Single-voxel long TE 1H-MR spectroscopy of the normal brainstem and cerebellum

To evaluate the feasibility of single voxel 1H-MRS of the CNS structures contained in the posterior cranial fossa and to determine the distribution of the normal metabolite ratios, concentrations, and T2 relaxation times in the midbrain, pons, medulla, dentate nucleus and cerebellar vermis.

A total of 147 single voxel 1H-MR spectra with a point-resolved proton spectroscopy sequence (PRESS) sequence and echo time (TE) of 136 or 272 msec were obtained in the midbrain, pons, medulla, dentate, and vermis of 31 healthy volunteers. In seven additional patients; the concentrations and T2 relaxation times of metabolites were obtained in the same locations (except the medulla) with an external phantom calibration method and a four TE PRESS technique.

Ten (27%) of 36 spectra acquired in the medulla were of poor quality. A similar ranking of the N-acetyl aspartate (NAA)/creatine (Cr) ratio and choline(Cho)/Cr ratios in the five locations for the two TEs was observed, with the highest values in the pons (mean NAA/Cr = 4.16 +/- 0.6 and Cho/Cr =2.66 +/- 0.6 at TE 272) and the lowest values in the dentate and vermis (mean NAA/Cr = 1.66 +/- 0.2 and Cho/Cr = 1.20 +/- 0.2 at TE 272). The analysis of variance showed significant regional differences of the NAA and Cr concentrations, which had the highest values in the dentate. Non-significant regional differences were observed for the concentration of Cho and for the T2 of the metabolites.

With the exception of the medulla, single voxel 1H-MRS enables an in vivo biochemical analysis of the CNS structures contained in the posterior cranial fossa. Regional differences in the metabolite ratios and concentrations must be considered when employing 1H-MRS for evaluation of diseases of the brainstem and cerebellum.