Non-invasive 1H NMR spectroscopy of the rat brain in vivo using a short echo time STEAM localization sequence

Fully localized proton NMR spectra were obtained from the brains of normal anaesthetized rats in vivo using stimulated echo (STEAM) spectroscopy sequences. Investigations were carried out at 2.35 T using a 40 cm bore magnet equipped with an actively shielded gradient system. Localized shimming resulted in water proton linewidths of 6.5-7.8 Hz permitting excellent water suppression. Thus, high-quality proton NMR spectra (TE = 20 ms) were acquired within measuring times of 1.5-6.4 min from 64 to 125 microL volumes-of-interest. The spectra show metabolite resonances due to N-acetyl aspartate, glutamate, creatine and phosphocreatine, cholines, taurine and inositols. The assignments of strongly spin-coupled resonances were confirmed by comparison with spectra from model solutions obtained under identical experimental conditions to those used in vivo. T1 relaxation times as well as relative metabolite concentrations were evaluated from spectra obtained for repetition times ranging from 900 to 6000 ms. Sequential acquisitions of 1.5 min spectra before, during and after killing the animals exhibited a rapid accumulation of lactate, but did not reveal significant changes in other metabolite levels for several hours post mortem.