RETURN TO ABSTRACT LISTING


Background
Placenta accreta spectrum (PAS) refers to a range of abnormally adhesive and penetrative placental tissue in the myometrium. Diagnosing placenta accreta spectrum is critical, as it can cause maternal morbidity and mortality due to catastrophic hemorrhage at the time of delivery if not anticipated by the obstetrical team. Ultrasound has traditionally been the first line imaging modality for the diagnosis of PAS, however MRI is a useful supplemental modality in the work-up and is a valuable tool in cases where ultrasound is limited or equivocal. It is also indicated in further assessment of PAS in cases of a positive ultrasound diagnosis.

Educational Goals / Teaching Points
The purpose of this presentation is to understand the pathology, risk factors, diagnosis, and management of placenta accreta spectrum, describe clinical scenarios where MRI is indicated as an adjunct to ultrasound, become familiar with MRI techniques for placental evaluation, review MRI findings of PAS, provide case examples of PAS highlighting MRI findings, with pathological correlation, when available, illustrate pitfalls, both technical and interpretive, in assessment for PAS on MRI, discuss potential advancements in imaging which can help to increase the performance of imaging in making the often challenging diagnosis of PAS.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
In cases where US is limited in the diagnosis of placenta accreta spectrum (PAS), MRI can aid in the diagnosis. The most commonly used sequences for the evaluation of the placenta are single-shot spin-echo/turbo spine-echo (ssFSE/ssTSE), half-Fourier acquisition single-shot turbo spin-echo (HASTE) T2 weighted, and balanced steady state free-procession (true FISP or FIESTA). A normal placenta appears as a 2-4 cm thick, discoid-shaped structure attached to either the anterior or posterior uterine cavity with a homogeneous high T2 signal in early gestation and a more heterogeneous appearance during later gestation. The normal myometrium has a tri-layered appearance on T2-weighted sequences, including a T2 hypointense thin inner layer, a T2 hyperintense thick middle layer, and a T2 hypointense thin outer layer. There are 3 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, placental protrusion into cervix, interface signs- myometrial thinning, loss of T2 hypointense interface, abnormal vascularization of the placental bed, placental infarction, and architecture signs- T2 dark bands, abnormal intraplacental vascularity, placental heterogeneity.

Conclusion
Given the increasing incidence of PAS and the risk of life-threatening hemorrhage in undiagnosed cases, it is important now more than ever to accurately diagnose at risk women in order to properly manage these pregnancies. While US is still a valuable tool in the work up of these women, MRI is a useful imaging adjunct in difficult cases and when more information needs to be obtained for proper pre-surgical planning.