices (29-55) are significant but more modest, ie, <5 Hz.…”