Concentrations and magnetization transfer ratios of metabolites in gray and white matter

The concentrations and magnetization transfer ratios (MTRs) in gray matter (GM) and white matter (WM) of N-acetyl aspartate (NAA), creatine (Cr), choline (Cho), myo-inositol (Ins), and glutamate plus glutamine (Glx) were investigated using magnetic resonance spectroscopic imaging (MRSI). The macromolecule (MM) baseline was studied separately using a metabolite-nulling inversion. Three data sets were collected from a point-resolved spectroscopy (PRESS)-selected volume (TE/TR = 30/3000 ms) of human frontal lobe in vivo: one with MT pulses applied, one with an inversion pulse to null small metabolites, and one with no inversion or MT pulses. The MM signal, which was analyzed by integrating the metabolite-nulled spectrum between 0 and 3 ppm, was estimated to be 38% higher in GM than in WM. MM subtraction decreased the signal-to-noise ratio (SNR) and also decreased the reliability of LCModel quantification of most metabolites, but may have improved the accuracy of quantification of Glx. Glx and Cr were both found to correlate strongly with the GM volume fraction of the voxels. Cr showed the highest MTR, but the other metabolites also showed some attenuation of signal when the MT pulses were applied. The MTRs did not correlate with the GM volume fraction, which implies that the local environment of metabolites does not differ markedly between GM and WM.