Cerebrovascular reactivity (CVR) measures the brain blood flow (CBF) response to a vasoactive stimulus such as carbon dioxide (CO₂). Simplistically, CVR can be thought of as a stress test of the brain vasculature. CVR is expressed as the ratio of the change in CBF and the change in the vasoactive stimulus. It measures the regulatory ability of the cerebral vasculature, and is used to assess cerebrovascular health; the ability of the brain to cope with changes in vascular anatomy (e.g. stenosis) and other factors impacting the CBF supply. For example, reductions in the amplitude of CVR are associated with an enhanced risk of stroke and delays in the timing of CVR are associated with dementia. Because CVR varies across the healthy brain and in focal pathologies, whole-brain neuroimaging techniques such as Magnetic Resonance Imaging (MRI) are used to map CVR parameters, providing insight into a number of clinical aspects of cerebrovascular health.

This Research Topic is intended to attract manuscripts that advance the use of MRI-based CVR parameters in clinical assessments of cerebrovascular health using CO₂ as a stimulus. In pursuit of this aim, a better understanding of a number of aspects are needed.

These include:
• Methods of administering the CO₂ stimulus during MRI (stimulus pattern, apparatus).
• The physiology involved in the vascular response to CO₂ and how it reflects regional and/or systemic vascular health.
• Techniques to analyse CVR data in different ways (e.g. speed of response, resistance changes) to provide more information about vascular health.
• Relationships between CVR data acquired using different MRI scan acquisitions and aspects of repeatability and reliability.
• The use of CVR to assess vascular health in various pathologies (e.g. concussion, dementia, Moyamoya).
• Methods of measuring CVR with MRI and a CO₂ stimulus.
• Improving the analysis of MRI data to provide more information about CVR.
• Characterizing or modelling the physiology involved in the vascular response to CO₂ and how this reflects vascular health
• Using MRI-based CVR to assess vascular health in various pathologies
• Addressing the challenges of repeatability, reliability, and specificity of CVR metrics, including within problematic cohorts, during extreme CO₂ stimuli, across research sites, or spanning imaging modalities.

Topic Editor Prof. James Duffin contributed to the development of an automated end-tidal targeting device, RespirAct^TM and is employed by Thornhill Medical Inc. (Toronto, Canada). RespirAct^TM is currently a non-commercial research tool assembled and made available by TMI to research institutions to enable CVR studies. All other Topic Editors declare no competing interests with regards to the Research Topic subject.

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