BOLD MRI in sheep fetuses: A non-invasive method for measuring changes in tissue oxygenation

Abstract

Objective: The purpose of this descriptive study was to correlate changes in the blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) signal with direct measurements of fetal tissue oxygenation. Methods: Seven anesthetized ewes carrying singleton fetuses at 125 days' gestation (term 145 days) underwent BOLD MRI, covering the entire fetus in a multislice approach. The fetuses were subjected to normoxic, hypoxic and hyperoxic conditions by changing the O2/N2O ratio in the maternal ventilated gas supply. The partial pressure of oxygen (pO2) in the fetal liver was measured using an oxygen-sensitive optode.Maternal arterial blood sampleswere simultaneously withdrawn for blood gas analysis. These measurements were compared with BOLD MRI signals in the fetal liver, kidney, spleen and brain. Results: We demonstrated a consistent increase in the BOLD MRI signal with increasing tissue pO2. For the fetal liver, spleen and kidney we observed a clear association between changes in maternal arterial blood pO 2 and changes in BOLD MRI signal. Interestingly, we found that the BOLD signal of the fetal brain remained unchanged during hypoxic, normoxic and hyperoxic conditions. Conclusions: This experimental study demonstrated that BOLD MRI is a reliable non-invasive method for measuring changes in tissue oxygenation in fetal sheep. The unchanged signal in the fetal brain during altered maternal oxygen conditions is probably explained by the brain-sparing mechanism. Copyright © 2009 ISUOG. Published by John Wiley & Sons, Ltd.