Mekle Ralf, Kühn Simone, Pfeiffer Harald, Aydin Semiha, Schubert Florian, Ittermann Bernd
Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, and Berlin, Germany.
Center for Stroke Research Berlin (CSB), Charité - Universitätsmedizin Berlin, Berlin, Germany.
NMR Biomed. 2017 Feb;30(2). doi: 10.1002/nbm.3672. Epub 2016 Dec 23.
The two-fold benefit of H magnetic resonance spectroscopy (MRS) at high B fields - enhanced sensitivity and increased spectral dispersion - has been used previously to study dynamic changes in metabolite concentrations in the human brain in response to visual stimulation. In these studies, a strong visual on/off stimulus was combined with MRS data acquisition in a voxel location in the occipital cortex determined by an initial functional magnetic resonance imaging experiment. However, 1) to exclude the possibility of systemic effects (heartbeat, blood flow, etc.), which tend to be different for on/off conditions, a modified stimulation condition not affecting the target voxel needs to be employed, and 2) to assess important neurotransmitters of low concentration, in particular γ-aminobutyric acid (GABA), it may be advantageous to analyze steady-state, rather than dynamic, conditions. Thus, the aim of this study was to use short-TE H MRS methodology at 7 T to detect differences in steady-state metabolite levels in response to a varying stimulation paradigm in the human visual cortex. The two different stimulation conditions were termed voxel and control activation. Localized MR spectra were acquired using the SPECIAL (spin-echo full-intensity acquired localized) sequence. Data were analyzed using LCModel. Fifteen individual metabolites were reliably quantified. On comparison of steady-state concentrations for voxel versus control activation, a decrease in GABA of 0.05 mmol/L (5%) and an increase in lactate of 0.04 mmol/L (7%) were found to be the only significant effects. The observed reduction in GABA can be interpreted as reduced neuronal inhibition during voxel activation, whereas the increase in lactate hints at an intensification of anaerobic glycolysis. Differences from previous studies, notably the absence of any changes in glutamate, are attributed to the modified experimental conditions. This study demonstrates that the use of advanced H MRS methodology at 7 T allows the detection of subtle changes in metabolite concentrations involved in neuronal activation and inhibition.
高磁场下氢磁共振波谱（MRS）的双重优势——增强的灵敏度和增加的谱线分散度——此前已被用于研究人类大脑中代谢物浓度随视觉刺激的动态变化。在这些研究中，强烈的视觉开/关刺激与在枕叶皮质体素位置的MRS数据采集相结合，该体素位置由初始功能磁共振成像实验确定。然而，1）为了排除全身效应（心跳、血流等）的可能性，开/关条件下这些效应往往不同，需要采用不影响目标体素的改良刺激条件，并且2）为了评估低浓度的重要神经递质，特别是γ-氨基丁酸（GABA），分析稳态而非动态条件可能更有利。因此，本研究的目的是使用7T的短回波时间氢MRS方法来检测人类视觉皮质中不同刺激范式下稳态代谢物水平的差异。两种不同的刺激条件分别称为体素激活和对照激活。使用SPECIAL（自旋回波全强度采集定位）序列采集局部磁共振波谱。数据使用LCModel进行分析。十五种个体代谢物得到可靠定量。比较体素激活与对照激活的稳态浓度时，发现GABA降低0.05 mmol/L（5%）和乳酸增加0.04 mmol/L（7%）是仅有的显著效应。观察到的GABA降低可解释为体素激活期间神经元抑制作用减弱，而乳酸增加暗示无氧糖酵解增强。与先前研究的差异，特别是谷氨酸没有任何变化，归因于改良的实验条件。本研究表明，在7T使用先进的氢MRS方法能够检测与神经元激活和抑制相关的代谢物浓度的细微变化。
