Determination of relative CMRO2 from CBF and BOLD changes: Significant increase of oxygen consumption rate during visual stimulation

Abstract

The blood oxygenation level-dependent (BOLD) effect in functional magnetic resonance imaging depends on at least partial uncoupling between cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) changes. By measuring CBF and BOLD simultaneously, the relative change in CMRO2 can be estimated during neural activity using a reference condition obtained with known CMRO2 change. In this work, nine subjects were studied at a magnetic field of 1.5 T; each subject underwent inhalation of a 5% carbon dioxide gas mixture as a reference and two visual stimulation studies. Relative CBF and BOLD signal changes were measured simultaneously using the flow-sensitive alternating inversion recovery (FAIR) technique. During hypercapnia established by an end-tidal CO2 increase of 1.46 kPa, CBF in the visual cortex increased by 47.3 ± 17.3% (mean ± SD; n = 9), and ΔR2/* was -0.478 ± 0.147 sec-1, which corresponds to BOLD signal change of 2.4 ± 0.7% with a gradient echo time of 50 msec. During black/white visual stimulation reversing at 8 Hz, regional CBF increase in the visual cortex was 43.6 ± 9.4% (n = 18), and ΔR2/* was -0.114 ± 0.086 sec-1, corresponding to a BOLD signal change of 0.6 ± 0.4%. Assuming that CMRO2 does not change during hypercapnia and that hemodynamic responses during hypercapnia and neural stimulation are similar, relative CMRO2 change was determined using BOLD biophysical models. The average CMRO2 change in the visual cortex ranged from 15.6 ± 8.1% (n = 18) with significant cerebral blood volume (CBV) contribution to 29.6 ± 18.8% without significant CBV contribution. A weak positive correlation between CBF and CMRO2 changes was observed, suggesting the CMRO2 increase is proportional to the CBF increase.