Motion correction in magnetic resonance spectroscopy

Abstract

In vivo proton magnetic resonance spectroscopy and spectroscopic imaging (MRS/MRSI) are valuable tools to study normal and abnormal human brain physiology. However, they are sensitive to motion, due to strong crusher gradients, long acquisition times, reliance on high magnetic field homogeneity, and particular acquisition methods such as spectral editing. The effects of motion include incorrect spatial localization, phase fluctuations, incoherent averaging, line broadening, and ultimately quantitation errors. Several retrospective methods have been proposed to correct motion-related artifacts. Recent advances in hardware also allow prospective (real-time) correction of the effects of motion, including adjusting voxel location, center frequency, and magnetic field homogeneity. This article reviews prospective and retrospective methods available in the literature and their implications for clinical MRS/MRSI. In combination, these methods can attenuate or eliminate most motion-related artifacts and facilitate the acquisition of high-quality data in the clinical research setting.

Keywords: MRS; MRSI; motion; navigated spectroscopy sequence; prospective correction; retrospective correction.

Figure 1:

a) Plot of water signal vectors during head motion. Each solid line represents the water signal of a single FID in the complex plane (total 32 FIDs). The length of some of the vectors is decreased (1a), probably due to motion-induced attenuation during fast movements; b) MR spectrum from coherent (blue) and incoherent averaging (red). When using the individual signals represented in 1a (incoherent averaging), the phase of the individual FIDs is random, leading to incoherent averaging.

Figure 2:

In vivo PRESS-localized data with i) baseline acquisition with no intentional motion performed in frontal white matter (solid box); ii) motion without correction shifted the voxel and intersected with the scalp (dashed box); and iii) motion with correction using motion navigators.

Figure 3:

Changes in water frequency of a mid-frontal voxel during continuous up-down head movements (“nodding”) of about ±10 mm translation and ±10° rotation for 32 individual FIDs. The nodding caused changes in the water frequency of approximately ± 0.06 ppm

Figure 4:

Changes in water line width as a function of x-rotation (“nodding”) during an MRS scan of a mid-frontal voxel. Prospective motion correction was enabled to ensure the voxel was locked relative to the moving brain. Δfwhm: change in full-width half-maximum.