Metabolite concentrations are valuable biomarkers in brain tumors (BTs). However, absolute quantification of metabolites using MR spectroscopy requires a correction of water relaxation using time-consuming quantitative MRI (qMRI) sequences in addition to a lengthy two-dimensional spectroscopic water-reference acquisition. The goal of this work was to develop and validate a fast quantification method where a two-dimensional spectroscopic water reference is obtained using qMRI and a single-voxel stimulated-echo acquisition mode (STEAM) sequence. The semi-adiabatic localization by adiabatic selective refocusing (sLASER) sequence was used for MR spectroscopy imaging (MRSI) acquisition. A single-voxel unsuppressed water signal was acquired using a STEAM sequence. A qMRI protocol was also acquired, and the H2O map was calibrated based on the STEAM signal to obtain the spectroscopic water reference (proposed method). Five healthy volunteers and one BT patient were scanned at 3 T. Concentrations obtained using the proposed and two reference methods-one where water-relaxation effects were corrected using literature values (reference method) and one where they were corrected using qMRI-derived values (reference method with qMRI)-were compared. In healthy subjects, white-matter metabolite concentrations obtained using water relaxation using literature values (reference method) significantly differed from those using individual-specific corrections (reference method with qMRI and proposed method). Bland-Altman analyses revealed a very low bias and standard deviation of the differences between the reference method with qMRI and the proposed method (bias < 0.5% and standard deviation < 10%). The BT regions showed an approximate 35% underestimation of metabolite concentrations using the reference method. For metabolite quantification, accurate water referencing with individual-specific corrections for water relaxation times was obtained in 8 min using the proposed method.
Thomas et al. (Wed,) studied this question.