Motion correction methods for MRS: experts' consensus recommendations

Abstract

Long acquisition times due to intrinsically low signal-to-noise ratio and the need for highly homogeneous B₀ field make MRS particularly susceptible to motion or scanner instability compared with MRI. Motion-induced changes in both localization and shimming (ie B₀ homogeneity) degrade MRS data quality. To mitigate the effects of motion three approaches can be employed: (1) subject immobilization, (2) retrospective correction, and (3) prospective real-time correction using internal and/or external tracking methods. Prospective real-time correction methods can simultaneously update localization and the B₀ field to improve MRS data quality. While localization errors can be corrected with both internal (navigators) and external (optical camera, NMR probes) tracking methods, the B₀ field correction requires internal navigator methods to measure the B₀ field inside the imaged volume and the possibility to update the scanner shim hardware in real time. Internal and external tracking can rapidly update the MRS localization with submillimeter and subdegree precision, while scanner frequency and first-order shims of scanner hardware can be updated by internal methods every sequence repetition. These approaches are most well developed for neuroimaging, for which rigid transformation is primarily applicable. Real-time correction greatly improves the stability of MRS acquisition and quantification, as shown in clinical studies on subjects prone to motion, including children and patients with movement disorders, enabling robust measurement of metabolite signals including those with low concentrations, such as gamma-aminobutyric acid and glutathione. Thus, motion correction is recommended for MRS users and calls for tighter integration and wider availability of such methods by MR scanner manufacturers.

Keywords: MRS; Metabolites-neurochemistry; NMR probes; motion correction; navigator; optical tracking; real time; shim correction.

Figure 1.

Prospective real-time motion correction for MRS. Left panel: External (optical camera) and internal (navigator) tracking systems. Note that NMR probes use a combination of external and internal tracking. Middle Panel: Localization update is possible with both external and internal tracking, however, B₀ shimming is possible only with navigators with B₀ field mapping. Right panel: Examples of MR spectra under stationary conditions and in the presence of motion with no correction (NoCo), motion correction only (MoCo), and with both motion and shim correction (ShMoCo). Decreased SNR and line broadening/splitting are noticed in NoCo spectra. MoCo recovers partially the SNR and linewidth, while the full spectral quality is restored only if both motion and shim corrections (ShMoCo) are performed.

Figure 2.

Real-time motion and B₀ field correction for MEGA edited MRS of GABA. Subtraction artifacts are eliminated with re-acquisition of corrupted TRs.

Figure 3.

Applications of real-time motion and shim correction for edited MRSI of metabolites: i) oncometabolite D-2-hydroxyglutarate (2HG) in mutant IDH glioma; ii) antioxidant glutathione (GSH) in amyotrophic lateral sclerosis; iii) inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in Parkinson’s disease. Arrows indicate regions with abnormal metabolite levels in metabolic maps. Examples of edited spectra are shown at the bottom.