MR spectroscopy using static higher order shimming with dynamic linear terms (HOS‐DLT) for improved water suppression, interleaved MRS‐fMRI, and navigator‐based motion correction at 7T

Abstract

Purpose: To interleave global and local higher order shimming for single voxel MRS. Single voxel MR spectroscopy requires optimization of the B 0 field homogeneity in the region of the voxel to obtain a narrow linewidth and provide high data quality. However, the optimization of local higher order fields on a localized MRS voxel typically leads to large field offsets outside that volume. This compromises interleaved MR sequence elements that benefit from global field homogeneity such as water suppression, interleaved MRS-fMRI, and MR motion correction. Methods: A shimming algorithm was developed to optimize the MRS voxel homogeneity and the whole brain homogeneity for interleaved sequence elements, using static higher order shims and dynamic linear terms (HOS-DLT). Shimming performance was evaluated using 6 brain regions and 10 subjects. Furthermore, the benefits of HOS-DLT was demonstrated for water suppression, MRS-fMRI, and motion corrected MRS using fat-navigators. Results: The HOS-DLT algorithm was shown to improve the whole brain homogeneity compared to an MRS voxel-based shim, without compromising the MRS voxel homogeneity. Improved water suppression over the brain, reduced image distortions in MRS-fMRI, and improved quality of motion navigators were demonstrated using the HOS-DLT method. Conclusion: HOS-DLT shimming allowed for both local and global field homogeneity, providing excellent MR spectroscopy data quality, as well as good field 1102 | BOER Et al. K E Y W O R D S1 H MR spectroscopy, B 0 shimming, EPI distortions, prospective motion correction, ultra-high field, water suppression

Motion correction methods for MRS: experts' consensus recommendations

“…Several other image‐based navigator techniques have been proposed, including fat navigators, 58 collapsed fat navigators, 59 and iNavs (image navigators), 60 but these techniques have not yet been extended to include field maps. The use of fat navigators has been recently shown at 7T for motion correction of SVS in combination with dynamic update of linear shim terms 61 …”

“…for motion correction of SVS in combination with dynamic update of linear shim terms. 61 The use of image-based navigators may be more challenging at ultra-high field. For instance, vNavs are based on fast EPI readout, which may suffer geometric distortions and other imaging artifacts.…”

Estimation and Removal of spurious echo Artifacts in single-voxel MRS using Sensitivity Encoding

“…In addition, due to the use of localized high-order shimming, brain regions remote from the target voxel location often showed large amounts of unsuppressed water, often including the frontal or parietal lobes, and posterior fossa. These regions could potentially be reduced by the use of dynamic B 0 shimming, with a globally optimized shim set used during water suppression, and a regionally optimized shim set during data acquisition (24,25). WSI could potentially also be used online during scanning to plan acquisition-specific parameters such as order of slice selection gradients or OVS bands,to further reduce these artifacts.…”

B0 shimming for in vivo magnetic resonance spectroscopy: Experts' consensus recommendations

“…This is because local field homogeneity is desired to achieve narrow linewidth for signals originating from the targeted MRS volume. At the same time, larger scale homogeneity is desired for optimal performance of frequency‐selective radio frequency (RF) pulses for water suppression, lipid suppression or spectral editing across the MRS volume and beyond 13,14 . Thus, MRSI in the frontal and temporal lobes remains challenging due to the necessity to obtain homogeneous B 0 conditions in these areas and their surroundings (Figure 1).…”

“…At the same time, larger scale homogeneity is desired for optimal performance of frequency-selective radio frequency (RF) pulses for water suppression, lipid suppression or spectral editing across the MRS volume and beyond. 13,14 Thus, MRSI in the frontal and temporal lobes remains challenging due to the necessity to obtain homogeneous B 0 conditions in these areas and their surroundings (Figure 1). Lastly, residual eddy currents originating from the gradient or shim system can have a severe impact on the spectral quality (Figure 1E).…”