Rapid multi-orientation quantitative susceptibility mapping

Abstract

Three-dimensional gradient echo (GRE) is the main workhorse sequence used for susceptibility weighted imaging (SWI), quantitative susceptibility mapping (QSM), and susceptibility tensor imaging (STI). Achieving optimal phase signal-to-noise ratio requires late echo times, thus necessitating a long repetition time (TR). Combined with the large encoding burden of whole-brain coverage with high resolution, this leads to increased scan time. Further, the dipole kernel relating the tissue phase to the underlying susceptibility distribution undersamples the frequency content of the susceptibility map. Scans at multiple head orientations along with calculation of susceptibility through multi-orientation sampling (COSMOS) are one way to effectively mitigate this issue. Additionally, STI requires a minimum of 6 head orientations to solve for the independent tensor elements in each voxel. The requirements of high-resolution imaging with long TR at multiple orientations substantially lengthen the acquisition of COSMOS and STI. The goal of this work is to dramatically speed up susceptibility mapping at multiple head orientations. We demonstrate highly efficient acquisition using 3D-GRE with Wave-CAIPI and dramatically reduce the acquisition time of these protocols. Using R=15-fold acceleration with Wave-CAIPI permits acquisition per head orientation in 90s at 1.1mm isotropic resolution, and 5:35min at 0.5mm isotropic resolution. Since Wave-CAIPI fully harnesses the 3D spatial encoding capability of receive arrays, the maximum g-factor noise amplification remains below 1.30 at 3T and 1.12 at 7T. This allows a 30-min exam for STI with 12 orientations, thus paving the way to its clinical application.

Keywords: Parallel imaging; Phase imaging; Quantitative susceptibility mapping; Susceptibility tensor imaging; Wave-CAIPI.

Fig. 1

R=15 fold accelerated 3D-GRE with Wave-CAIPI at 3T and 7T. The large FOV (255×255×180 mm³) allows imaging of the entire brain across head orientations without moving the prescribed acquisition volume. G-factor analysis reveals high-quality parallel imaging with reduced noise amplification penalty at both field strengths.

Fig. 2

Tissue phase and susceptibility map obtained from 15-fold accelerated Wave-CAIPI acquisition with 0.5 mm isotropic resolution at 7T. High encoding efficiency yields a 5:35 min acquisition per head orientation with long TR/TE = 29/19.5 ms.

Fig. 3

Zoomed views of magnitude, phase and susceptibility reconstructions at 7T. While phase and COSMOS yield higher contrast than the magnitude signal, QSM deconvolution further mitigates the non-local dipole effects seen in the frequency maps. This provides the susceptibility images with the ability to depict the cerebellum, basal ganglia and cerebral cortex with superb contrast.

Fig. 4

Zoomed views of the thalamic substructures at 7T with 0.5 mm isotropic resolution. The nuclei visible in COSMOS reconstruction from this view are (1) medial dorsal, (2) centromedian and parafascicular, (3) ventral posterior (lateral & medial), (4) ventral lateral, and (5) intralaminar nuclei.

Fig. 5

Tissue phase and COSMOS solution from 12 orientation data acquired using 15-fold accelerated Wave-CAIPI with 1.1 mm isotropic resolution at 3T. For each orientation, this led to a 90 s scan with long TR/TE = 35/25 ms.

Fig. 6

Susceptibility Tensor Imaging analysis from 12 orientations at 3T. Tensor eigenvalues are depicted on the left, where the principal component λ₁ corresponds to the most paramagnetic eigenvalue. The average of the eigenvalues yielded the Mean Magnetic Susceptibility, while the combination λ₁−(λ₂+λ₃)/2 revealed the Magnetic Susceptibility Anisotropy.

Fig. 7

Tractography solution following the main eigenvector of the STI eigensystem at each voxel. Sagittal and coronal views are shown, where color coding indicates the directionality of the fibers. The orientations are reflected in the cube displayed in each panel.

Fig. 8

The effect of off-resonance on Wave-CAIPI acquisition is a voxel shift in the readout direction identical to conventional acquisition. (a) Conventional GRE data acquired on-resonance. (b) Conventional GRE acquired at 500 Hz off-resonance. (c) GRE acquired using Wave-CAIPI trajectory at 500 Hz off-resonance. (d) Estimated B₀ map.

Fig. 9

Parallel imaging performances of normal GRE, 2D-CAIPI and Wave-CAIPI at 3T upon R=15 fold acceleration. Wave-CAIPI reduces the maximum g-factor by more than 2 fold while incurring only 10% noise amplification on average.

Fig. 10

Mean volumes computed over 7 averages for R=15 normal GRE and Wave-CAIPI, and time-matched, fully-sampled normal GRE. Time-SNR analyses revealed improved stability and robustness in Wave-CAIPI relative to accelerated normal GRE.