Placenta: angiogenesis and vascular assessment through three-dimensional power Doppler ultrasonography

Arch Gynecol Obstet DOI 10.1007/s00404-007-0453-y

REVIEW ARTICLE

Placenta: angiogenesis and vascular assessment through three-dimensional power Doppler ultrasonography Hélio Antonio Guimarães Filho · Lavoisier Linhares Dias da Costa · Edward Araújo Júnior · Luciano Marcondes Machado Nardozza · Paulo Martin Nowak · Antonio Fernandes Moron · Rosiane Mattar · Cláudio Rodrigues Pires

Received: 18 April 2007 / Accepted: 15 August 2007 © Springer-Verlag 2007

Abstract The placenta is fundamental for fetal development. It combines the functions of an endocrine organ, kidneys, lungs and intestines, purifying catabolites, oxygenating and nourishing the conceptus. Its fetal portion is the largest part develops from the chorionic sac. The maternal portion, which is smaller, is originated in the endometrium, more speciWcally in the decidua basalis. The placenta starts its function closer to the fourth week of gestation, when anatomical arrangements for the physiological exchanges are already established. The circulatory function of the placenta appears at an early stage of embryo-placental development and it is strongly related to fetal growth, to the placental size and to uterine and umbilical blood Xows. Therefore, an adequate placental angiogenesis is critical for the establishment of a normal placental vascularization with consequent normal development of the fetus. In this review article, the authors discuss about placental ontogeny, focusing on the main aspects of its normal development, and about the recent advances in ultrasonography for the study of the vascular architecture of the placenta through three-dimensional power Doppler ultrasonography.

H. A. Guimarães Filho · E. Araújo Júnior · L. M. M. Nardozza · P. M. Nowak · A. F. Moron · R. Mattar · C. R. Pires Obstetrics Department, São Paulo’s Federal University—Paulista School of Medicine (Unifesp-EPM), São Paulo, Brazil H. A. Guimarães Filho · L. L. Dias da Costa ECOCLÍNICA, Joao Pessoa, Paraíba, Brazil H. A. Guimarães Filho (&) Rua Reinaldo Tavares de Melo, 142, Apto. 901, Manaíra, CEP 58038-300 Joao Pessoa, Paraíba, Brazil e-mail: [email protected]

Keywords Placenta · Angiogenesis · Vascularization · Power Doppler ultrasonography · Three-dimensional imaging

Introduction The placenta is a fundamental structure for fetal development. It has the capacity of an endocrine organ; it works as kidney, lung and intestines, removing catabolites, oxygenating and nurturing the conceptus. It is the fetal structure in closer contact with the mother’s body. It invades the decidua and promotes fundamental modiWcations in uterine hemodynamics, besides producing substances like the gonadotropins that directly interact with the mother organism and allow the harmonious development of the fetus. The placenta is considered a fetomaternal organ and it has two basic parts: a fetal and a maternal portion. The fetal portion is the largest and develops from the chorionic sac. As for the smaller maternal portion, it is originated in the endometrium, more speciWcally in the decidua basalis. It starts functioning near the fourth gestational week, when the necessary anatomical changes have already been established [1]. The ultrasonography is a non-invasive method that allows the study of several placental parameters since its formation. Through two-dimensional ultrasonography, the placenta can be assessed according to thickness and maturity. The maturity is usually assessed through the Grannum’s criteria, which consider echogenicity and presence of placental calciWcation [2]. The three-dimensional ultrasonography (3DUS) is one of the most recent technological advances in diagnostic medicine. It appeared in 1987, with several limitations then, allowing only the static observation of the fetal

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surface. Only in 1992, devices capable of reconstructing two-dimensional planes and of showing transparent images, in addition to the surface ones, became available. With digitization, images started to be captured quite faster, and with better resources since 1998, the method became popular and brought about the concept of real-time threedimensional images. Currently, when we use the phrase “three-dimensional ultrasonography”, we refer to the group of new methods used to decode sound reXection. Methods that include surface-image capturing, the reconstruction of alternative two-dimensional planes, and also three-dimensional images in transparency [3–5]. One of the recent applications of 3DUS is related to the vascularization assessment of organs and structures through three-dimensional power Doppler (3D power Doppler), using Xow indexes (FIs) that are calculated from threedimensional data formed by the voxels (the basic information units of volume). Brightness grading, gray scale, and color Doppler are associated with voxels and the information obtained is plotted on a histogram [6–8]. The 3D power Doppler allows the assessment of the architecture of the placental tree. Such information is very important considering that problems on the normal development of the placenta, as well as the reduction on its FIs, are usually associated with alterations in fetal growth, amniotic Xuid volume, and Doppler Xowmetric parameters of the umbilical artery [9, 10].

Development of the placental angiogenesis Pregnancy initiates after fertilization of the ovum, which subsequently implants as the blastocyst between the 6th and 8th days after conception and Wnishes on the 11th day. In this phase the trophoblast is separated into two layers: an outer layer called syncytiotrophoblast, formed by fused cells sharing the same cytoplasm and creating a highly invasive plasmodial mass containing many nuclei (syncytium), and an inner layer called cytotrophoblast, consisting of clear and cuboidal cells. The syncytiotrophoblast invasion in decidual tissue erodes vessels and glands, forming lacunas Wlled with maternal blood and endometrial Xuids that nurture the embryo initially by diVusion. Those lacunas are the basis of the future intervillous space. Some authors have suggested that those lacunas do not have maternal blood, but only plasmatic fraction, due to the obstruction of spiral arterioles by trophoblast plugs. The passage of the blood and, therefore, the establishment of the hemochorial circulation, would only happen later, at 12–14 weeks [11–14]. Soon, the cytotrophoblast grows Wnger-like projections in those lacunas (primary villi), accompanied then by the extra-embryonic mesoderm (secondary villi). Around the

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15th day after conception, capillaries appear in those projections, and these are the tertiary villi. Around the 8th week, due to ovular expansion, only the villous portion in contact with the decidua basalis receives the appropriate nutrition, spreading and forming the chorion frondosum, which will be the placental body. The other portions in contact with the decidua capsularis atrophy and originate the plain chorion. Villous branches of the chorion frondosum subdivide themselves during all gestation, forming approximately 50–60 cotyledons, each derived from a primary villous. Each cotyledon divides itself later in up to Wve lobes. During the 3rd month, the decidua plate grows septa that divide the villous tree into 15–20 lobes. These lobes do not have functional meaning and are wrongly called cotyledons [15]. The placenta increases in volume up to the beginning of the third trimester, and stabilizes around the 30th week. It weighs around 488 g when mature, and has a villous surface of 11–14 m2, or 3.98–4.33 m2/kg of fetal weight (quite more than the pulmonary area of an adult, which is 1.28 m2/kg). The fetal surface of the placenta represented by the chorion is called chorionic plate, and it is where we observe fetal vessels. The portion in contact with the mother is called decidual plate. From 80 to 100 of the spiral arteries are opened on the decidual plate, near the central areas of the lobes, supplying with maternal blood the intervillous space. Those spiral arterioles are superWcially deteriorated by the trophoblastic invasion occurring in the ovular implantation phase. A second trophoblastic invasion occurs in the second trimester, destroying the elastic layers of the spiral arterioles up to their junction with the radial arteries. This remodeling allows an increase in the Xow from 50 ml, on the 10th week, to 500–600 ml at term. As the intervillous space in mature placenta conveys around 150 ml, its renovation happens three to four times per minute [16] (Fig. 1). In 1937, Mossman had already said that the mammals’ placenta was an organ with the primary objective of

Fig. 1 Schematic illustration of human placenta. a umbilical arteries, b umbilical vein, c corionic surface vessel, d main stem vessel, e stem vessel, f tertiary stem vessel, g arcuate artery, h radial artery, i spiral artery, j maternal veins, k maternal arteries; 1 corial plate, 2 intervillous space, 3 decidua, 4 myometrium (Adapted from Pretorius et al. [28])

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promoting physiological exchange [17]. Indeed, all the respiratory gases and nutrients that are transported between the maternal and fetal systems are done through the placenta. Therefore, the importance of the placental exchanges in providing the metabolic substrates necessary to support fetal growth is clear and has been recognized since long [18]. Problems in the development of placental vascularization, as well as an increase in its vascular resistance, have been associated with early embryonic mortality [19, 20]. The Wrst trimester of gestation is considered a critical period. It is when major events happen, such as the embryonic organogenesis and the placenta formation, or placentation [21, 22]. The placentation includes an extensive angiogenesis of the placental tissue, and a sharp increase in the uterine and umbilical blood Xow. These events allow the development of the conceptus through the appropriate uterine adaptation for the metabolic exchanges, and probably will aVect the rate of the physiological exchanges between the maternal and fetal systems to the end of the pregnancy [18, 23]. The circulatory function is established early during the embryo-placental development and it is strongly related to fetal growth, size of the placenta, and uterine and umbilical blood Xows. So, the appropriate placental angiogenesis is critical for the establishment of a normal placental circulation, and consequent normal development of the fetus. The most important angiogenic factors that regulate placental angiogenesis are the vascular endothelial growth factor and the Wbroblasts growth factor. The reduced genetic expression of these growth factors is normally associated with lethal embryonic defect [24–26]. Uterine and umbilical blood Xows grow exponentially during gestation and represent the circulation of the maternal and fetal portions of the placenta, respectively. Other factors that aVect fetal growth, such as maternal genotype, number of fetuses, maternal malnutrition, maternal age and parity, have a similar eVect on the size, angiogenesis, and blood Xow of the placenta [18].

Placental vascular assessment by three-dimensional power Doppler As for the assessment of placental vascularization, the 3D power Doppler allows the evaluation of the placental vascular tree architecture, and of its vascular indexes. The method apparently can show the villous vessels of the Wrst, second, and third order, with a higher percentage of visualization than the two-dimensional Doppler [27] (Fig. 2). This is relevant because an incomplete placental development is generally associated with fetal growth alterations [9, 10, 29]. Besides that, a reduction in the placental Xow may identify those alterations at an earlier stage than increases

Fig. 2 Three-dimensional planar power Doppler ultrasound image showing branching of intraplacental vessels. The right, in detail: corionic surface vessel (long arrow); Main stem vessel (short arrow) penetrating the intervillous space and branching in minors trunks; secondary stem vessel (curved arrow); tertiary stem vessel (arrowhead)

in the umbilical artery resistance [30, 31]. There are few studies in medical literature describing the use of 3D power Doppler in the assessment of placental vascularization [6–8], and only two correlated placental FIs with fetal biometric and Doppler Xowmetric parameters of fetoplacental circulation. In 1999, Pairleitner et al. described a new method to quantify the blood Xow and the vascularization of adnexal masses by three-dimensional imaging [32]. Using threedimensional ultrasound and the amplitude Doppler (power Doppler), they created three indexes derived from the analysis of the gray and Doppler color voxels contained in a volumetric cube previously selected as the region of interest of the analysis. Those indexes measured the vascularization (vascularization index—VI), the blood Xow (Xow index—FI), or its relation (vascularization Xow index— VFI). The three-dimensional volume is formed by basic units called voxels. The voxel contains all the information about the gray scale and the color, according to an intensity scale that ranges from 0 to 100. According to these values, this measurement system obtains the three indexes of power Doppler to assess the blood Xow and vessels. The VI measures the number of color voxels from the studied volume that represents the tissue blood vessels, and it is expressed as a percentage (number of placental vessels). The FI is the mean color value of all color voxels, and shows the mean blood Xow intensity (placental Xow). The VFI is the mean color value of all gray and color voxels present in the spherical volume acquired, and represents the behavior of the other two indexes (VI and FI) that assess blood Xow and vascularization of the organ. Therefore, it is calculated and derived from them. Some years ago, with the development of the threedimensional ultrasound technology, a new program for volumetric calculations was created: the Virtual Organ

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Computer-aided Analysis (VOCAL™) [33–35]. That program is an extension of the SonoView Pro™ software from Accuvix XQ (Medison Co., Ltd, Korea). In 2003, Yu et al. analyzed the blood Xow volume from a small part of the placenta using the FIs described by Pairleitner et al., associated with the VOCAL method to obtain a region of interest on the placenta [6]. The placental vascularization of 100 healthy pregnant women was studied then by 3D power Doppler. They observed that the calculation of the VIs through VOCAL showed a positive correlation with the fetal biometric data at 20–40 weeks, and an increase of all indexes as pregnancy developed. The 3D power Doppler cannot scan the whole placental vascular tree except on the Wrst trimester of gestation. To overcome this problem, Mercé et al. developed a method called “vascular placental biopsy” [7, 8]. This technique includes the acquisition of the three-dimensional image through VOCAL using the sphere mode to deWne the region of interest in the placenta, and the automatic calculation of the VIs through 3D power Doppler pressing the HISTOGRAM key on the ultrasound device (Fig. 3). Those authors studied the method reproducibility and proved that it had good intraobserver reproducibility, concluding that the 3D power Doppler parameters can be applied to the study of the placental vascular tree in normal pregnancies [7]. In 2005, Mercé et al. evaluated 86 gestations from 15 to 40 weeks using the placental vascular biopsy technique, with the objective of studying the normal development of placental vascularization through 3D power Doppler [8]. They also tried to correlate the placental vascularization results obtained, with fetal biometry data and the umbilical artery resistance indexes. The study showed that all the indexes of the 3D power Doppler had a signiWcant correlation with gestational age. The FI was the index that showed

Fig. 3 Placental biopsy by 3D power Doppler ultrasonography. A placental tissue sphere was obtained and the three power Doppler indexes (VI, FI and VFI) were calculated by histogram

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the most signiWcant relation with gestational age, increasing linearly with the gestation age. The VI showed a clear dispersion of values, stabilizing from the 30th week onward, and decreasing from the 37th week until term, showing a high variation coeYcient and reduced reproducibility. The VFI behaved as a combination of the VI and FI. They observed that all the 3D power Doppler indexes, signiWcantly correlated with the fetal biometric parameters (BPD, CC, AC and femur length) and fetal weight (FI was the index with the best correlation), except for the correlation coeYcient of the VI and fetal weight, which was not signiWcant. The VI and FI showed a positive correlation with the umbilical systole (especially the FI), and negative with the umbilical diastole. Finally, the VFI showed a signiWcant correlation with the umbilical systole and with the resistance index of the umbilical artery. The authors conclude that the 3D power Doppler allows an appropriate study of the placental vascularization tree with the identiWcation of the diVerent branches of the villous vessels, as well as the quantitative assessment of the number of vessels through the VI, and of the blood Xow through FI and VFI. They also conclude that the placental vascular biopsy technique is an appropriate tool for the assessment of the placental vascular tree during gestation.

Final comments The human placenta is directly related to the exchange of substances that happen between the mother and the fetus. For this reason, it is considered a fetomaternal organ. Its normal development during gestation ensures the necessary support for the formation of a healthy fetus. Such process depends on a group of complex genetic and environmental factors that directly interfere in the growth, angiogenesis and function of the placenta. These factors have not yet been completely revealed, but there have been advances in understanding the mechanisms related to its ontogeny, and the methodologies to study its anatomy and functionality. As an example, 5 years ago the Wrst studies, which tried to assess the placenta volume through 3DUS and correlate that volume with poor perinatal results [36], chromosomal diseases [37, 38], DNA concentration of fetal cells in maternal circulation [39] and prediction of fetal alpha-thalassemia [40], appeared. Recently, with the help of 3D power Doppler, it has been possible to assess the placental vascular tree architecture since the beginning of gestation. Such condition is very important because problems in placental development are generally associated with alterations in fetal growth [9, 10]. In our service we are developing a study involving pregnant women with and without risk factors for placentary insuYciency to determine the reliability and validity of this method as an early

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marker for placentary insuYciency. Our preliminary results suggest that a reduction in the placental Xow seen by 3D power Doppler (placental vascular biopsy) might became an earlier marker than the increase in the umbilical artery resistance [6, 8]. So, we continue to learn and consequently to advance towards a more reliable comprehension of this so important fetal attachment. Medical technology continues to advance in such a way that it is already possible to perform “virtual biopsies” of the placenta and to study its vascularization, as an alternative way to access alterations on its Xow. We are certain that the results of our study to be published in near future will contribute even more to a better knowledge about the issue, and increase the perspectives of treatment for several clinical conditions related to placental vascular insuYciency.

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