Fetal sheep brains: findings at functional blood oxygen level-dependent 3-T MR imaging--relationship to maternal oxygen saturation during hypoxia.

PURPOSE

To quantify the dependence of the signal intensity (SI) at blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging of fetal sheep brains on maternal oxygen saturation and to investigate the influence of positions of regions of interest (ROIs).

MATERIALS AND METHODS

All experimental protocols were reviewed and approved by the local authorities on animal protection. The brains of singleton fetuses of five anesthetized sheep were subjected to rapid sequences (single-shot echo-planar imaging) of BOLD measurements with a 3-T MR imaging unit. Maternal oxygen saturation and heart rate were recorded continuously. After a normoxic phase, hypoxia was induced by reducing the oxygen in a ventilated gas mixture. ROIs were placed in the cerebrum at a reference level and in the cerebellum. Normalized BOLD SI values were calculated from the mean values of steady-state BOLD SIs at the control (SI(c)) and hypoxic (SI(h)) plateaus as follows: normalized BOLD SI = (SI(h)/ SI(c)) x 100. Normalized BOLD SI values were correlated with maternal oxygen saturation, and linear regression (slope) analysis was performed. Additionally, ROIs were varied in section level and position. Differences in normalized BOLD SI values for ROI placements were calculated by using analysis of variance. A t test was performed to evaluate differences.

RESULTS

Mean maternal oxygen saturation (as the percentage of oxygen in the blood) was 88% (95% confidence interval [CI]: 80%, 96%) in the control period. During hypoxia, it was reduced to 62% (95% CI: 50%, 75%), while fetal normalized BOLD SI decreased to 64% (95% CI: 44%, 85%) in the cerebrum and 56% (95% CI: 32%, 80%) in the cerebellum. Correlations between normalized BOLD SI values and maternal oxygen saturation were as follows: r2 = 0.84 and slope = 1.27 (95% CI: 1.17, 1.36) in the cerebrum and r2 = 0.83 and slope = 1.54 (95% CI: 1.44, 1.63) in the cerebellum. Normalized BOLD SI was 4% lower in the section above the reference level. Variations in normalized BOLD SI for different ROI positions ranged between 0% and 12%.

CONCLUSION

The depletion of oxygen supply is reflected by decreases in fetal brain BOLD SIs that are more distinct in the cerebellum than in the cerebrum. Normalized BOLD SI is influenced only slightly by ROI position.