Placental Morphologic and Functional Imaging in High-Risk Pregnancies

Placental Morphologic and Functional Imaging in High-Risk Pregnancies
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:  Author's personal copy Placental Morphologic andFunctional Imaging in High-Risk Pregnancies Saemundur Gudmundsson, MD, PhD,* Mariusz Dubiel, MD, PhD, † and Povilas Sladkevicius, MD, PhD*  The placenta is vital for fetal growth and development. Improvement in ultrasound andmagnetic resonance imaging have improved our understanding of placental morphologythat can be important as in the case of placental accrete/percreta. Functional imaging ispresently mainly performed by the use of Doppler ultrasound and can give information onplacental perfusion, which can be vital for clinical diagnosis. This review summarizes thepresent knowledge on placental imaging and it’s clinical value in high-risk pregnancies.Semin Perinatol 33:270-280 © 2009 Elsevier Inc. All rights reserved. KEYWORDS  placenta, ultrasound, MRI, Doppler, pregnancy, imaging, review T he placenta is a highly vascularized organ of fetal srcinthat attaches the growing fetus to the uterine wall. Nor-mal function and perfusion of the placenta is vital for theexchangeofoxygenandnutrientsbetweenmaternalandfetalcirculations and for discharge of waste substances from thefetal blood.Ultrasoundhasopenedupnewwaysofstructuralimaging,and the Doppler modality can provide information on perfu-sion, which is strongly related to function and thus related tofetal growth and development. Although the importance of the placenta is evident to researchers, in clinical obstetricalpractice the fetus receives most of the attention during imag-ing.The aim of this review is to provide an overview of ourpresent understanding of both placental morphologic andfunctional imaging, and its possible clinical use in perinatalmedicine. Morphologic Imaging Ultrasound scanning of the placenta is supposed to be a rou-tine part of the prenatal ultrasound examination. It can pro-vide information on the location and size of the placenta. Inthe first trimester, the placenta can be defined at about 8weeks of pregnancy. Placental volume increases from 83 cm 3 at 12 weeks to 430 cm 3 at term. 1 Hafner et al 2 measuredplacental volume using 3-dimensional sonography between12 and 22 weeks of gestation. The results suggested that arelationship exists between reduced placental volume and asmall for gestational age newborn.The mature term placenta weighs about 600 g and has adiscoid shape with a diameter of 16-20 cm. During ultra-sound scanning in the second trimester of pregnancy, theplacenta is homogenous in echostructure. 3 Placental thick-ness increases throughout pregnancy and as a rule of thumb,the thickness should be approximately equal in millimetersto gestational age in weeks,  10 mm. 4 Placental thickness of more than  2 SD (  40 mm) has been reported in associa-tion with higher frequency of both small for gestational ageand large for gestational age fetuses, hydrops fetalis, and in-creased perinatal mortality. 4,5  A velamentous insertion of the cord directly enters themembranes, whereas a marginal umbilical cord insertion isdefinedasaninsertionwithin1cmoftheplacentaldiscedge.Focal cystic or hypoechogenic lesions are frequentlyseen during ultrasound dating and anomaly screening atmid-gestation (Fig. 1). The most common are maternallakes, which are villous vascular spaces with swirling jetsof maternal blood flow at a low velocity. It is unclear howthese lakes are formed, but speculations are that it is aresult of vascular thrombosis or trauma caused by fetalkicks. The frequency of this finding varies between reports(8%-60%). Thompson et al 6 reported this finding in17.8% in a large series. *Department of Obstetrics and Gynaecology, University Hospital MAS,Malmö, Sweden.†Department of Perinatology and Pathology of Pregnancy, Collegium Medi-cum Nicolaus Copernicus University, Torun, Poland. Address reprint requests to Saemundur Gudmundsson, MD, PhD, Depart-mentofObstetricsandGynecology,UniversityHospitalMAS,SE-20502Malmö, Sweden. E-mail: 270  0146-0005/09/$-see front matter © 2009 Elsevier Inc. All rights reserved.doi:10.1053/j.semperi.2009.04.005  Author's personal copy  Venous lakes have also been described. These have a moreirregular appearance and blood flow is difficult to demon-strate on power Doppler recording. The srcin of these lakesis also unclear, but fibrin deposition is frequent and throm-bosis might also be a part of the etiology.Characteristicultrasoundappearanceinplacentalinvasionof the uterus (see later in the text) is the formation of numer-oushomogenousspacescalledlacunas.Theoriginisunclear,but the placenta is usually thickened. Infarctions and abrup-tion can also be expressed as hypoechogenic structureswithin or under the placenta in the acute phase. However,when organized, infarctions and thrombosis are usually vi-sualized as hyperechogenic areas within the placenta, espe-ciallywhencalcified.Othercommonlyfoundlesionsincludesubchorionic and septal cysts (Fig. 1). In general, hypoecho-genic lesions in the placenta, a part from lacunas in placenta Figure 1  Ultrasound placental imaging.  (A)  Normal placenta with minor calcifications on the chorionic plate and aseptal cyst (right) and a subchorionic cyst (left).  (B)  Placenta with a venous lake. ( C)  Placenta with a maternal lake(blood is flowing into the cyst).  (D)  Image in a case with placenta percreta and characteristic lagoon formations.  (E)  Venous cysts on the surface of the placenta. (Color version of figure is available online.) Placental morphologic and functional imaging in high-risk pregnancies   271  Author's personal copy percreta (see later in the text), are not related to adverseoutcome of pregnancy. 6 The placental echostructure changes during normal preg-nancy. This is probably secondary to microthrombosis andcalcification even in normal pregnancy, but is more pro-nouncedincomplicatedpregnancies,suchascasessuspectedfor fetal growth restriction. Grannum et al 7 proposed a scorefor classification of placental maturity changes based on areview of ultrasound evaluations of placental texture. Theclassification graded placentas from 0 to 3 according to spe-cific ultrasonic findings at the basal and chorionic plates aswell as within the substance of the organ itself. Clinical re-sultsusingthisscoringsystemwere,however,disappointing,showing abnormal placental appearance in many normalpregnancies with normal perinatal outcome. 8 Consequently,this scoring system is infrequently used in clinical practice. Magnetic Resonance Imaging There has been an increase in the use of fetal magnetic reso-nance imaging (MRI) in the last 20 years. Although ultra-soundisandpossiblywillremainthemaintoolforobstetricalimaging, MRI is creating a niche in the areas where ultra-sounddoesnotprovidecompletedetailsorasasecondopin-ion tool to confirm equivocal ultrasonographic findings.There are several obstacles that limit the use of MRI inobstetrics: first is fetal motion, still unclear safety consider-ations, particularly for new techniques and contrasts agents,education of radiographers to interpret the images of thefetus, local experience for use of MRI, and expenses. An at-tempt to overcome the problem of fetal motion has been thedevelopmentofultrafastMRItechnologywithsubsecondim-age acquisition. However, for imaging of placenta, MRI tech-nologyislargelyappropriateasmovementsarenotaproblemfortheplacenta.AlthoughsurveyofchildrenexposedtoMRIhas not shown adverse outcomes, additional studies on thesafety of different modalities of MRI are still needed. 9,10 Gen-eral rules are that patients should not be exposed to MRI inthe first trimester when the organ systems are developing,and consent from the pregnant women should always beobtained before the investigation is performed.The main technique used in obstetrics is a rapid T2-weighted sequence of which the single-shot fast spin-echosequence is most widely used. Using this technique, it ispossible to acquire images in axial, coronal, and sagittalplanes. Dynamic contrast-enhanced MRI quantifies the rateat which contrast material crosses from the intravascularcompartment into other compartments, such as interstitialspace of various tissues and tumors. 11 This may help in eval-uating the maternal placental interface, and placental perfu-sion may improve our understanding of a variety of vascularabnormalities affecting this interface. However, before itsclinical use many questions regarding the effects of intrave-nousMRIcontrastmaterialsontheunbornfetuseshavetobeanswered. Fetal MRI is usually carried out without intrave-nous contrast agent administration, as detection of fetal mal-formations does not require the use of a contrast medium.Furthermore, gadolinium agents that can cross the placentaare classed as category C drugs and, as such, should only be Figure 2  Magnetic resonance imaging. Anterior posterior view of placenta percreta. The wall between the uterus andurinary bladder is absent. 272  S. Gudmundsson, M. Dubiel, and P. Sladkevicius   Author's personal copy used if the benefits outweigh the risks. For instance, a com-bination of placenta, myometrium, bowels, and vertebraeform a complex image, which makes it almost impossible todiagnosis posterior myometrial placental invasion. There-fore, use of gadolinium is highly recommended to improve adiagnostic accuracy.The leading indication to perform MRI for pregnantwomenistheevaluationofcentralnervoussystemabnormal-ities. 12,13  Although the combination of transabdominal andtransvaginal ultrasound is a very good method to visualizethe placenta and placenta-related problems, there are situa-tions when these methods are still insufficient. One of theseconditions is placenta invasion disorders: placenta accreta,increta,andpercreta.Thisisparticularlydifficulttodiagnosewhen the placenta is located on the posterior wall due toprevious operations performed on the uterus rather than acesareansection. 14,15 TheMRIfindingsofplacentaaccretaaresimilartothoseobtainedsonographically:lacunarflowintheplacenta, focal exophytic masses, loss of the retroplacentalmyometrial zone, thinning or disruption of the uterine serosa/ bladder interface, and vessels extending from the placentaintotheorgansurroundingthebladder. 14,16-18 Toconfidentlyassess the uterine-vesical interface, MRI needs to be per-formed on patients with a moderately full bladder. Onestudy 15 showedthatMRIprovidednoadditionalinformationas compared with ultrasound examination in every case ex-cept one in which the placenta was located on the posteriorwall of the uterus. Another large study 19 comprising 300pregnant women showed that MRI adequately outlined thetopographic anatomy of an invasion, relating it to vasculardistribution.Thisstudyattemptedtoconfirmthepresenceof parametrial invasion in axial slices, which is not possible tovisualize with ultrasound. Parametrial invasion was associ-ated with the possibility of urethral damage during surgery.CharacteristicMRIimagesinacasewithplacentapercretaarepresented in Figures 2 and 3. Ultrasound certainly will continue to be the primaryscreening modality for the evaluation of pregnant womenbecause of its relatively low cost and real-time capability.However, MRI is increasingly used as a complementarymethodinmanycasesasaproblem-solvingtool.Itisprudentto use MRI in all cases of equivocal ultrasound or Dopplerexamination. MRI may rule out or confirm a diagnosis, forexample, the presence of parametrial invasion in cases of placentaladherence.TheMRImayalsohavearoleincasesof intrauterine fetal death, where parents are opposed to diag-nostic autopsy. Other Methods of Imaging Positronemissiontomography(PET)isascanningtechniqueusedinconjunctionwithsmallamountsofradiolabeledcom-pounds to visualize brain anatomy and function. PlacentalPET has been tested in animal models, and the amino acids Figure 3  Magneticresonanceimaging.Lateralviewshowingnormalwallbetweentheuterusandbladder(Z)andanareawhere the wall is obliterated (Y). Posteriorly, the normal fat layer has disappeared between the uterus and rectum (X),which suggests penetration of the uterine wall. Placental morphologic and functional imaging in high-risk pregnancies   273
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