Normal BOLD Response to a Step CO₂ Stimulus After Correction for Partial Volume Averaging.

Area of Science:

• Neuroimaging
• Cerebrovascular Physiology
• Medical Physics

Background:

• Cerebrovascular reactivity (CVR) assesses vascular function by measuring cerebral blood flow changes in response to vasoactive stimuli.
• BOLD MRI with end-tidal CO2 manipulation is a precise method for CVR assessment, improved by advanced gas delivery systems.
• Normative control ranges for CVR metrics are currently lacking in scientific literature.

Purpose of the Study:

• To calculate and report normative control ranges for key CVR metrics: magnitude (mCVR), steady state amplitude (ssCVR), speed (TAU), and time delay (TD).
• To implement and validate a partial volume correction method for accurate voxel-wise CVR quantitation in gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF).

Main Methods:

• CVR metrics (mCVR, ssCVR, TAU, TD) were analyzed in a cohort of 45 healthy controls using BOLD MRI and controlled end-tidal CO2.
• A linear regression method was applied to correct CVR maps for partial volume averaging based on adjacent voxel data and tissue composition.
• The method estimated CVR metrics specifically for GM, WM, and CSF by solving a system of linear equations.

Main Results:

• Partial volume correction revealed expected increases in mCVR (+25%) and ssCVR (+19%) in GM, and decreases in WM (-33% and -13%, respectively).
• TAU and TD decreased in GM (-33% and -13%), while TAU increased in WM (+24%).
• A surprising decrease in TD (-9%) was observed in WM post-correction.

Conclusions:

• The study provides essential normative ranges for CVR metrics in healthy individuals.
• Partial volume correction significantly improves the accuracy of voxel-wise CVR quantitation in different brain tissue types.
• These findings enable more precise comparisons of individual CVR assessments against normative data, particularly for GM and WM.