Quantitative magnetic resonance imaging in experimental hypercapnia: Improvement in the relation between changes in brain R2*and the oxygen saturation of venous blood after correction for changes in cerebral blood volume

Acute hypercapnia simultaneously induces increases in regional cerebral blo od volume (rCBV) and the oxygen saturation of cerebral venous blood (Yv). C hanges in both physiologic parameters may influence the changes in R2* (Del ta R2*) that can be measured in the brain with gradient echo magnetic reson ance imaging. The authors examined the effect of incorporating independent measurements of the change in rCBV (Delta rCBV) on the fidelity of the rela tion between Delta R2* and Delta Yv in the setting of experimental hypercap nia. A two-dimensional T2*-weighted gradient echo sequence was used to meas ure Delta R2* in the brain parenchyma of anesthetized rats in response to h ypercapnia with respect to the control state. In parallel, estimates of rCB V were obtained using a three-dimensional steady-state approach in conjunct ion with a paramagnetic contrast agent during both control and hypercapnic states so that a Delta rCBV could be calculated. Regional CBV values of 2.9 6 +/- 0.82% and 5.74 +/- 1.21% were obtained during the control and hyperca pnic states, respectively, and linear relations between rCBV and CO2 tensio n in both arterial (r = 0.80) and jugular venous (r = 0.76) blood samples w ere obtained. When correlating Delta R2* directly with Delta Yv, no clear r elation was apparent, but a strong linear relation (r = 0.76) was observed when correction for Delta rCBV was incorporated into the data analysis. The se results are consistent with the current understanding of the mechanisms of brood oxygen level-dependent (BOLD) contrast and underscore the potentia l importance of taking into account Delta rCBV when quantitative estimates of Delta Yv from the "BOLD effect" are intended.