Experimental hypoxemic hypoxia:: Changes in R2* of brain parenchyma accurately reflect the combined effects of changes in arterial and cerebral venous oxygen saturation

A two-dimensional T-2*-weighted gradient-echo sequence was used to image the rat brain before and during graded hyperemia. Changes in R-2* (Delta R-2*) with respect to the control state were calculated for brain parenchyma and were compared with changes in hemoglobin saturation measured from both arterial and jugular venous blood samples. Delta R-2* was first correlated with the changes in arterial (Delta Ya) and venous (Delta Yv) hemoglobin saturations individually. Although a general trend toward a linear relationship with Delta R-2* was observed for both Delta Ya and Delta Yv, neither alone was strong (correlation coefficients r = 0.71 and 0.75 for Delta Ya and Delta Yv, respectively, and standard errors of the regression (SER) = 0.52 and 0.48 for Delta Ya and Delta Yv, respectively), However, when an "effective" cerebral blood hemoglobin saturation change (Delta Yb) was constructed that: takes into account the approximate weighting of the contributions from the arterial and venous phases of the circulation (Delta Yb = 0.75 x Delta Yv + 0.25 x Delta Ya), a stronger correlation with Delta R-2* was obtained and there was less variance (r = 0.87 and SER = 0.35). It is concluded that an appropriate weighting of the contributions of arterial and venous phases of the circulation must be taken into account in modeling the volume susceptibility effects of deoxyhemoglobin on R-2* of brain parenchyma. In this way, a more accurate relationship between Delta R-2* and Delta Yb can be obtained.