Three-dimensional/Four-dimensional Ultrasound: The Key for the Precise Assessment of Fetal Malformations
Corresponding Author: Eberhard Merz, Center for Ultrasound and Prenatal Medicine, Frankfurt am Main, Germany, Phone: +49 6976806559, e-mail: email@example.com
Received on: 02 April 2023; Accepted on: 25 April 2023; Published on: 30 June 2023
Three-dimensional/four-dimensional (3D/4D) ultrasound has a high diagnostic potential in the detection and visualization of fetal malformations. Compared to two-dimensional (2D) ultrasound, which only allows the demonstration of individual planes, 3D/4D ultrasound allows the storage of volumes that can be examined using different visualization modes. As a result, fetal structures can be represented in controlled reformatted planes, in multiple parallel (tomographic) planes, or in rendered surface or transparent images. Fetal malformations can thus be demonstrated with the optimal visualization mode and from the best viewing angle. In the case of a presumed fetal malformation during a 2D ultrasound examination, or in the case of an increased recurrence risk of a certain fetal malformation, the different viewing modes, in particular the surface mode, can be used to convincingly show the parents the absence of such a malformation.
How to cite this article: Merz E, Pashaj S. Three-dimensional/Four-dimensional Ultrasound: The Key for the Precise Assessment of Fetal Malformations. Donald School J Ultrasound Obstet Gynecol 2023;17(2):158–164.
Source of support: Nil
Conflict of interest: None
Keywords: Prenatal diagnosis, Fetal malformations, Three-dimensional ultrasound, Four-dimensional ultrasound, Visualization modes
This paper has been previously published as Eberhard Merz, Sonila Pashaj. 3D/4D ultrasound: The key for the precise assessment of fetal malformations. In: Chervenak FA, Kupesic Plavsic S, Kurjak A. Donald School The Fetus as a Patient: Current Perspectives. Jaypee Brothers, New Delhi, India, 2019, pp 217–225.
There is no doubt that most fetal malformations can be detected with conventional 2D ultrasound. However, all of the described abnormalities can only be displayed in single planes. In contrast to 2D ultrasound, 3D/4D ultrasonography provides the examiner with several different visualization modes1-3 and displays images not previously viewed in 2D ultrasound. This gives the operator the opportunity to identify both, the normal and the abnormal anatomy in the most appropriate mode.1 Furthermore, 3D/4D ultrasound technology offers the possibility of digital volume storage without any quality loss. By reloading the volumes in the absence of the patient, virtual examinations can be performed by navigating through the volumes without causing any stress to the patient.2 In counseling patients, rendered images help the parents to understand the severity of an existing malformation or to give them the certainty of the absence of any fetal abnormality. This is particularly useful in cases with an increased recurrence risk for a specific fetal malformation.3
Despite the fact that most ultrasound companies have incorporated 3D/4D technology in their ultrasound units today and are offering both transvaginal and transabdominal 3D/4D probes, there are still distinct differences concerning routine handling of the equipment, in addition to differences in the image quality. However, an indispensable prerequisite for high-quality 3D/4D images is the availability of good-quality 2D images, considering that 3D/4D ultrasound technology is invariably based on 2D ultrasound.1
In any volume acquisition procedure, it has to be ensured that the region of interest (ROI) is completely inside the volume box. Structures that are not completely inside the volume box are cut and may thus simulate a malformation (e.g., missing hands or feet).
|3D display mode||4D display mode|
|Multiplanar mode||Multiplanar mode|
|Volume contrast imaging (VCI)||VCI|
|3D surface mode||3D surface mode|
|HDlive mode||HDlive mode|
|HDlive studio mode||HDlive studio mode|
|HDlive silhouette mode||HDlive silhouette mode|
|Transparent mode||Transparent mode|
|Minimum mode||Minimum mode|
|Inversion mode||Inversion mode|
|Glass body mode||Glass body mode|
|3D-animation (cine) mode||STIC|
The tri- or multiplanar mode is, in all cases, the basic display mode which shows all three perpendicular planes at the same time on the monitor.2,3 By navigating through the volume, the observed plane is always controlled by the two other orthogonal planes, thus allowing a precise demonstration of the preferred anatomical structure.
Omniview is a technique that allows the definition of a specific sectional plane by drawing a reference line, a polyline, or a curve in the A-plane. The corresponding perpendicular plane will be shown immediately next to the A-plane on the monitor.3
Volume contrast imaging represents a thick slice technique that enables imaging of thin volumes with high contrast resolution.2
The surface mode provides the examiner with 3D images of the fetal surface.1-3 For the application of this mode, it is important to ensure both the presence of sufficient amniotic fluid in front of the ROI and the absence of overlying structures.2
HDlive mode is a more recently introduced mode for the display of the fetal surface, providing the most realistic pictures of the embryo and the fetus.4-6 A virtual light source enables illuminating the fetus from different angles.
HDlive studio mode is similar to the HDlive mode. However, it allows the operator to illuminate the fetus using three different virtual light sources from different angles.3
HDlive silhouette mode enables two different demonstrations of the embryo and the fetus. The adjustment to “Low Silhouette” is useful in the demonstration of external structures, while the “High Silhouette” mode reveals internal structures.3
With the inversion mode, anechoic structures can be converted into hyperechoic structures.2
The glass body mode is a combination of greyscale 3D images and color Doppler, allowing the precise spatial demonstration of the blood flow in the fetus as if viewed in a glass model.2
The 3D animation mode or cine mode provides images of the ROI at different angles. In this manner, the physician and the patient can both see the object of interest rotating on the screen and obtain a better spatial impression thereof.2 The 3D cine mode can be used together with the different surface modes, the transparent mode, and the glass body mode.
Four-dimensional (4D) ultrasound (real-time 3D) enables the fast acquisition of volumes with the demonstration of the movements of the embryo and the fetus without artifacts.2
Spatiotemporal image correlation (STIC) allows an automatic volume acquisition of several heart cycles. According to spatial and temporal image correlation, they are merged together to form one single fetal heart cycle.1,7-9 Using this technology, the moving fetal heart can be analyzed offline. The combination of STIC and color Doppler provides the examiner with a detailed view of the cardiac blood flow.2
Assessment of Fetal Malformations
The use of the different display modes permits the operator to demonstrate a large number of visible abnormalities in the most appropriate mode, revealing the extent of the lesion interactively in all dimensions.3 Once a suspicious ROI is stored in a volume, even subtle anatomical defects can be demonstrated by a detailed examination of the volumes. Because any plane can be reformatted from the volume, 3D ultrasonography can depict image planes that are not accessible with conventional 2D ultrasound. Furthermore, rotation of the volume in all three directions enables the operator to align the fetus or the ROI into an exact upright position.
The usefulness of 3D ultrasound in the assessment of fetal malformations has been shown in a number of different studies.10-14 A comparison between 2D and 3D techniques in a study of 102 malformations showed 3D sonography to be advantageous in 60.8% of the defects.13
In the first trimester, the triplanar verification of the median plane allows precise control of the nuchal translucency (Fig. 1) and the demonstration of palatine clefts15 or retrognathia. The presence or absence of the nasal bone can be identified in a slightly paramedian plane3 (Fig. 1). Using the surface mode, early surface defects, such as cleft lip (Fig. 2), limb defects, or hexadactyly can be detected.
In the second trimester, the multiplanar view of the fetal head does not only reveal a flat profile, frontal bossing, a depressed nasal bridge, cleft lip and palate or micrognathia,16-23 but also confirms brain abnormalities such as agenesis or partial agenesis of the corpus callosum24-26 (Fig. 3). Further identifiable are another brain anomalies27,28 as, for example dilated ventricles, plexus cysts, arachnoidal cysts, the absence of gyri, holoprosencephaly, schizencephaly, hypoplasia, the absence of the vermis cerebelli, or encephalocele. The coronal and axial planes allow an exact comparison of symmetric structures, for example of the left and right brain ventricle, the left and right orbital diameter, as well as the precise demonstration of orbital and eye malformations.3 The transparent mode enables the identification of the presence of ossification of the skull, revealing abnormal width of the metopic suture, premature closure of the sutures29,30 (Fig. 4), or the absence or hypoplasia of the nasal bones.31
The surface modes enable the operator to verify surface defects of the head or face (exencephaly, cyclopia with proboscis, cleft lip/palate2,16,32-34 (Figs 5 and 6), or retrognathia), ear anomalies13 and protruding structures16 like an anterior or posterior encephalocele, epignathus, or a preauricular tag (Fig. 7). The surface mode can further be used to demonstrate surface views of cut planes of the brain. With the use of this mode, brain anomalies, such as holoprosencephaly, dilated brain ventricles, or plexus cysts3,13 can be seen from a bird’s eyes view.
In examinations of the spine, the transparent mode enables to verification of the demonstration of axis abnormalities (Fig. 8) or hemivertebrae, while spina bifida is best seen with one of the surface modes13 (Fig. 8).
For the assessment of thorax abnormalities, the transparent mode can be used to identify abnormalities in the ossification of the bony thorax.13 Lung abnormalities35 or diaphragmatic hernia are best seen with the multiplanar view, the tomographic view (Fig. 9), or with the surface view of specific cut planes.
To demonstrate heart malformations, the glass body mode, STIC technology, and the STIC color mode are the applications of choice.7-9 Complex anomalies can be demonstrated and the blood flow can be controlled three-dimensionally (Fig. 10).
Abdominal anomalies2,3,10 can be shown using the multiplanar mode, tomographic ultrasound imaging (TUI) mode, or one of the surface modes (Fig. 11). The High HDlive silhouette mode can be used to reveal the internal structures of an omphalocele (Fig. 11).
The multiplanar mode, TUI mode, minimum mode, inversion mode, and surface mode are used to visualize urogenital malformations. Cystic structures of the kidneys or dilated ureters can be converted to solid structures with the inversion mode3 (Fig. 12), while genital abnormalities3 can best be shown with one of the surface modes (Fig. 13).
Defects of the limbs13 (Fig. 14) or deviation of the limb axis3 (Fig. 15) are best demonstrated with the surface mode, while bone abnormalities like a missing bone (Fig.16), bowing of the bone, or fractures (Fig.16) can be visualized with the transparent mode.3 In addition, 4D ultrasound demonstrates any pathologic movement of the limbs, which may be observed in severe spina bifida.
SUMMARY AND OUTLOOK
Over the past three decades, 3D ultrasound has undergone tremendous development.38 In expert hands, 3D/4D ultrasound today represents an excellent complementary tool to conventional 2D ultrasound in prenatal diagnosis. The different display modes provide the operator with images that cannot be achieved with 2D ultrasound. This is of particular value in the demonstration of subtle defects in the embryo and the fetus. Moreover, the digital storage of volumes without any quality loss enables virtual ultrasound examinations while the patient is absent. Furthermore, the demonstration of fetal malformations to the parents or the pediatric surgeon is significantly easier with 3D than with 2D ultrasound, due to the fact that the future parents and the doctors are able to see such defects as a cleft lip or spina bifida immediately with their own eyes. This represents an invaluable aid in providing correct counseling to the parents.
The continuing development of high-end matrix probes will enable the operator to acquire volumes significantly more rapidly than with the use of mechanical devices and thus help to avoid artifacts caused by the moving fetus. With the ongoing development of computer technology, it might further become possible to provide the operator with extended 3D views of the fetus and thus serve as a helpful tool in the second half of pregnancy.
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