Chen W, Zhu X H, Gruetter R, Seaquist E R, Adriany G, Ugurbil K
Center for Magnetic Resonance Research, Radiology Department, University of Minnesota School of Medicine, Minneapolis, Minnesota 55455, USA.
Magn Reson Med. 2001 Mar;45(3):349-55. doi: 10.1002/1522-2594(200103)45:3<349::aid-mrm1045>3.0.co;2-8.
The relationships between brain activity and accompanying hemodynamic and metabolic alterations, particularly between the cerebral metabolic rate of oxygen utilization (CMR(O2)) and cerebral blood flow (CBF), are not thoroughly established. CMR(O2) is closely coupled to the rate of tricarboxylic acid (TCA) cycle flux. In this study, the changes in glutamate labeling during (13)C labeled glucose administration were determined in the human brain as an index of alterations in neuronal TCA cycle turnover during increased neuronal activity. Two-volume (1)H-[(13)C] MR spectroscopy (MRS) of the visual cortex was combined with functional MRI (fMRI) at 4 Tesla. Hemifield visual stimulation was employed to obtain data simultaneously from activated and control regions located symmetrically in the two hemispheres of the brain. The results showed that the fractional change in the turnover rate of C4 carbon of glutamate was less than that of CBF during visual stimulation. The fractional changes in CMR(O2) (Delta CMR(O2)) induced by activation must be equal to or less than the fractional change in glutamate labeling kinetics. Therefore, the results impose an upper limit of approximately 30% for Delta CMR(O2) and demonstrate: 1) that fractional CBF increases exceed Delta CMR(O2) during elevated activity in the visual cortex, and 2) that such an unequal change would explain the observed positive blood oxygenation level dependent (BOLD) effect in fMRI. Magn Reson Med 45:349-355, 2001.
大脑活动与伴随的血液动力学和代谢改变之间的关系，尤其是脑氧代谢率（CMR(O2)）和脑血流量（CBF）之间的关系，尚未完全明确。CMR(O2)与三羧酸（TCA）循环通量速率紧密相关。在本研究中，在人类大脑中测定了给予（13）C标记葡萄糖期间谷氨酸标记的变化，作为神经元活动增加期间神经元TCA循环周转率改变的指标。在4特斯拉下，将视觉皮层的双容积（1）H-[(13)C]磁共振波谱（MRS）与功能磁共振成像（fMRI）相结合。采用半视野视觉刺激，同时从位于大脑两个半球对称位置的激活区和对照区获取数据。结果表明，视觉刺激期间谷氨酸C4碳周转率的分数变化小于CBF的分数变化。激活诱导的CMR(O2)（ΔCMR(O2)）分数变化必须等于或小于谷氨酸标记动力学的分数变化。因此，结果为ΔCMR(O2)设定了约30%的上限，并证明：1）在视觉皮层活动增强期间，CBF分数增加超过ΔCMR(O2)；2）这种不平等变化可以解释功能磁共振成像中观察到的正血氧水平依赖（BOLD）效应。《磁共振医学》45:349 - 355，2001年。
