Study of tricarboxylic acid cycle flux changes in human visual cortex during hemifield visual stimulation using 1H-{13C} MRS and fMRI

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The relationships between brain activity and accompanying hemodynamic and metabolic alterations, particularly between the cerebral metabolic rate of oxygen utilization (CMRO2) and cerebral blood flow (CBF), are not thoroughly established. CMRO2 is closely coupled to the rate of tricarboxylic acid (TCA) cycle flux. In this study, the changes in glutamate labeling during 13C 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 1H-{13C} 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 dur ng visual stimulation. The fractional changes in CMRO2 (δCMRO2) 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 ∼30% for δCMRO2 and demonstrate: 1) that fractional CBF increases exceed δCMRO2 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.

Original languageEnglish (US)
Pages (from-to)349-355
Number of pages7
JournalMagnetic resonance in medicine
Issue number3
StatePublished - Mar 6 2001



  • Cerebral oxygen utilization
  • Functional MRI
  • H-{C} MRS
  • Hemifield visual stimulation
  • Human visual cortex
  • Neuronal activity
  • Tricarboxylic acid cycle flux

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