Dynamics of changes in blood flow, volume, and oxygenation: implications for dynamic functional magnetic resonance imaging calibration

Changes in cerebral blood flow (CBF), volume (CBV), and oxygenation (blood-oxygenation level dependent (BOLD)) during functional activation are important for calculating changes in cerebral metabolic rate of oxygen consumption (CMRO2) from calibrated functional MRI ( fMRI). An important part of this process is the CBF/CBV relationship, which is signified by a power-law parameter: gamma = ln (1 + Delta CBV/CBV)/ln (1 + Delta CBF/CBF). Because of difficulty in measuring CBF and CBV with MRI, the value of gamma is therefore assumed to be similar to 0.4 from a prior primate study under hypercapnia. For dynamic fMRI calibration, it is important to know if the value of gamma varies after stimulation onset. We measured transient relationships between Delta CBF, Delta CBV, and Delta BOLD by multimodal MRI with temporal resolution of 500ms (at 7.0 T) from the rat somatosensory cortex during forepaw stimulation, where the stimulus duration ranged from 4 to 32 secs. Changes in CBF and BOLD were measured before the administration of the contrast agent for CBV measurements in the same subjects. We observed that the relationship between Delta CBF and Delta CBV varied dynamically from stimulation onset for all stimulus durations. Typically after stimulation onset and at the peak or plateau of the Delta CBF, the value of gamma ranged between 0.1 and 0.2. However, after stimulation offset, the value of gamma increased to 0.4 primarily because of rapid and slow decays in Delta CBF and Delta CBV, respectively. These results suggest caution in using dynamic measurements of Delta CBF and Delta BOLD required for calculating Delta CMRO2 for functional stimulation, when either Delta CBV has not been accurately measured or a fixed value of gamma during hypercapnia perturbation is used.