MRS of pilocytic astrocytoma: The peak at 2 ppm may not be NAA

Abstract

Purpose: To determine whether the chemical shift of residual N-acetylaspartate (NAA) signal in pilocytic astrocytomas (PA) is consistent with the position of the NAA peak in controls. Methods: MR spectra from 27 pediatric World Health Organization (WHO) grade I pilocytic astrocytoma patients, fifteen patients with WHO grade II and high-grade (III–IV) astrocytomas, and 36 controls were analyzed. All spectra were acquired with a short echo time (35 ms), single voxel point-resolved spectroscopy sequence on clinical 3 tesla scanners. Fully automated LCModel software was used for processing, which included the fitting of peak positions for NAA and creatine (Cr). Results: The chemical shift difference between the NAA and Cr peaks was significantly smaller (by 0.016 ± 0.005 parts per million, P < 1e–10) in PAs than in controls and was also smaller than what was observed in infiltrative astrocytomas. Conclusion: The chemical shift position of the residual NAA peak in PAs is not consistent with NAA. The signal likely originates from an N-acetyl group of one or more other chemicals such as N-acetylated sugars. Magn Reson Med 78:452–456, 2017. © 2016 International Society for Magnetic Resonance in Medicine.