Magnetic Resonance Imaging Evaluation Of The Cervical Morphology Of Invasive Placenta Previa In The Third Trimester Of Pregnancy

In 2018,the International Federation of Gynecology and Obstetrics(FIGO)recommended that diseases such as abnormal adhesion of placental trophoblast and partial or complete invasion of uterine myometrium be collectively referred to as Placenta Accreta Spectrum(PAS),According to the depth at which the placental trophoblast invades the myometrium,PAS is classified into three types:placenta accreta,placenta increta,and placenta percreta.Placenta increta and placenta percreta are referred to as invasive placenta(IP).The incidence of PAS is on the rise in many middle-and high-income countries around the world.Previous cesarean section and placenta previa are recognized as the two most important risk factors,and PAS is often associated with placenta previa.Cesarean section rate in China is very high,and the incidence of PAS and placenta previa is high.In particular,there has been a turning point in the incidence of PAS since the birth control policy was gradually liberalized in 2013.Invasive placenta causes incomplete separation of placenta and uterus during delivery,resulting in a large amount of blood loss.If the mother is not given timely and appropriate treatment,life-threatening intrapartum or postpartum uterine hemorrhage may occur,with high morbidity and mortality.Therefore,accurate and detailed prenatal diagnosis and a mature and standardized multidisciplinary team are essential guarantees for maternal-fetal safety.Ultrasound and magnetic resonance imaging(MRI)are currently commonly used imaging methods for prenatal diagnosis of invasive placenta.Ultrasound is the preferred method and screening method.However,in the third trimester of pregnancy,when clinical protocols are particularly critical,ultrasound examination is restricted by many factors,making it difficult to assess the site and extent of invasion.In recent years,MRI has become an increasingly mature technique to evaluate placental lesions and become an effective supplement to prenatal ultrasound examination.Many studies have shown that both the depth and distribution of placental invasion have an important impact on surgical management,and cervical length has an impact on the difficulty of hysterectomy and patient clinical outcomes.However,there are few MRI studies on the cervical morphology of invasive placenta previa.During pregnancy,the cervical cavity located in the middle of pelvic cavity can reflect the changes of cervical load and surrounding structure.Ultrasound and MRI can be used to observe the cervical cavity visually and conveniently.Moreover,in the third trimester of pregnancy,MRI is more advantageous in the display of cervical morphology when ultrasound examination is restricted by many factors and the display is poor.Because placenta migrates during pregnancy,placenta previa must be identified in the third trimester.In addition,in the mature and standardized multi-disciplinary maternal-fetal management center,clinicians will apply for MRI examination in the third trimester of pregnancy according to the specific situation of patients,so as to evaluate the operation timing and formulate the operation plan.Therefore,this study intends to retrospectively analyze the cervical morphology in MRI images of placenta previa in the third trimester of pregnancy,to explore the correlation between cervical morphology and placenta previa and the depth and location of placenta previa invasion,so as to analyze the role of cervical morphology in evaluating the invasive placenta previa by MRI in the third trimester of pregnancy.The summary is as follows.Objective:To retrospectively analyze the cervical morphology in MRI images of placenta previa in the third trimester of pregnancy,to explore the correlation between cervical morphology and the depth and distribution of placenta invasion,and to analyze the role of cervical morphology in evaluating the invasive placenta previa by MRI in the third trimester of pregnancy.Materials and methods:Subjects:112 patients diagnosed with complete placenta previa by prenatal ultrasound and surgery and who underwent placental MRI in the third trimester of pregnancy were included in this study.Imaging methods:1.5T placental magnetic resonance imaging was performed in all patients.Imaging sequences used are half-Fourier acquisition single--shot turbo spin--echo(HASTE),true fast imaging with steady--state precession(true-FISP),t1-weighted imaging(T1WI)and diffusion weighted imaging(DWI).No sedatives or gadolinium contrast agents were administered to the fetus or mother during the examination.Image evaluation:Two senior radiologists with no knowledge of the surgical results independently evaluated the morphology of the cervical region on the image using a double-blind method.The median sagittal section of the cervix was selected as the standard section for observation and measurement of the cervix.If the cervix is difficult to identify,or if the placenta-cervical interface at the cervix is uneven and/or more than the expected curvature,the placenta is considered to have entered the cervix and cervical length measurement is not performed.The two ends of the low signal band in the cervical stroma area were selected as the identification points and the midpoint of the corresponding line between front and back lips was determined as the starting point of measurement.Clinical evaluation:Evaluation of the extent of placental invasion is based on published surgical and/or pathological criteria based on surgical records and pathological results.Patients with unclear cervix were divided into separate groups.Among the remaining patients,those diagnosed with placental penetration and placental implantation were included in the invasive placenta placenta previa(IPP)group,while those diagnosed with placenta placenta previa without placental invasion were included in the non-invasive placenta placenta previa group(non-IPP).The IPP group was further divided into placenta percreta(PP)and placenta implantation group(PI)according to the depth of invasion.The division of the placental invasion site is determined by the obstetrician according to the clinical and anatomical criteria of the operation.The uterus was divided into upper segment(S1)and lower segment(S2)on the mid-sagittal plane of the uterus,bounded by the midline of the posterior wall of the bladder.IPP group was divided into S1 group and S2 group according to the site of placental invasion of myometrium.For the cases with invasion in both S1 and S2,the cases were divided into groups according to the part with a higher proportion of invasion.Statistical analysis:The correlation between cervical length and surgical outcome evaluation(degree and location of placental invasion)was analyzed by independent sampleT-test.The receiver operating characteristic curve(ROC)is used to determine the optimal threshold,sensitivity and specificity of cervical length for evaluating the depth and location of placental invasion.The reliability of raters for cervical length measurements was assessed between two imaging physicians using intra-group correlation coefficients.The sensitivity and specificity of cervical invasion by Magnetic resonance evaluation was used to evaluate P<0.05 is considered statistically significant.Results:Of a total of 112 patients,7 had placental invasion of the cervix,and MRI images of the cervix were not complete enough to allow measurement of cervical length.Of the remaining 105 patients,57 were included in the IPP group and 48 were included in the non-IPP group.There was no significant difference in age and gestational weeks between the IPP group and the non-IPP group.The number of cesarean section and pregnancy in the IPP group was significantly higher than that in the non-IPP group(P<0.05).57 patients in the IPP group were grouped according to the depth and location of placental invasion.17 cases were included in PP group and 40 cases were included in PI group.27 cases were included in group S1 and 30 cases were included in group S2.There was no significant difference in gestation weeks between S1 group and S2 group(P=0.64).There were statistically significant differences in cervical length between non-IPP group and PI group(P=0.002,<0.05)and between non-IPP group and PP group(P=0.007,<0.05).There was no significant difference in cervical length between PP group and PI group(P=0.21,>0.05).The cervical length of S2 group(2.66±0.66 cm)was significantly shorter than that of S1 group(3.24±0.64 cm),and the difference was statistically significant(P<0.001).There was no significant difference in cervical length between SI group and non-IPP group(P=0.37,>0.05).All the 7 patients with endocervical placental penetration on MRI images were found to have placenta invasion into the cervix during surgery.The sensitivity and specificity of endocervical placental invasion were 100%and 100%,respectively.The two radiologists had a good reliability rating of cervical length(ICC>0.75).Conclusion:MRI cervical morphology evaluation can be used as an index to predict placenta previa.Cervical length can predict the site of invasive placenta invasion.When cervical length becomes shorter,it indicates the location of placenta invasion is lower,and surgery is more difficult and risky.Placental penetration in the cervix can be a reliable characteristic of cervical invasion.Our findings contribute to more accurate preoperative preparation and planning for multidisciplinary management in the clinical management of invasive placenta previa.