In vivo ¹H NMR spectroscopy of rat brain at 1 ms echo time

Using optimized, asymmetric radiofrequency (RF) pulses for slice selection, the authors demonstrate that stimulated echo acquisition mode (STEAM) localization with ultra-short echo time (1 ms) is possible. Water suppression was designed to minimize sensitivity to B₁ inhomogeneity using a combination of 7 variable power RF pulses with optimized relaxation delays (VAPOR). Residual water signal was well below the level of most observable metabolites. Contamination by the signals arising from outside the volume of interest was minimized by outer volume saturation using a series of hyperbolic secant RF pulses, resulting in a sharp volume definition. In conjunction with FASTMAP shimming (Gruetter Magn Reson Med 1993;29: 804- 811), the short echo time of 1 msec resulted in highly resolved in vivo ¹H nuclear magnetic resonance spectra. In rat brain the water linewidths of 11- 13 Hz and metabolite singlet linewidths of 8-10 Hz were measured in 65 μl volumes. Very broad intense signals (Δν( 1/2 ) > 1 kHz), as expected from membranes, for example, were not observed, suggesting that their proton T₂ are well below 1 msec. The entire chemical shift range of ¹H spectrum was observable, including resolved resonances from alanine, aspartate, choline group, creatine, GABA, glucose, glutamate, glutamine, myo-inositol, lactate, N-acetylaspartate, N-acetylaspartylglutamate, phosphocreatine, and taurine. At 9.4 T, peaks close to the water were observed, including the H-1 of α-D- glucose at 5.23 ppm and a tentative H-1 resonance of glycogen at 5.35 ppm.