Abstract
Acute normovolemic hernodilution (HD) was induced in anesthetized rats to assess the effect of changes in hematocrit (Hct) on signal intensity in T~2~*βweighted magnetic resonance (MR) images. Other relevant physiological parameters were maintained invariant. Two degrees of HD were induced: mild (Hct reduced from 42.6 Β± 2.2% to 33.4 Β± 2.1%) and moderate (Hct reduced from 44.6 Β± 2.7% to 26.2 Β± 1.7%). A twoβdimensional gradientβecho sequence was used to monitor signal changes with high temporal resolution before, during, and after HD protocols. The time course of signal intensity change was closely related to that of changes in Hct. Corresponding changes in R~2~* (Ξ__R__~2~*) with respect to the preβHD state were calculated for the brain parenchyma. Average ΞR~2~* values of β0.24 Β± 0.06 s^β1^ and β0.40 Β± 0.07 s^β1^ were obtained for the mild and moderate HD groups, respectively, during the final 2 min of MR imaging (proximal to correlative measurements of Hct). MR measured Ξ__R__~2~* values were in close agreement with the expected changes in R~2~* predicted from theory when the measured changes in Hct were used as independent variables. These data are in good agreement with the current understanding of the effects of changes in the intravascular concentration of deoxyhemoglobin on induced magnetic susceptibility and hold promise for quantitative measurement of brain oxygenation in vivo.