Abstract
Elucidating the brain regions and networks associated with cognitive processes has been the mainstay of task-based fMRI, under the assumption that BOLD signals are uncompromised by vascular function. This is despite the plethora of research highlighting BOLD modulations due to vascular changes induced by disease, drugs, and aging. On the other hand, BOLD fMRI-based assessment of cerebrovascular reactivity (CVR) is often used as an indicator of the brain's vascular health and has been shown to be strongly associated with cognitive function. This review paper considers the relationship between BOLD-based assessments of CVR, cognition and task-based fMRI. How the BOLD response reflects both CVR and neural activity, and how findings of altered CVR in disease and in normal physiology are associated with cognition and BOLD signal changes are discussed. These are pertinent considerations for fMRI applications aiming to understand the biological basis of cognition. Therefore, a discussion of how the acquisition of BOLD-based CVR can enhance our ability to map human brain function, with limitations and potential future directions, is presented.
Highlights
Cerebrovascular reactivity (CVR) reflects the ability of the cerebral blood vessels to respond to a vasoactive stimulus and is primarily sensitive to arterial compliance [1, 2]
We demonstrate that modulations to taskbased Blood oxygenation level-dependent (BOLD) activation are often found in such cases, and provide the motives for collection of cerebrovascular reactivity (CVR) data to aid the interpretation of fMRI activation maps
This study found that the brain regions with reduced CVR did not overlap with regions showing reduced baseline cerebral blood flow (CBF) in Alzheimer’s Disease (AD) patients, owing to vasoreactivity being sensitive to different pathology than CBF
Summary
Cerebrovascular reactivity (CVR) reflects the ability of the cerebral blood vessels to respond to a vasoactive stimulus and is primarily sensitive to arterial compliance [1, 2]. Blood oxygenation level-dependent (BOLD) functional MRI is an effective technique for measuring CVR. The change in the local concentration of dHb is predominantly driven by increases in cerebral blood flow (CBF), which reduces venous dHb via a dilution effect and increases the BOLD signal [3, 4]. In conventional task-based or resting-state BOLD fMRI, increases in the cerebral metabolic rate of oxygen (CMRO2) and cerebral blood volume (CBV) associated with increased neuronal activity both act to increase dHb and attenuate the CBF dominated BOLD signal increase. With a direct vasodilator stimulus used for CVR mapping, such as carbon dioxide (CO2), CMRO2 changes are assumed to be negligible and a robust positive BOLD signal is measurable that primarily reflects
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