Neurochemical and BOLD responses during neuronal activation measured in the human visual cortex at 7 Tesla

Abstract

Several laboratories have consistently reported small concentration changes in lactate, glutamate, aspartate, and glucose in the human cortex during prolonged stimuli. However, whether such changes correlate with blood oxygenation level-dependent functional magnetic resonance imaging (BOLD-fMRI) signals have not been determined. The present study aimed at characterizing the relationship between metabolite concentrations and BOLD-fMRI signals during a block-designed paradigm of visual stimulation. Functional magnetic resonance spectroscopy (fMRS) and fMRI data were acquired from 12 volunteers. A short echo-time semi-LASER localization sequence optimized for 7 Tesla was used to achieve full signal-intensity MRS data. The group analysis confirmed that during stimulation lactate and glutamate increased by 0.26 ± 0.06 μmol/g (~30%) and 0.28 ± 0.03 μmol/g (~3%), respectively, while aspartate and glucose decreased by 0.20 ± 0.04 μmol/g (~5%) and 0.19 ± 0.03 μmol/g (~16%), respectively. The single-subject analysis revealed that BOLD-fMRI signals were positively correlated with glutamate and lactate concentration changes. The results show a linear relationship between metabolic and BOLD responses in the presence of strong excitatory sensory inputs, and support the notion that increased functional energy demands are sustained by oxidative metabolism. In addition, BOLD signals were inversely correlated with baseline γ-aminobutyric acid concentration. Finally, we discussed the critical importance of taking into account linewidth effects on metabolite quantification in fMRS paradigms.

Figure 1

Representative ¹H MR spectra acquired from the occipital cortex during visual stimulation (A) and rest (B) periods. The typical pattern (narrow linewidth, sideband undershoot) of small residual signals of NAA (2 p.p.m.) and total creatine (3 p.p.m.) in the difference spectrum (C) clearly indicates the BOLD effect, which reduces the linewidth of spectra acquired during the stimulation period. MRS parameters: semi-LASER, TE=26 ms, TR=5 seconds, number of scans=32, data acquired during second halves of STIM, and REST periods. Inset: fMRI—typical VOI selection in the activated brain region. BOLD, blood oxygenation level dependent; Cr, creatine; Gln, glutamine; Glu, glutamate; GPC, glycerophosphocholine; Ins, inositol; Lac, lactate; NAA, N-acetylaspartate; PC, phosphocholine; PCr, phosphocreatine; REST, resting condition; STIM, stimulation period; VOI, volume of interest.

Figure 2

(A) Time courses of glutamate and lactate concentrations during the visual stimulation paradigm (number of scans=32, 2.7 minutes resolution) in individual subjects (N=12). The MRS data acquired in the second halves of the STIM and REST periods (black points) were used to calculate concentration differences between the STIM and REST conditions and for statistical analysis of fMRS data. (B) Time courses of glutamate and lactate concentrations with high temporal resolution. The fMRS data of four scans (20 seconds) were summed across all subjects (N=12). The resulting spectra (4 scans × 12 subjects=48 scans per time point) were quantified by LCModel. The error bars indicate CRLBs. CRLBs, Cramèr-Rao lower bounds; fMRS, functional magnetic resonance spectroscopy; REST, resting condition; STIM, stimulation period.

Figure 3

Time courses of glutamate (Glu), aspartate (Asp), lactate (Lac), and glucose (Glc) concentrations during the visual stimulation paradigm averaged across subjects (N=12 for Glu, Asp, and Lac; N=8 for Glc). Metabolite concentrations were quantified from the 32 scansums (2.7 minutes time resolution). Error bars indicate s.e.m., while shaded areas indicate the stimulation (STIM) periods. P-values correspond to statistical evaluation of differences between STIM and subsequent REST (resting) periods (paired t-tests, mean values from second half of each period).

Figure 4

LCModel analysis of the difference spectrum. (A) Magnetic resonance spectroscopy data acquired during the second half of either STIM or REST periods of all subjects were summed accordingly (32 scans × 2 blocks × 12 subjects=768 scans in total per condition) and then subtracted. (B) Difference spectrum without the BOLD effect: the spectral linewidth of summed STIM and REST data was matched (line broadening of the STIM spectrum by 0.46 Hz) before subtraction. (C) LCModel fit of the difference spectrum without the BOLD effect. (D) Residual of the LCModel analysis. (E) ¹H MR spectra of individual metabolites provided by LCModel analysis and quantified metabolite concentrations of the difference spectrum (mean±CRLB). Gaussian filter (σ=0.1 seconds) was applied on the spectra (A, B, and D) only for display purposes. Asp, aspartate; BOLD, blood oxygenation level dependent; CRLB, Cramèr-Rao lower bound; Glc, glucose; Glu, glutamate; Lac, lactate; REST, resting condition; STIM, stimulation period.

Figure 5

Relationships between spectral linewidth changes, blood oxygenation level–dependent functional magnetic resonance imaging (BOLD-fMRI) amplitudes, metabolite concentrations, and their changes during the visual stimulation studies. (A) Scatter plot showing linewidth changes of creatine (Cr) signals (3 p.p.m.) between REST (resting) and STIM (stimulation) conditions (ΔFWHM_Cr) assessed from the 27-minute functional magnetic resonance spectroscopy (fMRS) paradigm, relative to linewidth changes of water signals between REST and STIM conditions (ΔFWHM_water) determined from the short stimulation paradigm (1.5 minutes). Dashed line represents the line of identity. (B) Correlation between water signal–linewidth changes (REST−STIM) and BOLD signal amplitudes (averaged across the volume selected for fMRS) assessed from the fMRI data (R=0.93, P=0.0001). (C and D) Correlation of BOLD-fMRI amplitudes with the relative concentration changes of glutamate (Glu; R=0.73, P=0.01; C) and lactate (Lac; R=0.65, P=0.003; D) in response to visual stimulation. (E) Correlation between the baseline resting–state γ-aminobutyric acid (GABA) concentration and the BOLD-fMRI amplitudes (R=−0.65, P=0.043).

Figure 6

Suggested scheme of changes in metabolic fluxes and steady-state concentration of brain metabolites during increased brain activity induced by visual stimulation. Increased brain activity requires an increased energy production rate, which is achieved by an increase in glycolytic flux (accompanied by increased cerebral metabolic rate of glucose, CMR_Glc) and by an increase in tricarboxylic acid (TCA) cycle and mitochondrial respiration flux (accompanied by increased cerebral metabolic rate of oxygen (CMRO₂)). Arrows indicate changes in the steady-state concentration of intermediate brain metabolites and metabolites detectable by magnetic resonance spectroscopy (bold fonts). The decrease in glucose (Glc) is caused by increased CMR_Glc at a limited transport rate from plasma, changes in lactate (Lac) and glutamate (Glu) concentrations follow changes in steady-state concentration of pyruvate and α-ketoglutarate (α-KG), respectively, because of their dynamic exchange. Aspartate (Asp) decrease is associated with increased transamination reaction to Glu.