Synopsis

Measuring the brain perfusion responsive to the external vasodilators, termed cerebrovascular reactivity (CVR), is a useful tool towards better understanding of brain pathophysiological conditions, such as arterial stenosis, brain tumor, dementia, and traumatic brain injury. Moreover, CVR evaluation could serve as a novel approach to normalize the BOLD fMRI signal and quantify neural activity evoked by stimulation in calibrated fMRI as well, paving the way for neuroscience research.

Cerebrovascular reactivity or cerebrovascular reserve capacity (CVR), describing the ability of brain’s vasculature response to the external vasodilators by increasing the cerebral perfusion, has been widely used for mapping cerebrovascular function. In terms of MRI techniques, there are three useful methods for non-invasive assessment of CVR: phase-contrast (PC) MRI, arterial spin labeling (ASL) and blood oxygenation level dependent (BOLD). Besides being an research tools towards better understanding of the normal brain physiological conditions, CVR assessments have been applied to clinical in the following areas:

• Many degenerative brain physiological conditions have been associated with age. CVR is an important index to tell the age-related alternations.
• Understand the hemodynamic mechanisms in the brain, such constriction of blood flow or the blood clots in brain, e.g., moyamoya disease, and stroke-related factor, e.g., hypertension.
• Vascular function is one of the important causes of dementia in older individuals. Thus, the cerebrovascular dysfunction is expected to be related to the Alzheimer’s disease and poor cognition function.
• Clinical management and monitoring disease progress, telling the differences between responders and non-responders.

Since CVR is a fundamental parameter to delineate the basal physiologic condition, it can be considered as a powerful tool to normalize BOLD fMRI signals. One of the confounding factors in BOLD fMRI is the inter-subject variations, which leads to the challenge in finding the differences between two groups. Vessel elasticity across subjects contributes to the inter-subject variations in BOLD signals. Moreover, vessel elasticity shows different pattern between young and elderly subjects. The normalization approach to calculated a CVR-corrected fMRI signal is a more sophisticated way to carefully interpret the neural responses, rather than vascular responses [1].

Inhalation of external vasodilators, such as CO2, is so called as gas-challenge. Gas-inhalation MRI schemes have been used widely in calibrated fMRI to quantify the neural activity evoked by stimulation. Various calibration methods have been proposed, including mild hypercapnia, hyperoxia, and carbogen (a combination of hypercapnia and hperoxia). An assumption underlying these strategies is that the brain metabolism does not change during gas inhalation. However, this is datable. Caution is necessary when interpreting calibrated fMRI data obtained by different gas challenges [2].

Acknowledgements

References

1. Liu, P. et al. Age-related differences in memory-encoding fMRI responses after accounting for decline in vascular reactivity.

2. Peng, SL. et al. Searching for a truly “iso-metabolic” gas challenge in physiological MRI.