Research articleDirect measurement of oxygen extraction with fMRI using 6% CO2 inhalation
Introduction
The blood-oxygenation-level-dependent (BOLD) effect is a widely used functional magnetic resonance imaging (fMRI) technique to investigate changes in local brain activity following sensory stimulation. The BOLD signal is sensitive to the amount of paramagnetic deoxygenated hemoglobin, the concentration of which changes as a result of the interplay of cerebral blood flow (CBF), cerebral blood volume (CBV) and oxygen extraction into the tissue. The reliability of the BOLD signal is based on the assumption that there is a coupling between changes in neural activity, metabolism, oxygen extraction and the vascular response in the area under investigation. The sensitivity of the BOLD signal for a given neuronal activity change is determined by the amount of deoxygenated hemoglobin present in the voxel during baseline; hence, its amplitude can be modified independently of neuronal activity by changing the baseline CBF using, for example, caffeine prior to the experiment [1]. In addition, the neuronal and vascular responses can be decoupled by administering exogenous agents such as hypercapnia [2] or 7-nitroindazole [3], [4] and also in cases of cerebrovascular diseases [5]. Therefore, much effort has been expended to develop alternative noninvasive fMRI signals that are more closely coupled to the underlying neuronal activity and metabolism. An effective approach is to measure CBF with arterial spin labeling (ASL) MRI. In healthy brain, the measure of tissue perfusion faithfully reflects neuronal activity because it is regulated on a fine spatial scale and strongly coupled to cerebral metabolic rate of oxygen (CMRO2) [6], [7]. The CBF signal can be used to calibrate the BOLD signal and calculate CMRO2 (termed ‘calibrated BOLD approach’ [8], [9]). However, there are still numerous potentially confounding factors in using this approach [10], [11], [12]. Therefore, a more direct fMRI signal that is sensitive to neuronal activity or oxygen extraction without the influence of CBF and CBV is desirable. The CBF response to a sensory stimulus can be suppressed by injection of the vasodilator sodium nitroprusside in the cat [13] or with 7-nitroindazole in the rat [4]. Without any hemodynamic response, the concentration of deoxygenated hemoglobin and, thus, the BOLD signal can only be changed as a result of altered oxygen extraction following CMRO2. This allows the mapping of cortical columns in the anesthetized cat under sodium nitroprusside [14]. Here, we describe an alternative method for disentangling the vascular and metabolic response to a sensory stimulus in the anesthetized nonhuman primate using moderate hypercapnia.
Mild and moderate levels of hypercapnia are vasodilating challenges used in clinical applications and basic research and are assumed not to impair brain function. Hypercapnia can be induced either by decreased respiration frequency or by increased inhalation of CO2, termed ‘moderate’ for concentrations up to 6–8% CO2 in the inhaled gas. CO2 easily diffuses through the blood–brain barrier and the cell membrane into the intracellular space and, in very high concentrations (>20%), can cause anesthesia [15]. It has been shown, for example, that moderate and severe hypercapnia increases the threshold for electrically and chemically induced seizures [16], [17], and this effect is exploited clinically to suppress epileptic seizures.
We measured both BOLD and electrophysiological signals simultaneously in the primary visual cortex under hypercapnia and combined the results with independent measurements of CBF and CBV. CBF was obtained by a continuous ASL technique, and CBV was measured after intravenous injection of an exogenous contrast agent [monocrystalline iron oxide nanocolloids (MION)].
Inhalation of 6% CO2 completely abolished the CBF and CBV response to a visual stimulus. In contrast, the gamma band and multiunit activity measured by intracortical recordings remained unaffected by hypercapnia, and as a consequence, we observed a negative BOLD response in the stimulated visual field. This noninvasive method offers a novel methodology to directly measure oxygen extraction without the presence of confounding hemodynamic signals.
Section snippets
Animal preparation
Fifteen sessions with seven healthy adult monkeys (Macaca mulatta, 4–8 kg) were performed for this study. All experiments were approved by the local authorities (Regierungspräsidium) and were in full compliance with the guidelines of the European Community for the care and use of laboratory animals (EUVD 86/609/EEC). The experiments were performed under general anesthesia with a slightly different protocol than described previously [18], [19]. Briefly, the animal was sedated with ketamine (15
Results
The inhalation of 3% and 6% CO2 increased the end-tidal CO2 by approximately 12±2 mmHg and 23±4 mmHg. In the primary visual cortex, baseline CBF was increased by 35% (range 17–55%) for 3% CO2 and 60% (range 25–100%) for 6% CO2 (four sessions, compare baseline values of one session in Fig. 1A). The amplitude of the CBF response to a strong visual stimulus (full field flickering light-emitting diodes) under normocapnia is around 40–50% (Fig. 1A), comparable to the baseline CBF change induced by
Discussion
In this report, we show that oxygen extraction during stimulation can be imaged using MRI without concomitant changes in CBF and CBV applying moderate hypercapnia (6% CO2 inhalation). In a previous report, using the same setup, we showed that spontaneous baseline activity is depressed under moderate hypercapnia [27]. Baseline local field potentials in the gamma and theta range, as well as multiunit activity, decreased their power by up to 15% under 6% CO2 inhalation. In contrast, simultaneously
Acknowledgments
We are grateful to our colleagues Mark Augath and Axel Oeltermann, for technical assistance, Yusuke Murayama, for help with the analysis, and Josef Pfeuffer and Helmut Merkle for their involvement in the ASL measurements. MION was obtained from the Center for Molecular Imaging Research, Massachusetts General Hospital, Boston, USA. This work was supported by the Max-Planck Society.
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