OBSERVING N-ACETYL ASPARTATE VIA BOTH ITS N-ACETYL AND ITS STRONGLY COUPLED ASPARTATE GROUPS IN IN-VIVO PROTON MAGNETIC-RESONANCE SPECTROSCOPY

The similar to 2.6 ppm aspartate multiplet of N-acetyl aspartate (NAA) is considered a potential source of additional information on N-acety l aspartate in vivo. Because the aspartate multiplet is the AB part of a strongly coupled ABX system it gives rise, as is shown in the analy sis presented, to a significant field-strength dependence in the echo- time-dependent modulations of the response to typical spatial-localiza tion sequences. The echo-time dependence of this response is developed analytically, not only for the STEAM and the PRESS localization seque nces, but also for a spin-echo sequence. It is then verified experimen tally at 2.35 T. The field-strength dependence of the response is demo nstrated by evaluating the changes in the echo-time-dependent response s to each of the three sequences at field strengths of 1.5, 2.35, and 4.0 T. By means of these results, the preferred sequence (PRESS) can b e optimized for the NAA aspartate multiplet at each field strength, as is illustrated with the human brain spectra obtained in vivo at 1.5 T . These in vivo spectra compare the optimal, long TE timing (163 ms) w ith a suboptimal TE (70 ms), for the observation of the similar to 2.6 ppm aspartate resonances of NAA. (C) 1996 Academic Press, Inc.