High-fidelity, high-isotropic-resolution diffusion imaging through gSlider acquisition with B1+ and T1 corrections and integrated ΔB0 /Rx shim array

Abstract

Purpose: $B_1^{+}$ 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 $B_1^{+}$ 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 (R_inplane = 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 $B_1^{+}$ 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 $B_1^{+}$ 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.

Keywords: $B_1^{+}$ inhomogeneity; diffusion-weighted imaging; gSlider; shim array.

Figure 1

(a) Sequence diagram of gSlider acquisition and slab-by-slab triggers. (b) The 32-channel combined RF receive and B₀ shim array (“AC/DC” coil) with independent control of shim currents in each loop in the array.

Figure 2

(a) Five RF-encoded gSlider pulses and (b) their corresponding Bloch simulated slab profiles.

Figure 3

(a) Slab profiles (M_xy) of a RF-encoded gSlider pulse with B₁⁺ inhomogeneity and (b) Longitude Magnetizations (M_z) of the adjacent slab before and after TR/2 recovery. (c) The flowchart of B₁⁺ correction using the pre-scan B₁⁺ maps.

Figure 4

gSlider diffusion-weighted image results without correction, with B₁⁺ correction only, and with B₁⁺ & T₁ corrections. The white arrows highlight the further mitigation of striping artifacts by using T₁ correction. To demonstrate the efficiency of the proposed B₁⁺ & T₁ correction methods, the diffusion weighted gSlider data were acquired in axial orientation, with resolution of 1.5×1.5×1.0 mm³.

Figure 5

The comparisons of the B₀ field maps and MB-1 EPI distortions with Anterior-to-Posterior (AP) and Posterior-to-Anterior (PA) phase-encodings, with and without dynamic multi-coil B₀ shimming. The shimming is then combined with R_inplane=4 acceleration to achieve ~10-fold reduction in EPI distortion, bringing the AP and PA images into closer alignment. The resulting low-distortion EPI slices resemble the distortion-free T₂-TSE reference slices (see red brain mask outline).

Figure 6

Compared to baseline 2^nd-order spherical harmonic global shimming, dynamic MC shimming for MB-2 acquisitions jointly shims two slabs at the same time, while out-of-slab regions are unconstrained and are allowed to have poor shims. The shim results of MB-2 are performance closely to MB-1 slab-optimized shimming.

Figure 7

Comparison between MB-2 and MB-1. The MB-2 shimming performance closely resembles that of the MB-1 slab-optimized shimming. The red outlines show the contours of T₂-TSE reference brain mask.

Figure 8

(a) Averaged diffusion-weighted images and (b) directionally-encoded color fractional anisotropy (FA) maps of the 1 mm isotropic diffusion-weighted data with 64 directions obtained in ~20-minutes using the proposed correction and dynamic shimming framework. (c) The primary eigenvectors from diffusion tensor imaging in typically problematic temporal and frontal lobes of the brain were more accurately depicted by synergistically combining dynamic B₀ shimming with parallel imaging acceleration. The primary eigenvectors were color-encoded (red: left-right, green: anterior-posterior, blue: superior-inferior) and overlaid on FA maps.