Reference values for long echo time MR spectroscopy in healthy adults.

BACKGROUND AND PURPOSE There is a lack of information with regard to normal metabolic ratios acquired with MR spectroscopy utilizing a long echo time technique. Our purpose was to measure metabolic ratios in healthy adults to determine whether the metabolites varied across brain regions and by sex. METHODS Single voxel proton spectra were acquired with an echo time of 135 milliseconds in 10 brain regions of 72 healthy subjects ranging in age from 20 to 44 years. Six gray matter sites in the cerebrum included four cortical areas in the frontal, parietal, temporal, and occipital lobes, and two deep nuclear sites in the basal ganglia and the thalamus. Two subcortical white matter regions were in the parietal and the frontal lobes. Two posterior fossa sites included the pons and the cerebellum. All 10 brain regions were not studied in each subject. For each spectrum, the metabolites n-acetylaspartate (NAA), creatine (Cr), and choline (Ch) were identified and ratios of NAA/Cr and Ch/Cr calculated for each brain region. A multifactorial analysis of variance was performed with the two metabolic ratios as dependent variables and with brain region and sex as independent variables. Post hoc statistical analysis consisted of the Scheffé F statistic for significant difference between pairs of brain regions for both metabolic ratios. RESULTS There was significant regional variation for both the NAA/Cr ratio (P < .0001) and the Ch/Cr ratio (P < .0001). The NAA/Cr ratio was consistent within cortical gray and white matter but differed between cortical gray (smaller ratio) and white matter (larger ratio). The Ch/Cr ratio was variable in the gray matter, differed between some but not all gray and white matter regions, but was consistent within subcortical white matter regions. There was no difference between men and women for either metabolic ratio. CONCLUSION There was variation of the NAA/Cr ratio and the Ch/Cr ratio across brain regions, but no sex differences were found. These findings provide the requisite normative values to use single voxel, long-echo-time MR spectroscopy in adult patients with neurologic disorders.