Evaluation Of The Placental Blood Flow Perfusion And Vascular Anastomoses By Three-dimensional Ultrasonography In Monochorionic Diamniotic Twins

Objective:With the promotion of assisted reproductive technology,the incidence of twin pregnancies increased year by year.Among twin pregnancies,monochorionic diamniotic(MCDA)twin pregnancy has a unique placental pathology:Because the twins share a placenta with extensive vascular anastomoses on its surface,the incidence of perinatal complications and mortality are much higher than those of dichorionic diamniotic(DCDA)twins.The most common complications of MCDA twin pregnancies are twin-to-twin transfusion syndrome(TTTS)and selective intrauterine growth restriction(sIUGR).The placenta is an important organ to maintain pregnancy.Monitoring of placental function is an important index for clinical decisions.The evaluation of placental resistance is based mainly on the assessment of umbilical arteries by color Doppler.The pathologic basis of abnormal umbilical blood flow is the decrease in the number of secondary villi in the placenta.The reduction of blood perfusion in the placenta occurs earlier than the reported resistance index of the umbilical artery.Previous studies have found that three-dimensional power Doppler ultrasound(3D-PDU)can quantitatively evaluate normal placental perfusion and placental perfusion in fetuses with intrauterine growth restriction,preeclampsia and gestational diabetes mellitus,but current studies generally focus on singleton pregnancies.However,to the best of our knowledge,there are no reports about placental perfusion in twin pregnancies assessed by 3D-PDU.In the present study,we used 3D-PDU to obtain placental perfusion at the plane where each fetal umbilical cord entered the placenta in MCDA twins,analyzed the vascular indices of sIUGR and normal twins,and quantitatively provide more evidence for clinical monitoring of sIUGR twins.Since vascular anastomoses are almost universal in twin placentae.Several studies have attempted to observe the type of anastomoses and its relationship with the incidence of complications;the overall approach involves injecting colored dye into the placenta after birth.However,placenta injection could not show the internal structure of the placenta and reflect anastomotic changes in real time.In the present study,we applied three-dimensional high-definition flow(3D-HDflow)technique to collect volumetric data of the placenta in MCDA twins and observed the vascular anastomoses in combination with tomographic ultrasound imaging(TUI).We compared the ultrasound results with the postnatal placentas after dye injection to determine the diagnostic value of 3D-HDflow in combination with TUI for placental vascular anastomosis.Methods:This was a prospective study conducted in the authors’ hospital,and involved normal MCDA twins and twins with sIUGR in one twin.30 sIUGR twin pregnancies(60 fetuses)were enrolled.The marternal age was ranged 20-36 years old.The gestational age(GA)was ranged 15-32 weeks.We divided the twins with sIUGR into two groups:in group I(19 twins),the smaller fetus had continuous forward end-diastolic blood flow;in group II(11 twins),the smaller fetus had a loss or a reverse end-diastolic blood flow.104 normal MCDA twins were enrolled.The marternal age was ranged 22-40 years old.The GA was ranged 15-32 weeks.The normal twin pregnancies were divided into 19 subgroups according to GA;each group included 5-7 cases.All subjects were confirmed MCDA twins after delivery.The ultrasound equipment was the Voluson E8 Ultrasound Instrument with a 4.0-8.0 MHz transabdominal transducer.The same power Doppler signal settings were used to standardize our evaluation of placental vasulariztion:pulse repetition frequency:0.9 kHz,wall motion filter:lowl,quality:norm,frequency:low,Balance,170;filter,2;Smooth,4/5;Artefact:on.The standard technique used a 35° scanning angle to capture a placental 3D volume close to the implantation of the umbilical cord including the entire vascular tree from basal to chorionic plates.The stored volumes were further analyzed using the VOCAL Sphere mode.The placental 3D power Doppler indices,vascularization index(VI),flow index(FI),and vascularization flow index(VFI),were automatically calculated.Each measurement was performed three times for each twin,and the mean values were used for statistical analysis.SPSS Version 23.0 was used for data analysis.Differences of placental vascular indices,VI,FI,and VFI,between the twins in sIUGR and control groups were compared using paired t-tests.The indices of small and large sIUGR twins in groups I and II were compared with those of control group to determine whether the differences were statistically significant or not.The regression equations of the placental indices and GA were obtained based on the control group.All values were divided by their GA-specific median based on the regression equations into multiples of median(MOM)values to eliminate a possible confounding effect of GA for comparisons between groups.MOM values of each group were expressed as mean±standard deviation,and the independent t-tests were used for comparisons between groups.P<0.05 was considered to be statistically significant.22 monochorionic twin pregnancies,which continuously came to our hospital for prenatal ultrasound examination,were enrolled in this longitudinal study.The maternal age were ranged 19-36 years old.The GA at the first visit was ranged 16-27 weeks.All subjects underwent ultrasound examination every four weeks from the first examination until the 32nd week to confirm any complications during twin pregnancies.The ultrasound equipment used for inspection included Voluson E8 and Accuvix A30.The frequency of the transabdominal volumetric probe was 4-8 MHz,and the frequency of the conventional abdominal convex probe was 1.0-5.0 MHz.Two-dimensional HDflow(2D-HDflow)identified the arterio-arterial anastomoses(AAA)and the arterio-venous anastomoses(AVA).Because of the suspected vascular anastomoses,3D-HDflow was used to get additional data.The technique used a 85° scanning angle to capture a placental 3D volume including the entire vascular tree from basal to chorionic plates.The stored volumes were further analyzed using the TUI mode.The TUI mode was initiated to concurrently display multiple parallel sections of the inspection site.The convergence of vascular anastomosis was taken as the initial plane,and the thickness of the image layer was set to 0.5mm.The inner part of the anastomosis and the surrounding blood vessels were shown in a series of parallel section images to detect changes in various vascular anastomoses in the placenta.We compared the ultrasound results with the postnatal placentas after dye injectionResults:1.There were no significant differences between two fetuses in the control group in terms of the placental vascular indices(P>0.05).The bivariate correlation analysis of placental vascular indices and GA in the control group showed that VI(r=0.785),FI(r=0.675),and VFI(r=0.745),were positively correlated with GA(P<0.05).The regression equations between indices and GA were as follows:VI=EXP(4.369-28.533/GA),R2=0.699;FI=EXP(3.916-13.003/GA),and R2=0.511;VFI=EXP(3.577-37.468/GA),R2=0.675.2.The VI and VFI of the small fetuses in both sIUGR groups were significantly smaller(P<0.05)than those in the control group,but only FI of the small fetuses in sIUGR group II was significantly smaller(P<0.05)than those in the control group.There were no significant differences(P>0.05)between the larger fetuses in the sIUGR groups and the normal fetuses in the control group in the values of VI,FI,and VFI.3.After delivery,5 twin pregnancies were confirmed sIUGR,4 were TTTS,and the rest had no complications.Prenatal fetoscopic surgery and injection specimens identified anastomoses.We confirmed that 18 cases had AAA and 19 cases had AVA.The detection rate of anastomoses by ultrasound increased with GA.2D-HDflow detected an AAA in 10 cases(55.6%)and AVA in 3 cases(15.8%).3D-HDflow in combination with TUI detected an AAA in 13 cases(72.2%)and AVA in 7 cases(36.8%).3D-HDflow images of AAA displayed 3D locations of anastomoses from two umbilical cords on the surface of the placenta.The TUI showed that AAA was present only on the surface of the placenta and did not enter the inside of the placenta.3D-HDflow images of AVA showed that the unaccompanied umbilical artery and vein from two twins entered the same placental lobule with two blood vessels converged at a certain angle.The TUI showed that,after converging on the surface of the placenta,AVAs anastomosed in the deep part of the placenta.Conclusions:3D-PDU could be applied to the study of placental perfusion in MCDA twin pregnancies.This study presented the vascular indices distribution with GA in normal twin pregnancies.The vascular indices could sensitively display the varying degrees of changes in placental perfusion.3D-HDflow combined with TUI could clearly show the 3D structures of AAA and AVA and their relationship with surrounding blood vessels,and the morphology of vascular anastomoses deep inside the placenta in MCDA twins.