Three- and Four-Dimensional Ultrasound and Magnetic Resonance Imaging in Pregnancy
Sifa Turan
Ozhan M. Turan
Introduction
Advances in imaging technology have extended the capabilities of prenatal diagnosis beyond two-dimensional (2D) imaging to provide a wide array of analytic possibilities via three- and four-dimensional (3D and 4D) ultrasound and magnetic resonance imaging (MRI). New applications in ultrasound technology have been integrated into almost all ultrasound equipment and are available in virtually all obstetric settings, and continue to be a mainstay of prenatal imaging and diagnosis.
On the other hand, MRI is performed as a complementary problem-solving tool and can provide additional information resulting in better counseling, management, and perinatal outcomes.
When both are performed to delineate a fetal anomaly, the MRI examination is often interpreted in conjunction with an ultrasound examination done just before the MRI study. As this practice becomes more and more widely accepted, it is becoming imperative that those who deal with prenatal diagnosis become familiar with these imaging tools, as they increasingly gain acceptance as part of the routine armamentarium in fetal imaging1 (Table 19.1).
Three-Dimensional and Four-Dimensional Ultrasound
During the conventional 2D ultrasound, a single image plane is acquired. A 3D ultrasound includes acquired volumes, which is composed of a series of 2D planes. If these volumes are captured over time, then a 4D image is obtained (Figure 19.1). A transvaginal and transabdominal approach is used to acquire the 3D or 4D images. A transvaginal transducer (5-9 MHz) is used for volume acquisition in early pregnancy. Transabdominal 3D/4D probes (4-8 MHz) are used in the second and third trimesters.
Main Principles of the 3D and 4D Ultrasound
In general, 3D ultrasound consists of four main steps: (1) volume acquisition, (2) 3D or 4D visualizations of the volume, (3) optimization of the volume acquisition? with different modes, and (4) storage of volume images, rendered images, or image/volume sequences (Table 19.2).
Volume Acquisition
The main determinants for the ideal volume acquisition are the presence of adequate amniotic fluid in front of the target structure, identifying the area of interest, and using proper angle and time. The region of interest (ROI) box determines the height and width of the acquired volume. The lateral dimensions of the ROI box have the most significant influence on the frame rate; therefore, the narrower the angle, the better the image quality. After identifying the ROI, the volume is acquired symmetrically upwards and downwards toward the targeted image. The acquisition angle determines how many planes above and below the acquisition plane will be acquired. If a larger acquisition angle is chosen, a more substantial portion of the fetus is sampled, and the time to obtain this sample is longer. Therefore, the acquisition angle should be adjusted to the area of interest (Figure 19.2A).
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Table 19.1 Comparisons of Ultrasound and MRI as a Fetal Assessment Method | ||||||||||||||||||||||||
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Three- and Four-Dimensional Visualization of the Volume
Multiplanar Display
At the end of the ideal volume acquisition, there will be A, B, and C planes on display. These planes are also called the multiplanar display or orthogonal planes. The image that is displayed in the A plane corresponds to the 2D starting image and, therefore, has the highest image resolution. In contrast, planes B and C are reconstructed images and have lower image resolutions (Figure 19.2B). Two main postprocessing applications can be used after obtaining a multiplanar display: tomographic ultrasound imaging (TUI) or virtual organ computer-aided analysis (VOCAL).
Figure 19.1 Schematic presentation of a three-dimensional (3D) image. During 3D acquisition, multiple two-dimensional (2D) images are obtained to create a 3D image. The single circle represents the 2D image. When the multiple circles are combined, a tridimensional object can be compiled as depicted with a cylinder. The four-dimensional (4D) image can be obtained when the 3D images are captured over time. |
TUI is an automated display modality where parallel images in a 3D volume are shown in a multi-image display. TUI allows the examiner to simultaneously display multiple cross-sectional images at specific distances from each other. The number of slices as well as distances between the slices can be adjusted by the position of each plane within the ROI. The advantage of this application is the ability to display sequential parallel planes (Figure 19.3).
VOCAL utilizes computer technology to provide accurate volume calculations. The initial acquisition can be a static 3D volume in traditional or inversion modes. Once the VOCAL is activated, the quantification of the volumes can be calculated. This postprocessing application allows us to delineate the organs and provides a more accurate assessment of the volumes. Manual rotation or constant degree rotation selection, such as 12°, 15°, and 30°, can be selected, and multiple adjacent and sequential planes can manually be measured (Figure 19.4).2
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Table 19.2 Steps in 3D/4D Ultrasound | ||
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Figure 19.2 Steps of a volume acquisition. The first step is the identification of the area of interest by using the three-dimensional (3D) box (A). Following a volume acquisition, a multiplanar display is obtained (B). Plane A: corresponds to the two-dimensional (2D) starting image and therefore have the highest image resolution; Planes B and C are the transverse and coronal planes, respectively. |
Optimization of the 3D Volume With Different Modes
After the volume has been acquired, optimization of the acquired image can be performed using several different display modes. This allows the operator to choose the ideal mode to visualize the fetal structures and detect any abnormalities (Table 19.2).
The surface mode is the most common mode to visualize the 3D structures. Adjusting filters and thresholds in the surface mode can change the mode to the maximum surface mode or skeletal mode, which can delineate the details of the bone structures (Figure 19.5A-C).
The high-definition (HD)-live mode is a recent addition to the optimization options and gives a more realistic visualization of the fetus. HD-live uses a moveable virtual light source that can illuminate the examination object from all aspects.3 In the HD-live mode, the human skin-based color spectrum and movable virtual light source allows almost photographic imaging of the target structure and can show pathological changes that were not detectable with previous 3D surface modes (Figure 19.5D and E).3
Figure 19.3 Tomographic ultrasound imaging (TUI) is a postprocessing tool. After obtaining the volume, the TUI button can be activated, and slices can be arranged. The slice thickness is 2 mm in the example. |
Inversion mode is another postprocessing tool that inverts the gray scale of the volume voxels. With this mode, anechoic structures, such as the heart chambers, vessel lumen, stomach, gallbladder, renal pelvis, and bladder, appear echogenic in the rendered images, whereas structures that are normally echogenic before grayscale inversion (eg, bones) appear anechoic (Figure 19.5G and H).4
Figure 19.4 Virtual Organ Computer-Aided Analysis (VOCAL). This postprocessing application provides an accurate volume measurement. Manual rotation with 15° selected for measurement of the brain hemisphere. |
Glass body is another option in which color and power Doppler are used during volume acquisition. This mode allows visualization of the details in blood vessels (Figure 19.5I).5
Volume contrast imaging (VCI) is another tool for the optimization of the image. This tool enhances the appearance of the fetal structures via increasing the balance in contrast (Figure 19.5F).
Figure 19.5 Different optimization modes used in three-dimensional (3D) and four-dimensional (4D) ultrasonography. A, Surface mode. Using the surface mode, and adjusting threshold and filters, it is possible to display the surface of the fetal face. B, Maximum mode of the same volume and with the same region of interest. C, Skeletal mode with details of the skeleton of the face. D, High-definition live mode. E, High-definition live mode with a light application. F and G, Surface mode with volume contrast imaging (VCI) of the kidneys demonstrates renal pelvis size and kidney tissue surround. H, Inversion mode of the renal pelvis and bladder. I, Demonstration of the pericallosal artery with glass body color. |
Clinical Applications of the 3D and 4D Ultrasound in Pregnancy
3D and 4D ultrasound play an essential role in the accurate and more realistic view of normal and abnormal findings in the first, second, and third trimesters. The various display options give the operator the ability to demonstrate the target structure.
Fetal Face and Ears
The most common mode for visualizing the face is the surface-rendering mode (Figure 19.6A). It is usually used to examine the external structures, such as the nose, lips, ears, and profile, and it is an excellent method to show cleft lip, low-set ears, and profile abnormalities such as micrognathia. In some studies, it has also been used to evaluate facial expression. This method can be used starting from 13 weeks; however, the best time is the 23- to 30-week interval. For an ideal image, sufficient amniotic fluid in front of the ROI is essential.
The maximum mode is the second most common mode used to visualize the fetal face and allows practitioners to assess the bone and sutures of the fetal head and nasal bone (Figure 19.6B). The reverse-rendering mode is also used in facial evaluations, and the principle of this rendering is to assess the fetal face posteriorly, which allows more precise palate evaluation (Figure 19.6C). TUI of the fetal profile at the eye socket view has been used widely to assess the eye sockets, lenses, palate, tongue, and mandible in one display in all gestational weeks. Postprocessing with the VCI application on top of the TUI improves the appearance of the above structures and helps to delineate the details (Figure 19.7).
Fetal Central Nervous System
Fetal Brain
Brain assessment with 2D ultrasound requires multiple views and experiences. Fetal lie and acoustic shadowing from bony structures make evaluation using 2D ultrasound challenging. The transventricular and transcerebellar planes are two standard 2D planes for the basic brain examination. Coronal views and sagittal views are required if detailed neurosonography is needed. Potential benefits of 3D ultrasound in the fetal brain include the ability to identify the location, degree of severity, and extent of central nervous system (CNS) abnormalities; and the possibility of reconstructing and visualizing all the corpus callosum, thalamus, cavum septum pellucidum, and posterior fossa in the sagittal plane from volume datasets. In addition, 3D ultrasonography can increase the speed of fetal neurosonography performed by 2D transvaginal ultrasonography.
Figure 19.6 Demonstration of the fetal face. Surface-rendering mode (A) is ideal for external structures. The maximum mode (B) helps to examine the bone and sutures. The reverse-rendering mode (C) demonstrates the fetal face posteriorly. Green arrows depict the direction of the acquisition. |
Figure 19.7 Assessment of fetal face using tomographic ultrasound imaging (TUI) (A) and addition of volume contrast imaging (TUI-VCI) (B). TUI-VCI improves visualization of the images in the slices by adding more contrast. |
Volume acquisition at the level of the transventricular, transcerebellar, and coronal planes allows assessment of the fetal brain by multiplanar and TUI methods (Figure 19.8). VOCAL can evaluate the hemisphere volumes if asymmetry is suspected (Figure 19.4). The color mode by color Doppler or power Doppler allows visualization of brain circulation and abnormalities of the blood vessels in the brain such as vein of Galen malformations (Figure 19.9).
Figure 19.8 Assessment of fetal brain using tomographic ultrasound imaging (TUI) using transventricular (A), transcerebellar (B), and coronal planes (C). CC, corpus callosum; CSP, cavum septum pellucidum; Vp, posterior horn of the lateral ventricle. |
Fetal Spine
The 3D volume of the spine and application of the multiplanar plane enables assessment of the sagittal, transverse, and coronal view of the spine in one picture. Rendering of the volume will show the spine and ribs very clearly (Figure 19.10A). The skeletal mode is the preferred mode for optimization to evaluate fetal spine abnormalities such as hemivertebra and scoliosis (Figure 19.10B-D). In addition, it is also helpful to identify the level of the defect in spina bifida cases, which is essential for prognosis and treatment (Figure 19.10E and F).
Figure 19.9 Assessment of vascular anomalies using three-dimensional (3D) ultrasound. The multiplanar image from volume acquisition with color Doppler (A) and glass body postprocessing image (B) demonstrates an arteriovenous malformation. The arrow shows the vein of Galen abnormality. |
OmniView (GE Medical Systems, Kretztechnik GmbH, Zipf, Austria) is a new display technology for the 3D ultrasound that allows interrogation of volume datasets and simultaneous display of up to three independent (nonorthogonal) planes by manually drawing lines from any direction or angle. This has the potential to simplify 3D volume examination, thus further reducing dependence on the operator’s skills.6 This modality has a lot of applications in spine (Figure 19.11A) and brain assessment (Figure 19.11C). In addition, if volume contrast imaging is applied on top of the initial image, more detail can be seen (Figure 19.11B), and definitions of the structures can be more precise.7
Figure 19.10 The normal spine demonstrated in multiplanar (A) and skeletal mode-high-definition (HD) live (B). Scoliosis (C) and hemivertebra (D) were diagnosed in skeletal mode. A spina bifida with skeletal mode was seen in the axial (E) and sagittal views (F). |
Fetal Chest and Abdomen
Fetal chest lesions, rib anomalies, intrathoracic masses, abdominal wall defects, and abdominal structures also can be assessed by 3D ultrasound. Minimum mode is constructive to determine cystic structures of the thorax and abdomen. The application of TUI can display thorax and abdominal structures in one picture (Figure 19.12A-D). Volume calculation of the fetal lungs are used to diagnose diaphragmatic hernia, skeletal dysplasia, and risk of pulmonary hypoplasia.8,9
Fetal Heart
Two-dimensional echocardiography is the gold standard for prenatal imaging of the fetal heart and situs. Obtaining the four-chamber view, left and right outflow tract, and the three-vessel and trachea views are the recommended views in obstetric settings.10 Examining the fetal heart in motion is a critical component of assessing cardiac structure and function. Displaying 3D image information as a moving image can be presented as 3D cine-looping and/or 4D real-time imaging. When these modalities are chosen, the spatial and temporal resolution of the image is limited. Motion artifacts can occur if the volume scan is relatively slow compared to rapid cardiac motion and if the probe moves during the scan. Because the image acquisition and cardiac motion with the cardiac cycle are not synchronized, a simple cine-loop display of serial images may not display the motion of cardiac structures optimally.11 To achieve the best anatomical display throughout the cardiac cycle, a fixed point of the image should be correlated spatially over a period of time. One of the most significant developments in fetal cardiac scanning is, therefore, spatiotemporal image correlation (STIC) imaging. This technology allows practitioners to obtain all of the standard planes from a single volume dataset. Eventually, this will lead to the ability to examine the fetal heart offline without fetal movement and potentially to make the whole process less operator dependent.
Figure 19.11 Examination of the spine using OmniView. Manually drawn nonorthogonal planes demonstrate normal spine (A). Activation of volume contrast imaging (VCI) enhanced image quality (B). Omniview of the fetal brain with manually drawn nonorthogonal lines from transventricular (yellow line), transcerebellar (blue line), and thalamic level (pink line) (C). |
Spatiotemporal Image Correlation
STIC is an automated volume acquisition that allows synchronization of the imaging data to specific times or phases of the cardiac cycle so that fetal heart motion can be incorporated into the final volume dataset. The transducer array performs single sweep, recording one single 3D dataset over a 7.5- to 15-second time period. The volume of interest is acquired at a sweep angle (approximately 20°-40°) to obtain orthogonal planes, depending on the size of the fetus and gestational age (Figure 19.13A and B). As an example, a frame time of 10 seconds and a sweep area of 25° stores 1500 2D images in the ultrasound machine’s memory. During this acquisition time, the fetal heart beats 20 to 25 times, which means that within these 1500 B-mode frames, there are 20 to 25 images showing systolic peaks.11,12 Sudden changes in the heart rate or fetal movements may interfere with the quality of the acquisition, and artifacts can occur. This particular problem may require another acquisition.
STIC volumes can also be obtained with or without color or power Doppler. The preferred method is obtaining an STIC volume with color or power Doppler. This could be removed by the color off button at the time of postprocessing. Notably, in the first-trimester cardiac evaluation, color or power Doppler is essential and very useful. It allows the practitioner to assess all the cardiac landmarks more precisely and reveals any abnormalities in a precise way.
Figure 19.12 Visualization of the thorax and abdomen by rendered three-dimensional (3D) image and application of the postprocessing tomographic ultrasound imaging (TUI) (A). TUI assessment of congenital pulmonary airway malformation (CPAM) (B), omphalocele (C), and diaphragmatic hernia (D). ST, stomach and heart at the same level. |
There are tremendous benefits of applying one of the postprocessing options, such as TUI, in the fetal heart examination. This application allows the examiner to simultaneously display multiple cross-sectional images at specific distances from the four-chamber view, so all the cardiac landmarks can be displayed in one picture (Figure 19.13C and D).
Overall, cardiac evaluation performed by 4D ultrasound using STIC and TUI during the first and second trimesters is advantageous in cardiac screening programs as well as in patients with cardiac anomalies (Figures 19.14 and 19.15).11,13
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