Changes in cerebral blood flow and cerebral oxygen metabolism during neural activation measured by positron emission tomography : comparison with changes in blood oxygeration level-dependent contrast measured by functional magnetic resonance imaging

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The discrepancy between the increases in cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) during neural activation causes an increase in venous blood oxygenation and, therefore, a decrease in paramagnetic deoxyhemoglobin concentration in venous blood. This can be detected by functional magnetic resonance imaging (fMRI) as blood oxygenation level-dependent(BOLD) contrast. However, the relation between cerebral oxygen extraction fraction (OEF), that corresponds to the ratio of CMRO2 to CBF, and BOLD contrast duaring neural activation has not been shown directly in human subjects. In the present study, changes in the OEF and in the BOLD signal during neural activation were measured by both positron emission tomography (PET) and fMRI in the same human subjects. PET studies were performed in each of seven healthy men at rest (baseline) and during parformance of a right hand motor task fMRI studies were then performed to measure the BOLD signal under the two conditions. Regions of interest (ROIs) were defined on all CBF, cerebral blood volume (CBV), OEF, CMRO2, and BOLD images for areas showing statistically significant changes in CBF. Significant relatire hyperperfusion indicating neural activation during the motor task activity was observed in the left precentral gyrus, left superior frontal gyrus, right precentral gyrus, right cingulate gyrus, and right cerebellum. Significant relative hypoperfusion indicating neural deactivations during the motor task activity was observed in the left anterior part of cinglate gyrus and right occipital cuneus. A significant positive correlation was observed between changes in the CBF and the BOLD signal, and a significant negative correlation was observed between changes in the OEF and the BOLD signal for all ROIs. This supports the assumption on which BOLD contrast studies are based, that the discrepancy between increases in CBF and CMRO2 during neural activation causes an increase in venous blood oxygenation.