Figure 1.

Methodology for quantitatively characterizing the brain’s reactions to the CO2 challenge. A relative arrival map (rAT) was obtained using systemic low frequency oscillation from BOLD (Top panel). After shifting the P_ETCO2 waveform (obtained from RespirAct™) based on the rAT map and convolving it with a set of candidate hemodynamic response functions (HRFs: see Figure 2), the voxel-wise optimal HRF was identified by the maximal correlation between observed BOLD signal and the convolved P_ETCO2 waveform (Middle panel). An improved cerebrovascular reactivity (CVR) map was then obtained by performing a linear fit between the BOLD percentage change and the convolved P_ETCO2 change (in mmHg), with the slope of this fit line being the CVR value (unit: %/mmHg) (see *; Bottom panel).