MRI evaluation of invasive placenta: “Cool” answers to radiologists’ “hot” questions

1. Introduction The term “placenta” originates from the Greek word plakuos, which means “flat cake”. Indeed, placenta is the “cake” for the fetus, as it is responsible for its nutrition and its respiratory and excretory function. Prenatal evaluation of the abnormal placenta is a “hot” topic for both gynecologists and radiologists; early identification of invasive placenta and accurate preoperative diagnosis regarding the degree of myometrial invasion and extrauterine spread, are critical for optimal management. Magnetic Resonance Imaging (MRI) may provide important information for pre-delivery planning in patients with invasive placenta; however, accurate interpretation of the MRI appearance of invasive placenta requires expertise. The aim of this study is to review current literature data regarding MRI diagnostic performance for the evaluation of invasive placenta and to familiarize radiologists with common MRI features of abnormal placentation. We have conducted a literature search using MEDLINE (PubMed) Library. Applied key words included: Invasive placenta; placenta accreta; placenta increta; placenta percreta; MRI; US. The search period extended from July 1985 to March 2016. Prospective or retrospective original research studies and review articles with or without meta-analysis data were reviewed; selection of the studies was performed by consensus from all authors and it was based on the presence of the following criteria: Appropriate study design, adequate study population (20 patients), use of clear diagnostic evidence, reliable statistical data and reproducibility of the results. A total of 33 studies were finally included in our review report. 2. Clinical information Invasive placenta is a serious pregnancy condition, characterized by a defect of the desidua, through which the fetal trophoblast (chorionic villi) extends to the myometrium. There are three types of invasive placenta, based on extent of myometrial invasion: (a) placenta accreta (the least invasive type), where the villi attach to the myometrium and may superficially invade it, (b) placenta increta, where the villi partially invade the myometrium and (c) placenta percreta, where the villi completely penetrate the myometrium, reaching to the uterine serosa, with or without invasion of the surrounding extrauterine tissues. Well-established risk factors associated with invasive placenta include a previous Cesarean section (C-section), presence of placenta previa (i.e. location of the placenta at the lower uterine segment, within 2 cm from the internal cervical os; two main types of placenta previa are defined: Complete previa, when placenta completely covers the internal os and marginal previa, when the leading edge of the placenta is less than 2 cm from the internal os and advanced maternal age (35 years). Minor risk factors associated with invasive placenta include Asherman’s syndrome (i.e. the presence of adhesions within uterine cavity), uterine fibroids [1] and history of uterine surgeries, including curettage, abortions or myomectomy [2]. Invasive placenta may spontaneously develop in a small percentage (0.4%) of the general population. Its incidence increases significantly (5%) when placenta previa is present. If the patient also has a history of a single C-section, the incidence rises to 24%; when placenta previa is associated with multiple prior (3) C-sections, the risk for abnormal placentation becomes extremely high (67%) [3]. Invasive placenta may be a life-threatening condition during delivery, because of the increased risk of massive intra- or postpartum haemorrhage, as the abnormal placenta is strongly attached to the myometrium and cannot be completely separated from the uterus, potentially causing uncontrollable bleeding. Massive blood loss (3-5 l) may cause disseminated intravascular coagulopathy (DIC), renal failure, adult respiratory distress syndrome (ARDS) and death. Reported maternal mortality rates due to invasive placenta reach 7% [4-7]. Therefore, accurate prenatal diagnosis becomes an important issue for clinicians, in order to appropriately schedule delivery and minimize maternal and neonatal risks. Patients with invasive placenta are usually scheduled for C-section at 34-35 weeks, in an attempt to limit the risk of fetal lung immaturity. The management of placental invasion requires a multidisciplinary approach with participation of a well-trained surgical team (e.g. gynecologists, urologists), dedicated anesthesiologists and pediatricians. Blood products should be readily available. Interventional radiology techniques, such as perioperative internal iliac artery occlusion, may be employed, in order to reduce blood loss during surgery and subsequent need for transfusion [8, 9]. 3. Discussion 3.1 Sonography: Is it enough for the diagnosis of invasive placenta? Transabdominal gray scale and Color Doppler sonography is the first-line imaging modality used to evaluate VOLUME 2 | ISSUE 1 41 HR J invasive placenta, since it is a widely available, low-cost imaging method, which can be safely applied to the pregnant patient, due to lack of ionizing radiation. Typical ultrasonography (US) screening for evaluation of the placenta is performed during week 18-20. In cases of placenta previa and prior C-section, the presence of abnormal placentation should be highly suspected. Evaluation of the lower uterine segment may be difficult with conventional sonographic examination; a high-frequency transducer can be used for more detailed imaging. A transvaginal approach may be helpful in some cases, for better evaluation of the myometrium of the anterior lower uterine segment, the morphology of the placenta, and the evaluation of the myometrial-placental interface [7, 10]. Sonographic features indicative of invasive placenta were initially described by Finberg and Williams [11]. Highly specific (80% specificity) sonographic signs of abnormal placentation include: Loss of the retroplacental hypoechoic zone (i.e. the venous network within the stratum spongiosium of the desidua), presence of large and irregular dilated intra-placental vascular spaces (commonly known as placental lacunae) observed as early as at the 15th week of gestation, extensive vascularization in the utero-placental interface, and myometrial thinning (1 mm) [10, 12]. In the case of placenta percreta with extrauterine spread, US may demonstrate a focal uterine bulge with a vascular mass extending beyond the uterus, marked thinning or loss of the normal utero-bladder interface and presence of a prominent vascular network between the uterine serosa and the bladder. Recently, Shih et al. [13] reported three-dimensional (3D) power Doppler criteria for the diagnosis of invasive placenta, focused on the evaluation of the utero-placental vascularity pattern; extreme intraplacental vascularity and presence of numerous, tortuous vessels with a chaotic pattern (mimicking the neovascularity observed in ovarian malignancies), located at the base of the placenta proved to be accurate signs of invasive placenta [13]. The overall diagnostic accuracy of sonography for the evaluation of the placenta is high, with reported sensitivity (SE) and specificity (SP) values equal to 85.7% and 88.6%, respectively [14]. Difficulties in the evaluation of invasive placenta include posterior location, presence of a postoperative uterine scar (as it is often associated with an acoustic shadow) and poor imaging quality due to patient’s body habitus or operator’s inexperience. Sonographic evaluation of extension of the placenta to the surrounding tissues and organs (e. g. parametrial space, bladder and bowel) may be limited because of their location deep in the pelvis and the transducer’s relatively small field of view. 3.2. MR imaging for evaluation of the placenta: When is it needed? MRI is an excellent imaging modality for the evaluation of pelvic tissues, due to its inherent ability to discriminate between tissues with similar consistencies. MRI can accurately assess the location of the placenta in the uterus, although this is easily evaluated with sonography, with the exception of a posteriorly located placenta [15]. Literature data support the important role of MRI for delineating the overall topography of invasive placenta prior to surgery. Lateral extension into the parametrial fat is rather uncommon (16%); however, such informat