Background: Magnetic Resonance Spectroscopy (MRS) is a non-invasive advanced imaging technique for assessing the brain's biochemical and metabolic states. For normal brain development, the fetal circulation preferentially delivers oxygen and nutrient-rich blood to the brain through unique adaptations within the fetal circulation. If these adaptations fail, poor neurodevelopment may ensue. With the overall goal of detecting altered neurodevelopment earlier in pregnancy, we aimed to develop an MRS method that overcomes the imaging complexities introduced by maternal respiration and fetal movement, to understand how variations in cerebral oxygen delivery can affect cerebral metabolism.
Methods: At 105–110 days gestation (term, 150 days), pregnant ewes (n=5) underwent fetal surgery where catheters were implanted in the fetal femoral artery and vein, and the amniotic cavity under aseptic conditions. Anaesthesia was induced with intravenous diazepam (0.3 mg/kg) and ketamine (5 mg/kg) and maintained with isoflurane (1.5%–2.5% in 100% oxygen). All ewes received an analgesic, meloxicam (0.5 mg kg−1, subcutaneously) on the day before and the day of surgery. MRI scans were performed on a 3T Siemens clinical system (Magnetom Skyraa, Siemens Healthineers, Erlangen) while the ewe was ventilated. Proton MRS was performed using PRESS at an intermediate echo time (135ms) and a voxel size 15x15x15, gated to maternal respiration. To measure blood flow and oxygenation within the major fetal vessels, phase-contrast MRI and T2 oximetry were performed as previously described (Darby et al., 2020; Saini et al., 2020). Measures were performed during a normoxemic (Nx) and a hyperoxemic (Hyx) fetal state, achieved by maternal hyperoxygenation and confirmed with blood gas analysis (PO2; Siemens RAPIDPOINT). After MRI, ewes and their fetuses were humanely killed with an overdose of sodium pentobarbitone, and fetal brains were collected. Data is presented as mean±SD and analysed using a Students’ t-test; P<0.05 is considered statistically significant.
Results: During MRI, maternal Hyx (22.933±1.712) increased fetal PO2 compared to Nx (17.983±3.058; P=0.0061). Cerebral blood flow (P=0.6534) as well as oxygen delivery (P=0.8236) and consumption (P=0.1604) were the same in Nx and Hyx. MRS detected total choline, creatine and N-acetyl aspartate peaks, key molecules for brain metabolism and development, that were quantified utilising Tarquin. There was no significant difference in total choline, creatine or N-acetyl aspartate concentrations between Nx (2.561±0.589; 2.528±1.274; 4.142±0.875) and Hyx (2.411±0.587; 2.438±0.773; 2.639±1.839; all P>0.05). However, Hyx (0.896±0.404) lowered the N-acetyl aspartate to choline ratio when compared to Nx (1.528±0.404; P=0.0442).
Conclusions: This study demonstrated that acute maternal hyperoxygenation elevated fetal oxygenation without affecting blood flow in the fetal circulatory system. Despite the lack of change in cerebral oxygen delivery or consumption, we showed for the first time that MRS in fetal sheep is feasible for measuring metabolic biochemicals that have a crucial role in neuronal density and function, myelin integrity and overall fetal brain metabolism. Further validation of this technique may lead to the development of a novel tool for assessing the early biochemical changes in the fetal brain of complicated pregnancies.