Investigating the dependence of BOLD contrast on oxidative metabolism.
Most functional magnetic resonance imaging (fMRI) studies are based on measuring the changes in the blood oxygenation level-dependent (BOLD) contrast that arise from a complex interplay between cerebral hemodynamics and oxidative metabolism. To separate these effects, we consecutively applied two different stimuli: visual stimulation (black/white checkerboard alternating with a frequency of 8 Hz) and hypercapnia (inspiration of 5% CO2). Changes in cerebral blood flow (deltaCBF) and the effective transverse relaxation time (T2*) were measured in an interleaved manner by combining a previously described spin-labeling technique with BOLD-based fMRI. In six healthy volunteers, T2* was significantly longer during hypercapnia than during visual stimulation, whereas the corresponding deltaCBF values were the same at the given level of significance (P<0.01). This finding is explained by a significant increase in oxygen consumption under visual stimulation. The average T2* changes in the visual cortex related to cerebral hemodynamics and oxidative metabolism were 10.6+/-3.0% and -4.7+/-1.2%, respectively, resulting in a net increase of 5.9+/-2.3%. Although the hemodynamic effect is dominant, the increase in oxidative metabolism gives rise to a significant decrease in BOLD contrast. The calculated average change in the cerebral metabolic rate of oxygen (CMRO2), 4.4+/-1.1% (N = 6), is in excellent agreement with previous results obtained by positron emission tomography.
Schwarzbauer
C
C
Max-Planck-Institut für neuropsychologische Forschung, Leipzig, Germany.
Heinke
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W
D001810:Blood Volume; D001931:Brain Mapping / Q000379:methods*; D002560:Cerebrovascular Circulation; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008955:Models, Cardiovascular; D008959:Models, Neurological; D010101:Oxygen Consumption / Q000502:physiology; D010775:Photic Stimulation; D012016:Reference Values; D012680:Sensitivity and Specificity; D014793:Visual Cortex / Q000033:anatomy & histology / Q000502:physiology*