IMAGING OF THE PLACENTA

Dr.Thambidurai,

Resident-Radiology,

Stanley Medical College.

Importance

Abnormalities of the placenta are important to recognize owing to the potential for maternal and fetal morbidity and mortality. Pathologic conditions of the placenta include

Placental causes of hemorrhage, Gestational trophoblastic disease, Retained products of conception, Nontrophoblastic placental tumors, metastases,

and Cystic lesions..

Imaging modalities

Sonography remains the imaging modality of choice for evaluation of the placenta.

Magnetic resonance (MR) imaging can be of added diagnostic value when further characterization is required, particularly in the setting of invasive placental processes such as

placenta accreta and gestational trophoblastic disease.

Role of CT

Computed tomography (CT) has a limited role in the evaluation of placental disease owing to limited tissue characterization, compared with that of MR imaging, and the radiation risk to the fetus;

The primary role for CT is in the evaluation of trauma and gestational trophoblastic disease, for which it allows characterization of the primary lesion and distant metastases

Introduction

The placenta is named for its appearance (Greek plakuos, meaning “flat cake”) and is responsible for the nutritive, respiratory, and excretory functions of the fetus.

The placenta is often overlooked in the routine evaluation of a normal gestation, receiving attention only when an abnormality is detected.

Embryologic Features

Both fetal and maternal components contribute to the structure of the placenta. The villi of the chorion frondosum are fetal in origin and contain arterial plexuses supplied by the umbilical artery. These chorionic villi protrude into the intervillous space, where they are bathed in maternal blood.

The maternal portion of the placenta is composed of the decidua placentalis, which lines the intervillous space. Fetal trophoblastic invasion of the endometrium induces decidual changes. Maternal decidual septa separate groups of villi within the intervillous space.

Normal placenta. (a) US image shows a placenta (P) that is relatively homogeneous in echo-texture. The retroplacental clear space is hypoechoic (arrowheads). 

 Sagittal single-shot fast spin-echo (SSFSE) T2-weighted MR image shows a placenta (P)with intermediate signal intensity. The dark line represents the retroplacental clear space (arrowheads).

Morphology and variants

Typically, the placenta is located along the anterior or posterior uterine wall, extending onto the lateral walls.

Although usually discoid, the placenta can be variable in morphology. Variant placental shapes include bi-lobed, succenturiate, circumvallate, and placenta membranacea.

Succenturiate placenta. (a) Diagram shows a placenta with a succenturiate lobe. (b) US image shows a placenta (P) with a succenturiate lobe (S). The main body of the placenta is located along the posterior uterine wall. A second soft-tissue structure of the same echogenicity but located anteriorly is the succenturiate lobe.

Bilobed placenta. (a) Diagram shows a bilobed placenta. (b) US image shows a bilobed placenta. The two lobes of the placenta (P1 and P2) are separated by a thin bridge of placental tissue that covers the internal os. In this case, the umbilical cord (arrowhead) inserts into the bridge of tissue.

Circumvallate placenta.--US image shows a circumvallate placenta. The chorionic plate (the fetal surface of the placenta) (black arrowheads) is smaller than the basal plate (the surface interfacing with the uterus), with rolling and shouldering of the placental margins (white arrowheads). F = fetus.

placenta membranacea.

The umbilical cord typically inserts centrally, but eccentric and velamentous (outside the placental margin) insertions also occur .

Eccentric insertions are cord insertions that are less than 1 cm from the placental edge. These are distinguished from a velamentous insertion, where the umbilical cord inserts on the chorioamniotic membranes rather than on the placental mass. This membranous insertion results in a variable segment of the umbilical vessels running between the amnion and the chorion, unprotected by Wharton jelly

Velamentous insertion of the umbilical cord. Doppler US image shows insertion (I) (white arrow) of the umbilical cord into a thin membrane of tissue extending from the margin (black arrow) of the placenta (P).

Placental size is expressed in terms of thickness in the midportion of the organ and should be between 2 and 4 cm.

Placental thinning has been described in systemic vascular and hematologic diseases that result in microinfarctions.

Thicker placentas (>4 cm) are seen in fetal hydrops, antepartum infections, maternal diabetes, and maternal anemia. Placental thickening can be simulated by myometrial contractions and underlying fibroids.

Chorioamniotic separation

The placental and fetal membranes (chorion and amnion, respectively) are separate early in gestation, accounting for the appearance of the amniotic sac. After approximately 14 weeks gestation, these membranes fuse and are no longer separately distinguishable (12).

In rare cases, chorioamniotic separation can occur later in gestation. This can be focal or extensive, with the amniotic membrane becoming either free floating or adherent to the fetus. Extensive cases pose a risk to the fetus, with increased rates of both preterm delivery and the development of amniotic bands (12).

Chorioamniotic separation is most commonly related to prior intervention such as amniocentesis or surgery but can occur sporadically. Sporadic cases have been associated with increased rates of underlying fetal chromosomal and developmental abnormalities

Chorioamniotic separation is usually detected with US and is visible as a free-floating or adherent membrane surrounding the fetus. Separation can extend throughout the entire uterine cavity and over the surface of the placenta.

Chorioamniotic separation. Transverse (a) and sagittal (b) images from obstetric US performed at 20 weeks gestation show a free-floating membrane (arrowheads) surrounding the fetus (F). This membrane is the amnion, which is completely separated from the underlying chorion; there is even separation (arrow) over the surface of the placenta (P). 

Twin Gestations-twin peak sign

Twin peak sign in dichorionic-diamniotic twin gestations. (a) US image of an early twin gestation shows the separate placentas converging at the insertion of the amniotic membrane (arrowhead), forming the so-called twin peak that is characteristic of a dichorionic-diamniotic gestation.

 (b) Sagittal SSFSE MR image shows similar findings, with the twin peak (*) formed by the two placentas. Arrowhead = intertwin membrane.

Placental Causes of Antepartum Hemorrhage

Placenta previa and placental abruption account for more than one-half of cases of antepartum hemorrhage and are increasing in prevalence as the rate of cesarean section increases.

Placental Hematoma

placental hematomas appear as well-circumscribed masses with echogenicity that varies according to chronicity.

They are hypoechoic or anechoic in the acute phase, heterogeneously echogenic in the subacute phase, and anechoic in the chronic phase.

Doppler interrogation should reveal absence of internal blood flow; this finding allows differentiation of hematomas from other placental masses

Placental hematoma. (a) US image shows a rounded collection of mixed-echogenicity material (arrowheads) deep to the chorion along the lateral margin of the placenta. There is no internal Doppler signal to suggest blood flow. This appearance is consistent with a subchorionic hematoma.

Placental Abruption

Placental abruption represents premature separation of the placenta from the uterine wall

US is frequently performed to confirm the presence of abruption and assess the extent of subchorionic or retroplacental hematoma . The presence of blood in large enough volumes to be visible sonographically indicates retained hemorrhage that may remain symptomatic.

False-negative results can occur when blood dissects out from beneath the placenta and drains through the cervix.

 US image shows placental abruption. A crescenteric collection of predominantly hypoechoic fluid lifts the edge of the placenta (P) away from the underlying myometrium (M). 

Placenta Previa

Placenta previa refers to abnormal implantation of the placenta in the lower uterine segment, overlying or near the internal cervical os

Normally, the lower placental edge should be at least 2 cm from the margin of the internal cervical os.

The diagnosis of placenta previa should not be made before 15 weeks gestation.

Transvaginal US image obtained at 20 weeks gestation shows a low-lying placenta (P). The placental margin comes to within 0.7 cm of the internal cervical os

Transvaginal US image obtained at 19 weeks gestation shows marginal placenta previa. The placental tip (T) is located immediately at the internal cervical os (O) but does not cover it

Transvaginal US image obtained at 19 weeks gestation shows complete placenta previa. The placenta (P) entirely covers the internal cervical os (O).

Vasa Previa

Vasa previa refers to the presence of abnormal fetal vessels within the amniotic membranes that cross the internal cervical os. These vessels are unsupported by Wharton jelly or placental tissue and are at risk of rupture when the supporting membranes rupture; such vessels are also at risk of direct injury during labor. Rupture of these vessels can lead to catastrophic fetal hemorrhage.

The diagnosis of vasa previa is made with Doppler US, which demonstrates vascular flow within vessels overlying the internal cervical os .

There is vascular flow in a vessel (V) that is closely applied to the internal cervical os (O). 

Placenta Accreta, Increta, and Percreta

During the process of placental development and implantation, a defect in the normal decidua basalis from prior surgery or instrumentation allows abnormal adherence or penetration of the chorionic villi to or into the uterine wall .

The extent of adherence to and invasion of the placental tissue varies:

Superficial invasion of the basalis layer is termed placenta accreta (approximately 75% of cases);

deeper invasion of the myometrium is termed placenta increta ; and

even deeper invasion involving the serosa or adjacent pelvic organs is termed placenta percreta

Sonographic features of placenta accreta include

loss of the normal retroplacental clear space, anomalies of the bladder-myometrium interface, prominent placental lacunae, and increased vascularity at the interface of the uterus and

bladder . Of these various sonographic features, the

presence of prominent placental lacunae has the highest positive predictive value. Lacunae are characterized by ill-defined margins, irregular shape, and turbulent flow.

US images show disruption of the normal hypoechoic myometrium (black arrowheads) by invading placental tissue (white arrowheads). B = bladder, P = placenta.

 Sagittal SSFSE MR image shows intermediate-signal-intensity placental tissue (arrowhead) invading the normal dark myometrium (M) in the lower uterine segment, findings consistent with placenta accreta.