Placenta Accreta Spectrum on MRI

Placenta accreta spectrum (PAS) refers to a range of abnormally adhesive and penetrative placental tissues in the myometrium. It is critical to diagnose PAS before delivery, as maternal morbidity/mortality can occur due to life-threatening hemorrhage. Ultrasound has traditionally been the first-line imaging modality for the diagnosis of PAS; however MRI is a useful supplemental modality in the workup and is a valuable tool in cases where ultrasound is limited or equivocal. It is also indicated in further assessment of PAS in cases with a positive ultrasound diagnosis. There are three main categories of MRI findings of PAS, all of which involve disruption of the normal anatomic appearance of the placenta/myometrium and include gross morphologic signs (placental bulge, bladder wall interruption, exophytic mass, rolled-up placental edge, and placental protrusion into the cervix), interface signs (myometrial thinning, loss of T2 hypointense interface, abnormal vascularization of the placental bed, and placental infarction), and architecture signs (T2 dark bands, abnormal intraplacental vascularity, and placental heterogeneity). It is important for radiologists to be aware of these signs, and potential MRI imaging pitfalls to avoid false diagnosis. Numerous studies are currently being conducted to improve the diagnosis of PAS on imaging, including investigations looking at dynamic contrast gadolinium enhancement and machine learning.