Figure 1.

Generation of steady-state cerebrovascular reactivity (ssCVR) and time constant (τ) maps from a standardized abrupt two-step stimulus. (A) (i) Target (dotted line) and actual (solid line) end-tidal partial pressure of CO₂ (PETCO₂). (ii) The set of first-order exponentials with τ ranging from 2 to 100 seconds used to convolve the actual PETCO₂ to generate a set of convolved curves seen in (iii). (B and C) (i) Actual PETCO₂ (solid black line) and blood oxygen level-dependent (BOLD) signal (green and red lines) from voxels X and Y, respectively, shown in (D). (ii) Set of curves resulting from the convolution of PETCO₂. The highlighted line is that which has the best fit to the voxel BOLD signal shown in (i). (iii) The regression with the best fit between the convolved PETCO₂ and the BOLD signal is shown. The difference in the pattern of the data results from the extent of distortion the convolution imposes on the PETCO₂ input function. (D) The ssCVR map is generated from the slope of the line with the highest correlation coefficient as illustrated in the graphs in row (iii). The τ map is generated from the τ of the exponential used to generate the line with the highest correlation coefficient. The ssCVR and the τ values are color coded according to the color scales shown and mapped onto the corresponding voxels in the anatomic scan.