Fig. 1.

Representative breath-hold time series. A) Expelled carbon dioxide content (eCO2) along with estimated end tidal CO2 (EtCO2) during the roughly 7 min breath-hold paradigm. Grey shaded rectangles denote the 5 breath hold epochs. For estimation of cerebrovascular reactivity (CVR), EtCO2 was temporally shifted by time lag, $t_{xcorr}$ (EtCO2-shifted) to maximize the cross-correlation with the relative change in cerebral blood flow (rCBF, B). C) Measured intensity autocorrelation functions ( $g_2(\tau )$ , solid lines) and the corresponding best fits to the semi-infinite solution of the correlation diffusion equation (dotted lines) at baseline (blue) and in response to breath hold (red). D) Linear regression between rCBF and $\mathrm{\Delta }$ EtCO2 was used to estimate CVR as the slope of the line of best fit.