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.