Quantitative proton magnetic resonance spectroscopy of human precentral gyrus and hippocampus. Absolute concentrations of metabolites.

P magnetic resonance spectroscopy (1H-MRS) provides a noninvasive tool for assessing brain metabolites in vivo. The precentral gyrus (PG), which functions as the primary motor cortex,1 is considered relevant to some diseases such as amyotrophic lateral sclerosis (ALS), whilst, the hippocampus is a brain center that is sensitive to the effects of stress exposure, and has been demonstrated to be affected in a variety of disorders.2 Currently, relative quantification (ratio) has gained popularity in quantifying metabolite concentrations for its technical ease of measurement. This approach is based on the assumption that total creatine (tCr) concentration is both relatively constant throughout the brain and resistant to change. However, this viewpoint has been challenged by some findings, which revealed Cr distribution discrepancy, age-related changes, and alterations under pathological conditions. Furthermore, it is hard to determine which metabolite alters when observing a change in the ratio. In consideration of these drawbacks, a more objective and accurate methodology is required. A new developing approach named absolute quantification meets this requirement. The objective of this experiment is to study the metabolic characteristics and absolute concentrations of the PG and hippocampus using 1HMRS with LCModel software (S-Provencher Company, Oakville, Ontario, Canada).3 This study was carried out in the Medical Imaging Department of the 2nd Affiliated Hospital of Shantou University Medical College, Shantou, China between December 2008 and July 2009. Fourteen healthy adults (5 women and 9 men; age range 23-29 years old, mean 26 years) were enrolled in this study. All of them are right-handed and clinically asymptomatic. They all first underwent MRI study. Only those with normal MRI proceeded to MRS. No one was rejected from the study. The local ethics committee approved the study, and all volunteers gave informed consent. The MR study was performed on a 1.5T GE Signa HDX scanner (GE Healthcare, Wauwatosa, Wisconsin, USA) with the standard head coil. Routine MR imaging including transverse, sagittal, and coronal scanning were obtained using Fast Gradient Echo (FGRE) (TR 5.4msec, TE 1.6msec). Localized proton spectra were acquired using stimulated-echo acquisition mode (STEAM) sequence (TR 3000 ms, TE 20 ms, TM 13.7ms). Voxels (15×15×20 mm) were located in the left PG and hippocampus. We used 3 planes to determine the volumes of interest. Voxel of the PG was placed anterior to the central sulcus, while voxel selection of hippocampus was based on the maximal structure displaying. The full width at half maximum was less than 5 Hz, and water suppression reached 98%. After acquisition, MRS data were processed using the LCModel software. Absolute metabolic concentrations of N-acetylaspartate (NAA), total creatine (tCr), total choline (tCho), myoinositol (mI), Glx (glutamate [Glu] + glutamine [Gln]) were measured. Using the Statistical Package for Social Sciences (SPSS Inc, Chicago, IL, USA) version 10, the pairedsamples t-test was used to compare the metabolic concentrations between the PG and hippocampus. Bivariate correlations and linear regression analysis were performed to determine whether some metabolites were correlated with Cr levels. Statistical significance was set at p<0.05. The metabolic concentrations that were calculated from the STEAM spectral data are presented in Table 1. Results showed significantly lower NAA level and higher Cho and mI in the hippocampus than in the PG, while no significant difference for Cr and GLX concentrations in these 2 locations was observed. Linear regression analysis showed a linear relation between NAA and Cr both in the PG and hippocampus (PG, Pearson correlation coefficient: r=0.664, and p=0.013; hippocampus, r=0.632, p=0.015). Our study was consistent with the result of Geurts et al,4 whose quantitative study was performed with Brief Communication