Brian C. Allen, MD∗bcallen2@wakehealth.edu and John R. Leyendecker, MD,     Abdominal Imaging, Department of Radiology, Wake Forest University School of Medicine, Wake Forest Baptist Medical Center, Medical Center Boulevard, 3rd Floor MRI, Winston-Salem, NC 27157-1088, USA

∗Corresponding author.

Because of the high maternal morbidity and mortality of undiagnosed placental abnormalities, there is a need for accurate antenatal diagnosis. Important placental features amenable to investigation with magnetic resonance (MR) imaging include variant placental location and morphology, and abnormal implantation or invasion of placenta into the myometrium. MR imaging features permit the diagnosis of abnormal placentation include placental lobulation with uterine contour deformity, interruption of the inner low signal-intensity myometrial layer, and placental heterogeneity resulting from dark intraplacental bands and abnormal vascularity.

Keywords

Placenta • Accreta • Increta • Percreta • Previa • Magnetic resonance

Key points

Introduction

Abnormal placentation is becoming more prevalent, largely attributable to increasing rates of cesarean delivery. Because of the potential maternal morbidity and mortality associated with some undiagnosed placental abnormalities, there is a need for accurate antenatal diagnosis.

Ultrasonography remains the primary method of imaging the placenta, as it is relatively inexpensive and widely available, and evaluation of the placenta is routinely and easily performed during a fetal screening examination at 18 to 20 weeks’ gestation. The high negative predictive value of ultrasonography for placental abnormalities relegates magnetic resonance (MR) imaging to a supporting role, reserved for equivocal sonographic findings or incomplete evaluation, as in cases of posterior placenta.

Placenta accreta, increta, and percreta describe a continuum of placental attachment disorders associated with incomplete postpartum detachment of the placenta and postpartum hemorrhage. During the MR imaging evaluation of suspected placenta accreta, other placental and umbilical cord anomalies may be identified, given their reported association with abnormal placentation. These disorders include placenta previa and vascular anomalies, such as velamentous cord insertion and vasa previa.

Normal anatomy

The normal disc-shaped placenta attaches to the anterior or posterior uterus, with the normal decidua basalis providing a plane of separation between the placenta and the uterine wall. The midportion of the placenta typically measures between 2 and 4 cm in thickness, and the placenta undergoes changes during gestation that are evident on MR imaging.1 Between 19 and 23 weeks gestational age, the placenta is typically relatively homogeneous on T2-weighted imaging (Fig. 1). Between weeks 24 and 31 the placenta becomes slightly lobulated, and conspicuous septae appear between placental lobules, leading to increased heterogeneity with increasing gestational age (Fig. 2). Following contrast administration, the placenta heterogeneously enhances before the uterus, and becomes more homogeneous over time.2,3

The normal myometrium has a trilayered appearance on T2-weighted images (Fig. 3). The middle layer is a heterogeneously hyperintense vascular layer, with thinner low signal-intensity layers on either side.4 The uteroplacental unit is of uniform, intermediate signal on unenhanced T1-weighted images, affording no opportunity to distinguish the placental-myometrial interface or to examine myometrial architecture.

The myometrium thins at sites of compression, such as adjacent to the spine and aorta, appearing as a single thin layer of uniform signal intensity (Fig. 4). Myometrial contractions are commonly imaged incidentally, visible as rounded or lentiform regions of transient myometrial thickening, demonstrating the low signal intensity typical of smooth muscle on T2-weighted images (Fig. 5). Contractions are easily distinguished from leiomyomas by the intermittent and temporary nature of the former entity. Subplacental contractions can also harmlessly and temporarily deform the overlying placenta.

Imaging protocols

The optimal timing of an MR imaging examination for evaluation of the placenta has not been clearly established. MR imaging is generally not performed during the first trimester, owing to theoretical concerns for the safety of the fetus and early stage of placental development. During the second trimester, most patients can tolerate supine imaging. However, in the third trimester, lateral decubitus imaging may be required to avoid the risk of impaired systemic venous return caused by uterine compression of the maternal inferior vena cava. Imaging late in the third trimester can be challenging, not only because of difficulties positioning the patient, but also because the placenta becomes more heterogeneous and the myometrium becomes thinner and more stretched late in gestation.

When evaluating the patient for placenta percreta, the bladder should be mildly distended. Anatomic landmarks can be difficult to identify in the setting of a completely collapsed bladder, and bladder-wall invasion can be difficult to exclude when a full bladder is closely apposed to the uterus. No other patient preparation is typically required.

A multichannel surface coil is used when possible. Initially, single-shot fast spin-echo/turbo spin-echo (ssFSE/ssTSE) or half-acquisition turbo spin-echo (HASTE), or balanced steady-state free-precession (true FISP or FIESTA) sequences are obtained in 3 imaging planes (Box 1). These sequences are relatively resistant to both maternal and fetal motion artifacts, and provide sufficient anatomic detail to be diagnostic in many cases. When possible, a radiologist can evaluate these images and decide whether additional imaging planes or sequences are required to further clarify suspicious findings. A fat-suppressed gradient echo T1-weighted sequence can help identify high signal-intensity blood products related to subchorionic hemorrhage or other abnormality (Fig. 6).