FMRI Approaches to Mapping Cerebrovascular Reactivity: Comparison of Gradient Echo BOLD and Spin Echo BOLD With...

To compare markers of cerebrovascular reactivity (CVR) measured with Gradient Echo (GE, T2*w) and Spin-Echo (SE, T2w) BOLD fMRI with quantitative physiological CVR measured using ASL.

CVR, the ability of blood vessels to dilate and increase cerebral blood flow (CBF), can be investigated with ASL fMRI but has a low signal-to-noise ratio (SNR). GE-BOLD provides semi-quantitative CVR assessment, has high SNR but is influenced by baseline deoxyhemoglobin and the macrovascular venous compartment. SE-BOLD is selectively more sensitive to microvasculature, mitigating contamination from larger vessels. We developed a pCASL GE-BOLD/SE-BOLD MRI sequence and collected all three fMRI modalities in 20 healthy subjects during a hypercapnic breath-holding task.

CVRs in GM were CVR_ASL = 5.3%/mmHg ± 1.8%/mmHg (mean ± SD), CVR_GE-BOLD = 0.18% BOLD/mmHg ± 0.05% BOLD/mmHg, and CVR_SE-BOLD = 0.09% BOLD/mmHg ± 0.03% BOLD/mmHg. The ground truth of physiological CVR was represented by the ASL measurements with higher SNR (CVR_ASL). CVR_GE-BOLD and CVR_SE-BOLD correlated across subjects with CVR_ASL in GM, with SE showing a stronger correlation (CVR_BOLD vs. CVR_ASL:r = 0.54, p < 0.05 for GE-BOLD and r = 0.70, p < 10^-3 for SE-BOLD). Region of interest (ROI) analysis based on previously reported macrovascular venous density maps derived from SWI showed that the lower correlation for GE-BOLD was driven by GM ROIs with high venous density. Similar results were observed for spatial correlations (across regions) of group average maps of CVR (CVR_BOLD vs. CVR_ASL:r = 0.44 for GE-BOLD and r = 0.58 for SE-BOLD, p's < 10^-3).

BOLD fMRI provides a semi-quantitative CVR assessment. Using SE-BOLD may be advisable, as it approximates physiological CVR more closely than GE-BOLD due to reduced sensitivity to larger veins while maintaining a similar group-level sensitivity.