LAUSR

Magnetic resonance spectroscopy (MRS) provides a powerful, in vivo method for evaluating the metabolic profile of the brain or other organs, noninvasively. Metabolite levels can be estimated from the size of the characteristic peaks (or sets of peaks) for each metabolite, but in order to derive quantitative estimates of the metabolite levels, it is necessary to calibrate the signal from each metabolite against that from a reference standard. A number of calibration methods have been suggested for MRS, but the two most widely used methods involve scaling either to the unsuppressed water peak or to the signal arising from the Creatine peak. There is an ongoing debate about the best calibration standard to use for quantitative MRS, but each has its merits, and it may be the case that different scaling methods might be preferred to address different clinical or research questions. In the present response to the commentary by Ostojic, we aim to discuss the relative merits of each method within the context of pediatric MRS studies, and particularly of our study published recently in Pediatric Research entitled “Altered brain metabolism contributes to executive function deficits in school-aged children born very preterm.” From a technical standpoint, the localization methods used for the Point RESolved Spectroscopy (PRESS) MRS sequence cause a chemical shift displacement (CSD) error, whereby the signals from different metabolites arise from different locations, shifted by a certain distance from the prescribed voxel. In practice, only the signal from one specific metabolite (typically N-acetyl aspartate (NAA) or sometimes an intermediate frequency between NAA and Creatine) originates from the intended location of the selected voxel of interest, as displayed on the magnetic resonance imaging (MRI) console. The signal for all other metabolites is displaced by a degree dependent on the difference in the spectral frequency of each metabolite from the reference frequency (e.g., of NAA or between NAA and Creatine). A recent methodological consensus paper from the Institute for Magnetic Resonance in Medicine (ISMRM) MRS study group discusses this issue in detail, and Fig. 2 of this consensus paper shows an example of the CSD error in practice. Since the chemical shift of water (4.7 ppm) is further removed from the chemical shift of most of the other neurometabolites, Creatine scaling is typically associated with a smaller CSD error than water scaling, for most metabolites apart from myo-inositol, whose multiple peaks lie between the Cr and water peaks but are slightly closer to the water peak. The CSD error increases with increasing magnetic field strength and is therefore more prominent at 3 T than at 1.5 T but is considerably improved with more recent MRS pulse sequences like the semiLaser sequence. In addition to the reduced CSD error for most metabolites, one advantage of Creatine scaling is that the Creatine signal originates from the same (water-suppressed) metabolite subspectra as the other neuro-metabolites, and the data are therefore acquired contemporaneously, while the unsuppressed water signal is either acquired in a separate scan or at the beginning or end of the MRS acquisition. Subject motion may therefore cause the water signal to be acquired from a different location, although it can also degrade the spectral quality overall, depending on the degree and duration of motion and when it occurs. For pediatric populations prone to motion during the scan, Creatine scaling is less sensitive to motion between the water-suppressed subspectra (the metabolite lines) and the water lines. However, a disadvantage of Creatine scaling in comparison to water scaling is that the Creatine signal is considerably smaller than the water signal (by a factor of 10,000) and therefore shows higher variability due to its lower signal-to-noise ratio. Due to the lower variability of the water signal, water-scaled concentrations can be more reproducible and hence are sensitive to more subtle changes. Another advantage of water scaling, as discussed in the commentary by Ostojic, is that it removes ambiguity in ascribing observed changes to the metabolite of interest (in the numerator of the Creatine ratio) or to the effects of Creatine in the denominator. Previous studies in both adults and children have shown that, while Creatine is arguably the most stable of the neurometabolites, it demonstrates significant changes with development and in the presence of pathology. This is an important point affecting MRS studies in pediatric patient groups and should be considered carefully when interpreting the results. As discussed in the commentary by Ostojic, previous studies have shown specific alterations in Creatine metabolism in prematurity, so it is possible that the Creatine signal may be altered in children and adolescents born very preterm. However, it is important to note that water-scaled concentrations are also effectively ratios to the water signal, which also shows developmental changes, as both the brain water concentration and the relaxation times change with age, and the unsuppressed water signal measured with MRS is sensitive to both of these effects. In addition to showing developmental effects, the relaxation times can also show persistent differences in children and adolescents born very preterm. In a previous study, we observed significantly increased