The prenatal detection of congenital heart defects remains one of the most difficult challenges for the sonologist/sonographer when performing the second- or third-trimester screening examination. The four-chamber view has been used for a number of years as the primary screening image for detection of heart defects, but the inclusion of the right and left outflow tracts increases the detection of cardiac malformations. One of the difficulties, however, is obtaining and interpreting two-dimensional images of the outflow tracts. This paper reviews a new technique using three-dimensional (3D) multiplanar imaging that allows the examiner to identify the outflow tracts within a few minutes of acquiring the 3D volume dataset by rotating the volume dataset around the x- and y-axes.
3D multiplanar imaging of the fetal heart using static 3D or spatio-temporal image correlation (STIC) imaging allows the examiner to obtain a volume of data that can be manipulated along the x- and y-axes using reference points from the four-chamber view, five-chamber view, three-vessel view at the level of the bifurcation of the pulmonary arteries, and three-vessel view at the level of the transverse aortic arch and trachea.
The full length of the main pulmonary artery, ductus arteriosus, aortic arch and superior vena cava could be identified easily in the normal fetus by rotating the volume dataset along the x- and y-axes. The vessels were identified using the four-chamber view, the five-chamber view, and the two three-vessel views. The technique was useful in identification of d-transposition of the great vessels and evaluation of the outflow tracts in hypoplastic left heart syndrome.
In 1998 the American Institute of Ultrasound in Medicine published a technical bulletin, entitled ‘Performance of the basic fetal cardiac ultrasound examination’, in which the concept of an extended cardiac examination was introduced, advocating that outflow tract evaluation be included in the screening examination of the heart1. In 2000, the Royal College of Obstetricians and Gynaecologists published ‘Ultrasound Screening’ in which guidelines for performing the 20-week anomaly scan suggested that the outflow tracts should be included in the ‘optimal’ examination of the fetus2. Although there are several approaches to examination of the outflow tracts, one method recently described by several authors appears easy to implement3–7. This approach requires the examiner to direct the ultrasound beam in a transverse plane from the stomach to the upper neck. One of the limitations of this approach is that the examiner does not always image the full length of the outflow tract vessels3–7. To accomplish this, the examiner must rotate and manipulate the transducer beam to image the full length of each vessel. This may be difficult in some situations because of fetal movement, fetal position, and the inexperience of the examiner4.
With the recent introduction of three-dimensional (3D) ultrasound it is now possible to obtain static and cineloop volume scans that contain images of the four-chamber view and outflow tracts that can be rotated and manipulated for optimizing the visualization of cardiac and great vessel anatomy8–12. The purpose of this paper is to present a simple technique, the ‘spin’ technique, for the evaluation of the outflow tracts using 3D ultrasound that enhances the diagnostic approaches described by Yoo et al.5–7 and Yagel et al.3, 4 (Figure 1).
The ultrasound system used for this study was a Voluson 730 Expert series (GE Medical Systems Kretz Ultrasound, Zipf, Austria) in which a real-time probe (RAB 4–8 MHz) for obstetric/gynecological and abdominal applications was used for acquiring the 3D image volumes. 3D static and spatio-temporal image correlation (STIC) volumes were acquired using commercially available software on the system13.
3D static volume
A 3D static volume of the heart and great vessels consists of a single sweep through the heart and great vessels. To accomplish this the sweep speed is selected from a list of options: ‘low’, ‘mid 1’, ‘mid 2’, ‘high 1’ and ‘high 2’. The low option selection has the fastest sweep speed, but the lowest image resolution, while the high 2 option has the slowest sweep speed, but the highest image resolution. The acquired volume is displayed in three planes simultaneously (Figure 2). The images are identified as A, B, and C; the A-image represents the acquired image and the B- and C-images are constructed from the volume data (Figure 2). The 3D sweep speed should be selected to provide the highest resolution while avoiding artifacts in the B-plane which may occur if the sweep speed is too slow (Figure 2).
3D STIC volumes
The 3D STIC volume is a single cardiac cycle that is displayed as a cineloop. 3D STIC volume image acquisition has been described previously13; once the 3D STIC volumes are acquired, the images can be viewed as still images, similar to the 3D static volumes, or as a cineloop13.
The examiner can simultaneously display images in plane A, B and C, and rotate each image around the x-, y- or z-axis. For this study, we examined the image in the A-plane and rotated the image around the x- and y-axes (Figure 3). In addition to rotating each image, the user can scroll through the volume data by adjusting the reference slice depth (Figure 1).
The images acquired for this study were obtained from over 1000 fetuses as part of their clinical ultrasound examination. The four-chamber view was imaged by directing the transducer beam in a transverse plane through the fetal chest14, 15. Fetuses were examined between 16 and 40 weeks of gestation. A technique was developed in which the outflow tracts and adjacent vessels were examined by placing a reference point over the vessel of interest and rotating the A-plane image until the full length of each vessel was identified. This was accomplished using software on the ultrasound machine or the offline software package that replicates the online environment (4D View, GE Medical Systems Kretz Ultrasound).
Techniques for imaging normal anatomy
Each volume sweep was initiated after imaging the four-chamber view in the transverse plane through the fetal chest. The volume sweep was directed from the fetal stomach to the neck (Figure 1). Four cardiovascular image planes were identified as follows: Level 1, the four-chamber view; Level 2, the five-chamber view; Level 3, the three-vessel view that includes the main pulmonary artery as described by Yoo et al.5; Level 4, the three-vessel view at the level of the trachea as described by Yagel et al.4. The four-chamber view (Level 1) should be imaged so that the interventricular septum is perpendicular to the ultrasound beam (apex at 9 o'clock) (Figure 1). If the four-chamber view is not imaged in this orientation, the 3D volume can be rotated along the x-, y- or z-axis until the correct orientation is obtained.
Ascending and transverse aortic arch
There are two approaches for identification of the ascending and transverse aortic arch, depending upon at which level the examiner is imaging the heart.
1.After identifying the four-chamber view (Level I), obtain the five-chamber view (Level 2) by moving through the 3D volume in a cephalad direction (Figure 1).
2.In the Level 2 image place the reference point over the aortic valve and rotate the image around the y-axis until the ascending and transverse aortic arch is identified (Figure 4).
1.Identify the main pulmonary artery and bifurcating right and left pulmonary arteries in the three-vessel view (Level 3) as depicted in Figures 1 and 5.
2.Identify the circular ascending aorta adjacent to the main pulmonary artery in the Level 3 image (Figure 5).
3.Place the reference dot in the center of the circular ascending aorta and rotate the image until the ascending and transverse aortic arch is observed (Figure 5).
Main pulmonary artery and bifurcation of the right and left pulmonary arteries
1.After obtaining the five-chamber view, scroll through the volume until the bifurcation of the main pulmonary artery (Level 3) is identified (Figure 1).
2.If the full length of the main pulmonary artery and the bifurcation are not clearly identified, place the reference dot at the level of the main pulmonary artery and rotate the image along the x-axis until the bifurcation is identified (Figure 6).
1.Identify the main pulmonary artery and bifurcation of the right and left pulmonary arteries in the transverse view (Level 3) as depicted in Figure 1.
2.Move cephalad through the 3D volume until the transverse arch, ductus arteriosus and superior vena cava (Level 4) are identified as described by Yagel et al.4 (Figures 1, 6 and 7).
3.Place the reference dot over the ductus arteriosus and rotate the image along the y-axis until the full length of the main pulmonary artery is identified exiting the right ventricle, merging with the ductus arteriosus (Figure 7).
Superior vena cava
1.Identify the superior vena cava in Levels 3 and 4 (Figures 1, 6 and 7).
2.Place the reference dot in the center of the superior vena cava (Figure 8).
3.Rotate the image along the y-axis until the superior vena cava is observed entering the right atrium (Figure 8).
Full aortic arch and thoracic aorta
1.Move cephalad through the 3D volume until the transverse arch, ductus arteriosus and superior vena cava (Level 4) are identified as described by Yagel et al.4 (Figure 1).
2.Rotate the image clockwise until the transverse aortic arch is parallel to the ultrasound beam (Figure 9a,b).
3.Place the reference dot in the center of the transverse aortic arch (Figure 9b) and rotate the image along the y-axis until the full aortic arch and thoracic aorta are identified (Figure 9c).
Examples of techniques for imaging pathology
d-Transposition of the great arteries: 24 weeks
1.Rotate the four-chamber view so that the interventricular septum is perpendicular to the ultrasound beam (Figure 10a).
2.Move cephalad through the volume to the five-chamber view (Level 2) (Figure 10b).
3.Place the reference dot over the aortic valve in the left ventricular outflow tract and rotate the volume along the y-axis (Figure 10b). This image (Figure 10c) demonstrates the bifurcation of the main pulmonary artery and the branching right and left pulmonary arteries.
4.Return to Level 2 and scroll through the volume cephalad to where the main pulmonary artery should be (Level 3, Figure 10d).
5.Identify the vessel and place the reference dot within the vessel nearest the spine (Figure 10d).
6.Rotate the image along the y-axis (Figure 10d) so that the vessel is parallel to the ultrasound beam (Figure 10e).
7.Rotate the image along the y-axis until two parallel outflow tracts are identified exiting their respective ventricles (Figure 10f).
Hypoplastic left ventricle: 17 weeks
1.Locate the stomach within the left side of the abdomen (Figure 11a).
2.The four-chamber view demonstrates disproportion between the right and left chambers of the heart (Figure 11b).
3.Move cephalad through the volume to Level 2 and identify a small vessel where the ascending aorta would be expected to be found (Figure 11c).
4.Place the reference dot over the vessel and rotate the image to demonstrate the hypoplastic ascending aorta with cephalic vessels branching from the arch (Figure 11d).
5.Move cephalad through the volume to Level 3 and identify the main pulmonary artery and ductus arteriosus exiting the right ventricle (Figure 11e).
6.To further elucidate the branching from the main pulmonary artery, place the reference dot next to the bifurcation and rotate the image until the full length of the right pulmonary artery is identified (Figure 11f).
7.Return to the Level 3 image, place the reference dot over the circular superior vena cava (Figure 11g) and rotate the image to identify the superior vena cava entering the right atrium (Figure 11h).
8.Return to the Level 3 image and move further cephalad to the Level 4 image (Figure 11i). Place the reference dot over the small circular vessels found where the transverse arch should be located.
9.Rotate the image to identify the hypoplastic aortic arch and the thoracic aorta (Figure 11j).
Since the introduction of fetal echocardiography in the early 1980s, the four-chamber view has become the standard approach used to screen for congenital heart defects14–19. However, a number of investigators have reported the limitations of this technique and the necessity to identify the outflow tracts to improve the sensitivity of the screening examination of the heart1, 15, 20–25. The outflow tract examination is one of the more difficult tasks that the fetal examiner encounters during the screening examination because of the myriad of fetal positions and views that must be imaged. To simplify this process Yoo et al.5 and Yagel et al.4 reported using a step-by-step process in which the four-chamber view, scanned in the transverse plane, is the point of reference for initiating the outflow tract examination.
Since the introduction of 3D ultrasound, a volume of image data is available from which the examiner can manipulate images and demonstrate anatomical views that were not in the original image acquisition plane10, 11. This technology is ideal for examination of the fetal heart because the examiner needs only to acquire a volume using either the static 3D or the STIC sweep. Using these techniques the examiner can first use the STIC technique to examine the beating heart. If an artifact is present because of fetal movement, then the static 3D method can be utilized. 3D ultrasound, as described in this paper, improves the examiner's ability to image cardiac structures and the full length of vessels that were not in the original acquired image plane. However, to accomplish this, reconstruction of images from the data volume is required. It is well appreciated by those familiar with this technology that the quality of the displayed 2D image from the 3D volume decreases from the A-plane, to the B-plane, to the C-plane (Figure 2). When the examiner rotates the images around the x- or y-axis to view the full length of the vessels, the image changes from one viewed in the A-plane to variations of images viewed in the B- and C-planes. Thus it is important for the examiner to acquire the original volume data with the greatest possible number of 2D images from which the volume dataset will be constructed. This is accomplished by decreasing the sweep speed of the image acquisition. Although the GE 730 Expert ultrasound machine has five settings for image acquisition using the static 3D sweep, we have found that mid-2 and high-1 settings can be used in almost all fetuses without observing any artifact, as illustrated in Figure 2. If the examiner uses the STIC technique, the 12.5-s sweep is preferable to maximize image quality.
For the individual who is not trained in fetal echocardiography, the techniques described in this paper will allow the examiner to further enhance the screening examination by being able to identify the aortic arch, main pulmonary artery, branching right and left pulmonary arteries and the superior vena cava. The 3D volume acquisition followed by image reorientation should decrease the examination time and improve the confidence of the examiner as to whether the outflow tract anatomy is normal.
In conclusion, this paper describes a step-by-step technique, the spin technique, that allows the user to efficiently identify the outflow tracts by simply modifying the techniques described by Yoo et al.5 and Yagel et al.4. The principle underlying the approach described is that the examiner has only to complete three tasks to identify the great vessels and the superior vena cava: (1) identify the vessel of interest in any of the transverse planes (Levels 2, 3 or 4); (2) place the reference point over the vessel of interest; (3) rotate the image along the x- or y-axis until the image displays the full length of the vessel. It is hoped that over time the methods described in this paper will lead to the improved prenatal diagnosis of congenital heart disease and ultimately improve neonatal outcome.