Rationale and Objectives
At lower magnetic field strengths, regional differences of cerebral metabolite distributions have been described, but these data are controversial. Magnetic resonance spectroscopy at 7 T is expected to deliver high spectral resolution and good differentiation, but there are problems arising at high magnetic field strengths that may diminish spectral quality. Because there have been only a few studies in humans so far, there are no standard values for 7 T concerning regional metabolite distributions and concentrations.
Materials and Methods
In the present study, the metabolites detectable with 1 H magnetic resonance spectroscopy, N-acetyl-aspartate, choline, and creatine (Cr), were evaluated with a single-voxel sequence. Five voxels were placed in the frontal and parietal white matter and the insular, thalamic, and occipital gray matter.
Results
For N-acetyl-aspartate, the lowest values were found in frontal white matter and the highest in thalamic gray matter. Choline displayed the lowest values in frontal white matter and the highest in insular gray matter. Cr showed the lowest values in frontal white matter and the highest in thalamic gray matter. The highest ratio of choline to Cr was found in parietal white matter and the lowest in thalamic gray matter. The highest ratio of N-acetyl-aspartate to Cr was found in thalamic gray matter and the lowest in frontal white matter.
Conclusions
In the present study, regional cerebral metabolite differences were verified with high-field magnetic resonance spectroscopy. Quantitative values and metabolite ratios could be a basis for further clinical studies.
In prior studies at 1.5 and 3 T, regional differences of cerebral metabolite concentrations have been described, but these data are contradictory . At 7 T, cerebral regional metabolite distributions have not yet been evaluated. High-field magnetic resonance spectroscopy (MRS) has the potential to provide enhanced neurochemical information on the basis of increased sensitivity and higher spectral resolution . However, problems arising in high-field magnetic resonance imaging, such as more pronounced B 0 and B 1 inhomogeneities, may decrease spectral resolution and minimize quantitation accuracy. The present 7-T study was an attempt to clarify existing uncertainties concerning cerebral metabolite distributions with established preprocessing software and quantization algorithm .
For this purpose, the three most commonly used metabolites, N-acetyl-aspartate (NAA), choline (Cho) and creatine (Cr), were evaluated. NAA is a marker for neuronal density, integrity, and viability and resonates at a frequency of 2.02 ppm . Cho is a marker of membrane density and integrity; its resonance frequency is at 3.22 ppm. Cr, which resonates at 3.02 ppm, is a marker for energy metabolism and is assumed to be quite stable in healthy but also pathologic brain tissue .
Materials and methods
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Results
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Table 1
Results in the 10 Volunteers for Metabolite Concentrations Quantified Using the Advanced Method for Accurate, Robust, and Efficient Spectral Fitting (Arbitrary Units)
Region NAA Cho Cr Cho/Cr NAA/Cr FWM 13.60 ± 5.45 7.51 ± 2.76 7.97 ± 2.44 0.97 ± 0.33 1.67 ± 0.26 PWM 19.10 ± 3.50 9.00 ± 7.48 10.32 ± 1.88 0.94 ± 0.91 1.88 ± 0.35 IGM 12.33 ± 5.45 8.87 ± 8.46 7.73 ± 2.36 1.04 ± 0.73 1.57 ± 0.46 TGM 38.27 ± 8.60 9.38 ± 6.27 20.79 ± 4.99 0.48 ± 0.31 1.86 ± 0.26 OGM 31.57 ± 6.00 3.68 ± 2.48 18.48 ± 3.76 0.20 ± 0.10 1.73 ± 0.27
Cho, choline; Cr, creatine; FWM, frontal white matter; IGM, insular gray matter; NAA, N-acetyl-aspartate; OGM, occipital gray matter; PWM, parietal white matter; TGM, thalamic gray matter.
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Discussion
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Conclusions
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