Observing N-acetyl aspartate via both its N-acetyl and its strongly coupled aspartate groups in in vivo proton magnetic resonance spectroscopy.

The approximately 2.6 ppm aspartate multiplet of N-acetyl aspartate (NAA) is considered a potential source of additional information on N-acetyl 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 analysis presented, to a significant field-strength dependence in the echo-time-dependent modulations of the response to typical spatial-localization sequences. The echo-time dependence of this response is developed analytically, not only for the STEAM and the PRESS localization sequences, but also for a spin-echo sequence. It is then verified experimentally at 2.35 T. The field-strength dependence of the response is demonstrated by evaluating the changes in the echo-time-dependent responses 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 be 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) with a suboptimal TE (70 ms), for the observation of the approximately 2.6 ppm aspartate resonances of NAA.