IMAGE-DIRECTED PROTON SPECTROSCOPY OF GERBIL BRAIN AT 7-TESLA

Image directed localized H-1 NMR spectra were obtained at 7 T (300 MHz ) from cubic volumes of less than 40 mu L in the gerbil brain. Signals from cerebral metabolites commonly detected in other rodent species w ere easily assigned, and high resolution spectroscopy (360 MHz) of aqu eous brain extracts assisted the tentative identification of partially overlapping resonances from lower concentration compounds like alanin e, lysine, gamma-aminobutyrate, valine, leucine and isoleucine. Weak c oupling at 7 T was manifest in the resolution of signals from the gamm a-CH2 groups of glutamine and glutamate. Down-held of water, signals a ssigned to purine nucleotides were conspicuous in the extract spectra, but localized spectra acquired routinely in vivo, using selective exc itation and gradient crushing (SUBMERGE) for water suppression, exhibi ted little or no signal from purines. When localized in vivo spectra w ere acquired without water suppression, however, or using a low power binomial excitation sequence rather than SUBMERGE, a broad signal appe ared at the resonant frequency of purine aromatic protons. NMR experim ents on the nucleotide adenosine 5'-monophosphate (AMP) in 90% glycero l/10% D2O solution demonstrated that pre-irradiation of the water sign al even for less than 100 ms attenuated the nucleotide signal apprecia bly, This implies that the soft pulses required for selective excitati on of water in sequences such as SUBMERGE induce spin-diffusion which eliminates or diminishes the signal from nucleotides in vivo.