The failure of the endometrium to decidualize in the area of the uterine scar is a critical disorder of PA[10].This allows abnormally deep anchorage of the chorionic villi and uncontrolled myometrial invasion by the extra- chorionic villi. Therefore, uterine scarring from previous surgery (e.g., cesarean section) is a risk factor. The worldwide increase in cesarean rates is accompanied by an increase in the incidence of PA, which increases 7-fold after one cesarean and 56-fold after three or more cesarean deliveries[11].Several studies have shown that in cases of PA with uterine scarring, the abnormal vascularity in the area between the uterine scar and the placenta can be observed in early pregnancy[12]. Placenta previa or low lying placenta may overlie the cesarean scar and thus promote the development of PA. The most typical PA cases are placenta previa with previous cesarean deliveries[13]. Advanced maternal age, uterine malformation and myomectomy are relatively minor risk factors.
In a normal placenta, the endometrial decidua provides a barrier to prevent the myometrium from being invaded by the placental villi. After delivery, the myometrium can be completely separated from the placental villi. When the chorionic villi invade the myometrium through defects in the underlying deciduas, these are called placental accreta. In this case, the placenta does not spontaneously separate from the myometrium after the delivery of the fetus[14].Placenta accreta refers to the spectrum of abnormal placental invasion named placenta accreta vera, increta, and percreta according to the depth of myometrial invasion.In placenta accreta (also known as placenta accreta vera ), the villi are attached to the myometrium without an intervening decidua or trophoblast, and are diagnosed on either the placenta or on hysterectomy specimens from cesarean sections. With placenta increta, the villi partially infiltrate into myometrial wall. The most severe form is placenta percreta, in which the villi penetrate uterine serosa or adjacent structures. Regarding the management of PAS, the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine recommend that, because of the high risk of postpartum hemorrhage, hysterectomy is not attempted by cesarean section after delivery of the fetus but is performed as planned. In our group, 33 patients underwent hysterectomy[15–16]. A recent report illustrates that removal of the placenta only, followed by insertion of a Bakri balloon, successfully prevented hysterectomy in 84% (16/19) of patients with PA[17]. It is worth noting that these conservative approaches have been reported in only a few cases and, therefore, efficacy remains uncertain.
The definitive placenta is formed at the end of the first trimester (12–16 weeks). Later in the second trimester, the placenta develops regular thin septa that divide the placenta into individual cotyledons (24–31 weeks) [18–19]. The thickness of the normal placenta is about 2–4 cm in the middle portion and has a smooth contour. The thickness of the placenta becomes progressively thinner and shows up as a smooth angle on axial imaging.During 24–30 weeks of gestation, which is the optimal gestational age for MRI diagnosis of PA, the normal placenta is usually homogenously intermediate T2 signal in contrast to the homogenous low T2 signal of the myometrial layer. These two structures can’t been recognized on T1WI [20].At less than 24 weeks, the placenta is immature and MRI is not effective in assessing abnormal placenta.Beyond 30 weeks, the placenta becomes increasingly large and heterogeneous as it matures. Throughout pregnancy, the signal of the myometrium changes dynamically on MRI.During the 24–30 weeks of gestation, the myometrium usually maintains its trilaminar wall appearance.The inner and outer layers of the myometrium presented as slightly hypointense on T2WI. The middle layer of the myometrium presented as hyperintense. Beyond 30 weeks, the myometrium becomes thinner as the gestation progresses.
The recently released Society of Abdominal Radiology and European Society of Urogenital Radiology (SAR-ESUR) consensus statement is a key step in the harmonization of MRI studies with PA.We classified the entire known MRI signs into the following three categories based on the underlying pathophysiological changes: gross morphological signs, interfacial signs, and histological signs. There are four gross morphological signs, four interfacial signs, and three histological signs. The above seven MRI signs are strongly and unanimously recommended by SAR-ESUR as follows:i) Myometrial thinning:This sign is known as focal PA, and it has been reported that when the placenta is focally accreted to the overlying myometrium, the myometrium may thin or become undetectable due to this accreta.ii) Loss of T2 hypointense interface: This sign is consistent with the loss of retroplacental clear zone on ultrasound. This sign implies the disappearance of the hypointense interface between the placenta and the myometrium.iii) Abnormal vascularization of the placental bed:This sign can be seen on MRI as flowing voids, or color Doppler-positive vessels extending from the placenta to the serosa or bladder wall, also known as "bridging vessels".iv) T2-dark bands: These are linear or nodular thick (> 1 cm) T2 hypoinense bands that are pathologically associated with fibrin deposition as a sequel to prior hemorrhage or infarction.v) Placental/uterine bulge:
In patients with placental anomalies, the expected "inverted pear-shaped" uterine contour is replaced by an "hourglass-shaped" appearance due to an enlarged, low lying placenta occupying the width of the lower uterine segment, which is overlain by a thinning and remodeling myometrium consistent with the placental contour.vi) Bladder wall interruption :
This is also known as the "bladder tent" sign and refers to an abnormal upward traction of the placenta on the bladder in the supine position due to the placenta invading the bladder wall and causing an interruption of the bladder wall contour.vii) Focal exophytic mass: This corresponds to a severe invasion of the placenta beyond the serosa of the uterus into the parametrium as a focal mass.Four other diagnostic features including placental ischemic infarction, placental heterogeneity, abnormal intraplacental vascularity, and placental protrusion into the cervix were designated as "indeterminate" categories and the data were considered to be below the recommended threshold[21].
In the presence of MRI signs ii,v,vi,vii, MRI can clearly diagnose placenta percreta, and all 22 cases in this group were clearly diagnosed before delivery. The presence of ii,ii,ivMRI signs strongly indicated placenta increta, which was clearly diagnosed in 13 cases and misdiagnosed in 3 cases in this group prenatally. If combined with other four MRI signs, and the placenta is combined with the uterus with deep scattered patchy short T2 signal foci, then it suggests placenta accreta appearing ischemic necrosis. Placental bulge type II and uterine serosal hypervascularity are useful MRI features to distinguish PP from PA[22].
In the past few years, the author has observed the PA in the patients with the image observation and medical history follow-up, and found that when the MRI sign of Loss of T2 hypointense interface appeared,the placenta was confirmed to be at least placenta accreta vera intraoperatively, and the placental tissue had to be removed manually. In this group, 8 cases were correctly diagnosed prenatally and 4 cases were misdiagnosed.
Enhanced T1WI can clearly indicate the presence or absence of blood in the placenta. The use of gadolinium contrast in pregnant women is controversial because its half-life in the fetus is unknown. Some animal studies have shown that it will retard the development of rats. Gadolinium-based MRI contrast agents cross the placental-fetal barrier and are FDA Class C[23].The American Academy of Radiology recommends that gadolinium should not be administered to pregnant women unless specifically needed. Gadolinium enhancement is not recommended for pregnant women unless they decide to terminate their pregnancy, and none of the 50 patients in this group underwent enhancement scans.
US often the primary imaging tool for evaluating women at risk for PA, such as those presenting with placenta previa and prior cesarean or uterine surgery, whereas MRI is used to confirm US diagnosis and delineate the depth and topography of placental invasion, which may significantly affect maternal outcomes and influence surgical approach[24].US may be limited in detecting more severe placental penetration, especially in assessing the extent of extrauterine involvement of the placenta.In terms of evaluating the degree of invasiveness, MRI outperforms US due to its better inherent contrast.In addition, although the lower uterine segment is the most common site of PA and can usually be evaluated with a high-frequency ultrasonic probe because of its relatively superficial location in pregnancy, other locations of PA may be more difficult to identify by US.When PA is located far from the probe, such as posterior accreta or when the patient is obese, the resolution obtained will not be as good as the former.In this case, US is difficult to identify PA. On MRI, the location of PA abnormality and the patient's body size do not affect the resolution or image quality. In contrast to ultrasound, MRI can provide versatile contrast and may provide additional information about placental structure and function to assist in the diagnosis of PA[25, 26]. Sun et al[27]. used texture features and automated machine learning to identify suspected PAS disease based on anatomical MRI and achieved sensitivity, specificity, and accuracy of 100%, 88.5%, and 95.2%., respectively. Bao et al[28]. recently investigated the diagnostic value of intravoxel incoherent motion (IVIM) for PA and showed that pseudo-diffusion fraction could predict PA with an accuracy of 85.7%.Yan et al[29]. reported that diffusion MRI showed superior diagnostic performance in the evaluation of PA compared to conventional anatomical MRI markers. In combination with anatomical MRI features, diffusion MRI has the potential to improve diagnostic accuracy and could assist in the clinical management of patients with PA. Although there is no published literature comparing the effects of 3T and conventional 1.5TMRI, 1.5T MRI is adequate for clinical diagnosis and there is long-term evidence of their safety during pregnancy[30].
In conclusion, MRI has the advantages of superior soft tissue resolution, large imaging range, and is not affected by placental position, and is currently an important and more advanced examination method for the diagnosis of PA. Currently, MRI is particularly useful in cases where US is inconclusive and to assess the extent to which the placenta penetrates the uterine serosa and invades outward into surrounding tissues[31]. Signs such as loss of T2 hypointense interface and placental/uterine bulge on MRI images are of great value in the diagnosis of PA. Although ultrasound is the first-line imaging method for placental evaluation, MRI now plays an important role in the prenatal diagnosis of invasive placenta and allows for multidisciplinary treatment planning to reduce maternal morbidity and mortality.The present group of cases also confirmed its excellent diagnostic value.
