Time-dependent correlation of cerebral blood flow with oxygen metabolism in activated human visual cortex as measured by fMRI

The aim of this study was to investigate the relationship between relative cerebral blood flow (δCBF) and relative cerebral metabolic rate of oxygen (δCMRO₂) during continuous visual stimulation (21 min at 8 Hz) with fMRI biophysical models by simultaneously measuring of BOLD, CBF and CBV fMRI signals. The δCMRO₂ was determined by both a newly calibrated single-compartment model (SCM) and a multi-compartment model (MCM) and was in agreement between these two models (P > 0.5). The duration-varying δCBF and δCMRO₂ showed a negative correlation with time (r = - 0.97, P < 0.001); i.e., δCBF declines while δCMRO₂ increases during continuous stimulation. This study also illustrated that without properly calibrating the critical parameters employed in the SCM, an incorrect and even an opposite appearance of the flow-metabolism relationship during prolonged visual stimulation (positively linear coupling) can result. The time-dependent negative correlation between flow and metabolism demonstrated in this fMRI study is consistent with a previous PET observation and further supports the view that the increase in CBF is driven by factors other than oxygen demand and the energy demands will eventually require increased aerobic metabolism as stimulation continues.