title: Regional and tissue-specific differences in brain glutamate concentration measured by in vivo single voxel MRS.
abstract: BACKGROUND
  There is growing interest in characterizing spatial distribution of glutamate (Glu) in brain disorders. Comparing the differences in Glu concentration using magnetic resonance spectroscopy (MRS) is hampered by the confounding effects of different anatomical regions and tissue composition.
  
  
  METHODS
  Effect of tissue composition on Glu concentrations was studied by selecting closely adjacent voxels within a designated cortical region. Glu regional differences were assessed using voxels comprising essentially the same tissue composition from different cortical regions.
  
  
  RESULTS
  Using point-resolved-spectroscopy (PRESS)-based averaged echo time method, Glu concentration in the anterior cingulate cortex (ACC) was found to correlate strongly with tissue gray matter (GM) fraction (r=0.87, p=10(-5)). No significant regional difference in Glu concentration was found between frontal and occipital lobes (p=0.23) when the two measured voxels had essentially the same tissue composition.
  
  
  METHODS
  The method of the current study is aimed to circumvent the difficulties in differentiating anatomical region from tissue composition, given that both can lead to Glu variations in brain. Glu concentration versus tissue composition was measured in the same anatomical region, while the comparison of regional differences was performed with the two regions that had essentially the same tissue composition.
  
  
  CONCLUSIONS
  In brain cortices, Glu level is significantly higher in GM than in WM. Glu level difference between frontal lobe and occipital lobe is insignificant.
authors.affiliation: MR Spectroscopy Core Facility, National Institute of Mental Health, National Institutes of Health, Bethesda, MD 20892, USA. Electronic address: zhangya@mail.nih.gov.
authors.forename: Yan
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: MR Spectroscopy Core Facility, National Institute of Mental Health, National Institutes of Health, Bethesda, MD 20892, USA; Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Bethesda, MD 20892, USA.
authors.forename: Jun
authors.initials: J
authors.lastname: Shen
mesh_terms: D000328:Adult; D002540:Cerebral Cortex / Q000033:anatomy & histology* / Q000378:metabolism*; D005260:Female; D018698:Glutamic Acid / Q000378:metabolism*; D005973:Glutamine / Q000378:metabolism; D006801:Humans; D008279:Magnetic Resonance Imaging; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008297:Male; D008875:Middle Aged; D013223:Statistics as Topic; D055815:Young Adult
chemical_list: D005973:Glutamine; D018698:Glutamic Acid
keywords: (1)H MRS; Averaged echo time; Glutamate; Human brain; Metabolite quantification; Regional difference; Tissue composition