High-field localized 1H NMR spectroscopy in the anesthetized and in the awake monkey

Abstract

Localized cerebral in vivo 1H NMR spectroscopy (MRS) was performed in the anesthetized as well as the awake monkey using a novel vertical 7 T/60 cm MR system. The increased sensitivity and spectral dispersion gained at high field enabled the quantification of up to 16 metabolites in 0.1- to 1-ml volumes. Quantification was accomplished by using simulations of 18 metabolite spectra and a macromolecule (MM) background spectrum consisting of 12 components. Major cerebral metabolites (concentrations >3 mM) such as glutamate (Glu), N-acetylaspartate (NAA), creatine (Cr)/phosphocreatine (PCr) and myo-inositol (Ins) were identified with an error below 3%; most other metabolites were quantified with errors in the order of 10%. Metabolite ratios were 1.39:1 for total NAA, 1.38:1 for glutamate (Glu)/glutamine (Gln) and 0.09:1 for cholines (Cho) relative to total Cr. Taurine (Tau) was detectable at concentrations lower than 1 mM, while lactate (Lac) remained below the detection limit. The spectral dispersion was sufficient to separate metabolites of similar spectral patterns, such as Gln and Glu, N-acetylaspartylglutamate (NAAG) and NAA, and PCr-Cr. MRS in the awake monkey required the development and refinement of acquisition and correction strategies to minimize magnetic susceptibility artifacts induced by respiration and movement of the mouth or body. Periods with major motion artifacts were rejected, while a frequency/phase correction was performed on the remaining single spectra before averaging. In resting periods, both spectral amplitude and line width, that is, the voxel shim, were unaffected permitting reliable measurements. The corrected spectra obtained from the awake monkey afforded the reliable detection of 6-10 cerebral metabolites of 1-ml volumes.