Current TopicThree-dimensional ultrasound evaluation of the placenta
Introduction
“The human placenta is a highly invasive and proliferative structure during the first half of pregnancy. While the growth rate of the human placenta decreases, its maturation continues throughout gestation” [1]. “The development and maturation of the placenta and the maintenance of successful pregnancy are dependent on the timely proliferation and differentiation of the villous cytotrophoblast cells into maternal decidua and myometrium in early pregnancy. This process leads to transformation of the spiral arteries supplying the intervillous space. These physiological changes alter the vascular supply to a low-pressure high-flow system, allowing adequate flow to the developing villous circulation, which undergoes progressive arborization until late in the third trimester. Consequently, all of the respiratory gases, nutrients, and wastes that are exchanged between the maternal and fetal systems are transported via placenta. The importance of transplacental exchange in supplying the metabolic substrates required for fetal growth is apparent and has long been recognized” [2]. Placental dysfunction is known to be a major cause of pregnancy complications, such as perinatal loss, pregnancy-induced hypertension (PIH), and fetal growth restriction (FGR) [2], [3], [4].
Conventional two-dimensional (2D) ultrasound has been widely used for the evaluation of the placenta during pregnancy. This 2D ultrasound evaluation includes the morphology, anatomy, location, implantation, anomaly, size, and color/power and pulsed Doppler ultrasound assessment of the placenta [5], [6]. Certainly, 2D ultrasound is useful to assess normal and abnormal placentae in most pregnancies, but information is insufficient on three-dimensional (3D) reconstruction and spatial relationships of the placenta, precise placental volume measurement, 3D placental vasculature, and placental blood flow.
3D ultrasound has the potential to provide improved visualization of the fetal anatomic morphology compared with conventional 2D ultrasound imaging [7]. Consequently, the introduction of 3D ultrasound would facilitate the novel assessment of the placenta, such as surface-rendered imaging and volume measurement [5]. With the recent advances in 3D power Doppler (3DPD) ultrasound as well as quantitative 3DPD histogram analysis, quantitative and qualitative assessments of the vascularization and blood flow of the placenta have become feasible [8], [9], [10].
The present paper reviews 3D ultrasonographic studies on the placental morphology, placental volume measurement, and placental vascularization and blood flow, and, on this basis, makes recommendations for future research on the placenta.
Section snippets
3D surface-rendered image
An exact antenatal diagnosis of placental abnormality can usually be performed in most cases using conventional 2D and/or color/power Doppler ultrasound. “By means of 3D surface-rendered imaging, however, more detailed information on placental abnormality is obtained. Especially, visualization of the continuity and curvature of placental abnormality is more easily accomplished with 3D surface-rendered imaging” (Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5) [11]. “The improved image clarity,
Volume measurement
Wolf et al. [15] were the first to report second-trimester placental volume measurement using a modification of the rectangular formula by means of conventional 2D ultrasound in 1989. With recent advances in 3D ultrasound, 3D sonographic placental volume measurements using multiplanar, VOCAL (Virtual Organ Computer-aided AnaLysis), and XI VOCAL (eXtended Imaging VOCAL) have been reported [16], [17], [18], [19]. Volume measurements using 3D ultrasound methods are much more accurate than those of
3D power Doppler (3DPD) ultrasound
3DPD ultrasound can depict intraplacental vessel characteristics such as the density of vessels, branching, caliber changes, and tortuosity [2], [8], [38], [39], [40]. Moreover, 3DPD ultrasound was found to be superior to 2D power Doppler (2DPD) ultrasound for the detection of secondary and tertiary stem vessels in the placenta, although there was no difference in the visualization of main stem vessels between 2DPD and 3DPD ultrasound [39]. It is rare to demonstrate more than the main stem of a
Quantitative 3DPD histogram analysis
3DPD indices, e.g., the vascularization index (VI), flow index (FI), and vascularization flow index (VFI), have been used to assess placental perfusion, and it has been accepted that these indices potentially reflect both utero-placental and feto-placental blood flows [8], [9], [10]. VI (0 – 100) means the proportion of the volume showing a flow signal in the placenta. FI (0–100) is the average flow signal intensity inside the placenta. VFI (0–100) is a combination of the information concerning
Conclusions
This review focused on 3D sonographic studies evaluating the placenta. 3D ultrasound facilitated a more precise evaluation of the placental surface features, the reconstruction of realistic 3D images of placental vascular trees, and assessment of the placental perfusion; conventional 2D ultrasound cannot provide the same information. Thus, this technique should be important in future research on placental characteristics and in evaluation of the placental function. If we can observe placental
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