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Validation of the hypercapnic calibrated fMRI method using DOT-fMRI fusion imaging
Meryem A Yücel1, Karleyton C Evans2, Juliette Selb1
1MGH/HST Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, 02129 MA, USA.
Neuroimage
|September 9, 2014
Summary
Hypercapnic calibration in functional MRI (fMRI) was validated using diffuse optical tomography (DOT). This alternative method confirms the reliability of fMRI calibration constants, suggesting variability reflects physiological differences.
Area of Science:
- Neuroimaging
- Physiology
- Biophysics
Background:
- Calibrated functional magnetic resonance imaging (fMRI) quantifies brain physiology, notably cerebral metabolic rate of oxygen (CMRO2).
- Hypercapnic calibration, a common fMRI calibration method, uses simultaneous BOLD and ASL signals during tasks and CO2 inhalation.
- A key assumption of hypercapnic calibration is constant CMRO2 during CO2 exposure, which has been debated.
Purpose of the Study:
- To cross-validate the calibration constant (M) estimation from traditional hypercapnic fMRI calibration.
- To assess an alternative calibration method using diffuse optical tomography (DOT) that bypasses gas manipulation and the constant CMRO2 assumption.
- To investigate the physiological basis of inter-subject variability in calibration constants.
Main Methods:
- Simultaneous BOLD and ASL fMRI recordings were acquired during functional tasks.
- Hypercapnic calibration was performed by CO2 inhalation to estimate the M value.
- Diffuse optical tomography (DOT) was employed as an independent method to estimate M, without gas manipulation.
Main Results:
- A high correlation (R=0.87, p<0.01) was observed between M values estimated by hypercapnic calibration and DOT.
- The findings validate the hypercapnic fMRI calibration technique.
- Inter-subject variability in M values obtained via hypercapnic calibration is reproducible with the DOT method.
Conclusions:
- The study validates the widely used hypercapnic fMRI calibration technique.
- The results suggest that the observed inter-subject variability in calibration constants reflects genuine physiological differences.
- DOT provides a reliable alternative for fMRI calibration, supporting the physiological interpretation of calibration parameters.
