Placenta accreta – MRI findings

A 40-year-old pregnant woman was admitted to the obstetric department after second trimester vaginal bleeding. The transvaginal ultrasound revealed a placenta previa, placental lacunae and a high suspicion for placenta accreta. The patient underwent a pelvic MRI that confirmed the diagnosis of placenta accreta.

T2-weighted images show a total posterior placenta with extension to the cervical internal os, related to total placenta previa. Also, on coronal T2-weighted images uterine bulging is depicted, especially at the lower anterior portion of the placenta, where focal myometrial disruption can be seen, related to placenta accreta. The typical placental heterogeneity in placenta accreta, with hypointense fibrous bands is present.

Placenta accreta occurs when the chorionic villi invade the myometrium because of a defect on the decidua basalis [1]. Placenta accreta is categorised according to the myometrial invasion: placenta vera – the villi are attached to the myometrium without invading it; placenta increta – the villi partially invade the myometrium and placenta percreta – the villi penetrate the entire myometrial thickness and beyond the serosa [1,2].  The two most important risk factors are previous cesarean section (present in this patient) and placenta previa [1,2]. Minor risk factors are previous uterine surgery, advanced maternal age, uterine anomalies, curettage, dilation and myomectomy [3]. 

Usually, these women present with abnormal vaginal bleeding during pregnancy and have high serum levels of alfa-fetoprotein and chorionic gonadotropin around the second-trimester [3]. The clinical concern about placenta accreta is massive haemorrhage (3-5L of blood) at the time of placenta separation, which can lead to intravascular disseminated coagulopathy, renal failure, adult respiratory distress syndrome and death [3]. Hysterectomy is often required in these cases [1,2,3,4].

Ultrasound has been the first diagnostic tool for placenta accreta and diagnosis is usually evident at the second-trimester scan [3]. Either transabdominal or transvaginal approaches are useful, being the later used for evaluation of low-lying placentas [4]. When the placenta is evaluated signs of placenta accreta, such as: placenta previa, placental lacunae and abnormal colour Doppler pattern, must be excluded. Placental lacunae are the most sensitive sign for placenta accreta diagnosis after 15 weeks' gestation and consist on vascular structures within the placenta that extend into the myometrium, giving the placenta an appearance of “Swiss cheese” [3]. Abnormal colour Doppler pattern is characterised by a disruption in the normal continuous colour flow in the myometrium, which represents the local of chorionic villi invasion [3].

On MRI studies, the T2 sequence is the best to evaluate the placenta [1,2,3,4]. On T2-weighted images, the placenta has a homogeneous isointense signal and is clearly delineated from the myometrium. Some normal, regular and thin placental septa can be seen within the myometrium [3,4]. In placenta accreta, dark, thick and irregular bands within the myometrium can be seen; as well as uterine bulging and heterogeneous signal intensity within the placenta [3,5]. Uterine bulging can be a focal outward bulge or can change the uterus’ pear-shape. The intraplacental dark bands are thought to represent areas of fibrin deposition and have a random distribution [3,5]. Although very thin, the myometrium is usually well-separated from placenta, but in placenta percreta focal areas of placental invasion of the myometrial thickness and beyond are seen [3,5].

Our patient underwent a cesarian section with uncontrolled massive bleeding and for that reason she underwent an emergency postpartum hysterectomy. She presented without complaints at the one-year-follow consultation.

Although rare, placenta accreta is a life-threating condition that should be promptly diagnosed.

Written informed patient consent for publication has been obtained.