Fast water concentration mapping to normalize (1)H MR spectroscopic imaging

To propose a fast and robust acquisition and post-processing pipeline that is time-compatible with clinical explorations to obtain a proton density (ρ) map used as a reference for metabolic map normalization. This allows inter-subject and inter-group comparisons of magnetic resonance spectroscopic imaging (MRSI) data and longitudinal follow-up for single subjects.

A multi-echo T 2 (*) mapping sequence, the XEP sequence for B 1 (+) -mapping and Driven Equilibrium Single Pulse Observation of T 1-an optimized variable flip angle method for T 1 mapping used for both B 1 (-) -mapping and M 0 calculation-were used to determine correction factors leading to quantitative water proton density maps at 3T. Normalized metabolite maps were obtained on a phantom and nine healthy volunteers. To show the potential use of this technique at the individual level, we also explored one patient with low-grade glioma.

Accurate ρ maps were obtained both on phantom and volunteers. After signal normalization with the generated ρ maps, metabolic concentrations determined by the present method differed from theory by <7 % in the phantom and were in agreement with data from the literature for the healthy controls. Using these normalized metabolic values, it was possible to demonstrate in the patient with brain glioma, metabolic abnormalities in normalized N-acetyl aspartate, choline and creatine levels; illustrating the potential for direct use of this technique in clinical studies.

The proposed combination of sequences provides a robust ρ map that can be used to normalize metabolic maps in clinical MRSI studies.