Fast mapping of the T₂ relaxation time of cerebral metabolites using proton echo-planar spectroscopic imaging (PEPSI)

Metabolite T₂ is necessary for accurate quantification of the absolute concentration of metabolites using long-echo-time (TE) acquisition schemes. However, lengthy data acquisition times pose a major challenge to mapping metabolite T₂. In this study we used proton echo-planar spectroscopic imaging (PEPSI) at 3T to obtain fast T₂ maps of three major cerebral metabolites: N-acetyl-aspartate (NAA), creatine (Cre), and choline (Cho). We showed that PEPSI spectra matched T₂ values obtained using single-voxel spectroscopy (SVS). Data acquisition for 2D metabolite maps with a voxel volume of 0.95 ml (32 x 32 image matrix) can be completed in 25 min using five TEs and eight averages. A sufficient spectral signal-to-noise ratio (SNR) for T₂ estimation was validated by high Pearson's correlation coefficients between logarithmic MR signals and TEs (R² = 0.98, 0.97, and 0.95 for NAA, Cre, and Cho, respectively). In agreement with previous studies, we found that the T₂ values of NAA, but not Cre and Cho, were significantly different between gray matter (GM) and white matter (WM; P < 0.001). The difference between the T₂ estimates of the PEPSI and SVS scans was less than 9%. Consistent spatial distributions of T₂ were found in six healthy subjects, and disagreement among subjects was less than 10%. In summary, the PEPSI technique is a robust method to obtain fast mapping of metabolite T₂.