Investigating the dependence of BOLD contrast on oxidative metabolism

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.