Normal placental structural anatomy: ultrasound and doppler features elucidated with US-MR image fusion and ferumoxytol-enhanced MRI.

Purpose: To elucidate ultrasound features of normal placental anatomy through correlation of gray-scale and ultrasound Doppler with ferumoxytol-enhanced MRI features using US-MR image fusion.

Methods: All patients referred to MR for ultrasound findings worrisome for PAS (placenta accreta spectrum) were included in this retrospective study. MR studies included a ferumoxytol-enhanced T1-weighted MRI. Ultrasound imaging included gray-scale, color Doppler, power Doppler, and spectral Doppler techniques. After the MR, US-MRI fusion was performed by co-registering a MR acquisition to real-time US, which allowed precise, point-to-point correlation of placental features.

Results: Fourteen patients at risk for PAS were studied using the US-MR image fusion. At delivery, there were six cases without PAS (gestational age range: 24 weeks 3 days to 34 weeks 0 days), and these composed the study cohort. Placental features that were on high signal intensity on post-ferumoxytol acquisitions represent spaces with maternal blood flow and corresponded to hypoechoic areas on ultrasound created by a paucity of reflective interfaces (villi). Color and spectral Doppler allowed the separation of maternal and fetal circulations in individual perfusional domains and demonstrated spiral artery inflow, circulation around the villous tree, and return of blood flow to the basal plate. Recognizable histopathologic features by ultrasound included the central cavity, villous tree, and venous return channels.

Conclusion: Internal placental architecture can be discerned on ultrasound. This anatomy can be correlated and confirmed with ferumoxytol-MR through US-MR fusion. Understanding this structural anatomy on ultrasound could serve as a basis to identify normal and abnormal placental features.