Regional and tissue-specific differences in brain glutamate concentration measured by in vivo single voxel MRS

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.

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.

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.

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.

In brain cortices, Glu level is significantly higher in GM than in WM. Glu level difference between frontal lobe and occipital lobe is insignificant.