Off-resonance metabolite magnetization transfer measurements on rat brain in situ

Off-resonance metabolite magnetization transfer (MT) experiments were performed on rat brain in vivo and post mortem, with short (18 msec) and long (144 msec) echo-time 1H nuclear magnetic resonance (NMR) spectroscopy. In vivo and post mortem, the methyl protons of total creatine and all protons from glutamate/glutamine showed a strong MT effect on off-resonance saturation, as well as the methyl protons from lactate post mortem. Other resonances, like that of A-acetyl aspartate, showed a much smaller, but detectable, MT effect. The results obtained were confirmed by combining off-resonance saturation with two-dimensional correlation spectroscopy. Three water suppression techniques, i.e., presaturation, chemical shift-selective (CHESS), and selective water eliminated Fourier transform (WEFT) were evaluated for their ability to generate an MT effect, to assess their possible influence on metabolite quantification. Presaturation and selective WEFT led to alterations of the total creatine, lactate, and N-acetyl aspartate resonance intensities, while CHESS had no effect. Finally, it was shown that water protons play an important role in the generation of the observed metabolite MT effects.