Motion correction methods for MRS: experts' consensus recommendations

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. MRS data quality is severely degraded by motion due to long scan times, low signal‐to‐noise ratio, the need for highly homogeneous B0 field to resolve spectral overlap, and the strong coupling between localization and B0 homogeneity. Effects of motion can be mitigated by retrospective correction, and prospective real‐time correction using internal and/or external tracking methods. The best MRS data quality can be obtained by prospective real‐time motion correction, which simultaneously updates localization and the B0 field.