Cerebral Time-of-Flight MRI in Third Trimester Fetuses: Visibility of the Circle of Willis, Diameter of the Middle Cerebral Artery, and Tolerability of the Technique Compared with Ultrasound

Background: The evaluation of the fetal intracranial arteries remains a challenge. A potential solution could be non-contrast enhanced MRI. Objectives: Evaluating the visibility and size of major intracranial arteries by time-of-flight (TOF) MRI in third trimester pregnancies and evaluate the acceptability and tolerability of MRI in third trimester pregnancies. Materials and methods: Cross-sectional observational study with ten healthy monofetal pregnancies. Participants underwent 3T fetal MRI and Doppler ultrasound to visualize the fetal cerebral vasculature and completed comfort and tolerability questionnaires. The visibility of the arteries forming the fetal circle of Willis (CoW) was evaluated on TOF MRI. Diameters of the middle cerebral arteries (MCA) were compared between MRI and ultrasound. Results: Median maternal age was 30 years, and median gestational age was 37 ⁺¹ weeks. The internal carotid arteries and the MCA were visible in all patients. The anterior cerebral arteries were visible in more than 75% of the cases and the posterior cerebral artery in 87.5%. Average MCA diameter was 2.36 mm (± 0.30) for grayscale ultrasound, 2.53 mm (± 0.33) for Doppler, and 1.56 mm (± 0.22) on TOF. Participants expressed high satisfaction levels regarding both examinations. Conclusions: Even if ultrasound is the most used fetal imaging modality, TOF MRI could become an additional tool to visualize and measure the fetal intracranial vasculature and to evaluate vascular anomalies in-utero. Vessel diameter measurements by TOF MRI were on average 1 mm smaller than their ultrasound counterparts.