Absolute quantification of cerebral metabolites using two-dimensional proton MR spectroscopic imaging with quantitative MRI-based water reference.

Purpose: 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.

Methods: 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.

Results: 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.

Conclusion: For metabolite quantification, accurate water referencing with individual-specific corrections for water relaxation times was obtained in 8 min using the proposed method.