Abstract
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 (ΔCBF) and the effective transverse relaxation time (T*2) were measured in an interleaved manner by combining a previously described spin-labeling technique with BOLD-based fMRI. In six healthy volunteers, T*2 was significantly longer during hypercapnia than during visual stimulation, whereas the corresponding ΔCBF 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 T*2 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. Magn Reson Med 41:537–543, 1999. © 1999 Wiley-Liss, Inc.
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