On the assessment of cerebrovascular reactivity using hypercapnia BOLD MRI

Cerebrovascular reactivity (CVR) reflects the capacity of blood vessels to dilate and is an important marker for brain vascular reserve. It may provide a useful addition to the traditional baseline blood flow measurement when assessing vascular factors in brain disorders. Blood-oxygenation-level-dependent MRI under CO₂ inhalation offers a non-invasive and quantitative means to estimate CVR in humans. In this study, we investigated several important methodological aspects of this technique with the goal of optimizing the experimental and data processing strategies for clinical use. Comparing 4 min of 5% CO₂ inhalation (less comfortable) to a 1 min inhalation (more comfortable) duration, it was found that the CVR values were 0.31 ± 0.05%/mmHg (N=11) and 0.31 ± 0.08%/mmHg (N=9), respectively, showing no significant differences between the two breathing paradigms. Therefore, the 1 min paradigm is recommended for future application studies for patient comfort and tolerability. Furthermore, we have found that end-tidal CO₂ recording was useful for accurate quantification of CVR because it provided both timing and amplitude information regarding the input function to the brain vascular system, which can be subject-dependent. Finally, we show that inter-subject variations in CVR are of physiologic origin and affect the whole brain in a similar fashion. Based on this, it is proposed that relative CVR (normalized against the CVR of the whole brain or a reference tissue) may be a more sensitive biomarker than absolute CVR in clinical applications as it minimizes inter-subject variations. With these technological optimizations, CVR mapping may become a useful method for studies of neurological and psychiatric diseases.