Asymmetric Hippocampal Metabolism Evaluated by Single-Voxel Spectroscopy at 7 T.

The hippocampus is known to be a key site of pathology for dementia and epilepsy, consistent with its role in memory formation. With this brain region also known to be vulnerable to metabolic injury, spectroscopic studies of the hippocampus are of relevance. However, the hippocampus is a challenging region for measurement, given its irregular small size and the high susceptibility gradients (e.g., anterior vs. posterior, medial vs. lateral) in the human temporal lobe. We acquired single-voxel spectra (3 × 2 × 1.2 cm3 encompassing the majority of the pes, body, and tail of the hippocampus) to separately measure the left and right hippocampi at 7 T using advanced RF (transceiver array with RF shimming) and B0 shimming methods (very high-order shim insert). N = 15 healthy right-handed middle-aged subjects (age 47-68 years) were recruited for this study. Based on the impact of the ApoE4 genotype on dementia risk, all subjects were genotyped as ApoE4 negative. The shim strategy resulted in equivalent Voigt linewidth (10.2 ± 2.9 Hz and 10.9 ± 3.7 Hz) and tCr SNR (27.2 ± 3.0 and 25.3 ± 4.6) for the left and right hippocampi, respectively, enabling direct comparisons. To further evaluate the spectra's accuracy for free creatine and phosphocreatine quantification, the data were fit with and without Gaussian broadening. This analysis showed that the original data shows no dependence of the creatine metabolites with the measured residual voxel field inhomogeneity σB0, whereas Gaussian broadening resulted in significant and undesirable changes in concentrations and statistically significant correlations with (the artificially broadened) σB0. The left hippocampus shows increased free creatine (3.3 ± 0.3 mM vs. 2.8 ± 0.4 mM p < 0.001) and glutamine (2.3 ± 0.2 mM vs. 2.0 ± 0.2 mM, p < 0.01) in comparison to the right. These two compounds were also found to be correlated in the left (R2 = 0.35 p < 0.05), but not in the right hippocampus. These results show that with advanced methods, 7 T spectroscopy detects neurometabolic differences between the hippocampi.