Regional Neurochemical Profiles in the Human Brain measured by 1H MRS at 7 Tesla using Local B1 Shimming

• Published in: NMR in biomedicine 2011-07, Vol.25 (1), p.152-160
• Main Authors: Emir, Uzay E., Auerbach, Edward J., Van De Moortele, Pierre-Francois, Marjańska, Małgorzata, Uğurbil, Kamil, Terpstra, Melissa, Tkáč, Ivan, Öz, Gülin
• Format: Article
• Language: English
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.1727

Increased sensitivity and chemical shift dispersion at ultra-high magnetic fields (UHF) enable precise quantification of an extended range of brain metabolites from 1 H MR spectra. However, all previous neurochemical profiling studies using single-voxel MRS at 7 T were limited to data acquired from the occipital lobe with half-volume coils. Challenges of 1 H MRS of the human brain at 7 T include short T 2 and complex B 1 distribution that imposes limitations in maximum achievable B 1 strength. In this study, the feasibility of acquiring and quantifying short-echo (TE = 8 ms), single voxel 1 H MR spectra from multiple brain regions was demonstrated by utilizing a 16-channel transceiver array coil with 16 independent transmit channels allowing local transmit B 1 (B 1 + ) shimming. Spectra were acquired from volumes-of-interest of 1 – 8 mL in brain regions that are of interest for various neurological disorders: frontal white matter, posterior cingulate, putamen, substantia nigra, pons and cerebellar vermis. Local B 1 + shimming substantially increased transmit efficiency, especially in the peripheral and ventral brain regions. By optimizing a STEAM sequence for utilization with a 16-channel coil, artifact-free spectra were acquired with a small chemical shift displacement error (< 5% /ppm/direction) from all regions. The high SNR enabled the quantification of neurochemical profiles consisting of at least 9 metabolites including GABA, glutamate and glutathione in all brain regions. Significant differences in neurochemical profiles were observed between brain regions. For example, GABA levels were highest in the substantia nigra, total creatine highest in the cerebellar vermis and total choline highest in the pons, consistent with known biochemistry of these regions. These findings demonstrate that single voxel 1 H MRS at UHF can reliably detect region-specific neurochemical patterns in the human brain and has the potential to objectively detect alterations in neurochemical profiles associated with neurological diseases.