The absolute CBF response to activation is preserved during elevated perfusion: Implications for neurovascular...

Area of Science:

• Neuroimaging
• Physiology
• Biophysics

Background:

• Functional magnetic resonance imaging (fMRI) measures blood oxygenation level dependent (BOLD) and cerebral blood flow (CBF) responses to neural activity.
• Estimating cerebral metabolic rate of oxygen consumption (CMRO2) changes is crucial for understanding neurovascular coupling.
• The coupling parameter 'n' (ratio of fractional CBF to CMRO2 responses) assumes relative changes adequately characterize flow-metabolism responses.

Purpose of the Study:

• To investigate the effect of baseline perfusion on the neurovascular coupling parameter 'n'.
• To assess the validity of using relative CBF changes in fMRI studies under varying baseline perfusion conditions.

Main Methods:

• 10 healthy subjects underwent fMRI with a dual-echo pulsed arterial spin labelling (PASL) sequence.
• BOLD and absolute CBF were measured during graded visual stimulation.
• Baseline perfusion was modulated using a hypercapnia challenge.

Main Results:

• Absolute CBF responses to visual stimuli remained constant across different baseline CBF levels.
• Fractional CBF responses were reduced under hyperperfused baseline conditions.
• The coupling parameter 'n' was significantly reduced during hypercapnia-induced hyperperfusion.

Conclusions:

• The neurovascular coupling parameter 'n' is sensitive to baseline CBF, potentially confounding fMRI studies.
• Using relative CBF changes may lead to misinterpretations of neurovascular coupling when baseline perfusion differs between groups.
• Absolute CBF changes might better reflect neural activity increases than relative changes.