We are often asked, "What is 3D Ultrasound?" The best way to understand this concept would be to compare ultrasound to a loaf of bread. The individual slices would represent a 2D ultrasound image. The 2D image is a single slice through the 3D loaf of bread. 3D ultrasound is an image that is composed of multiple 2D slices that, when placed together, create a 3D object, i.e. a loaf of bread. The most common example of this is the image of the fetal face, as illustrated below.
This illustrates the concept of 3D imaging. When a 3D or 4D ultasound is performed, thousands of 2D images are acquired (single slice of bread) and then processed to create a 3D volume dataset. This can be displayed as a 3D image (loaf of bread).
When a 3D image is captured over tiime, it has the appearance of movement. This type of image is called 4D ultrasound.
The image on the left is a 3D model of the heart. The 2D slice is represented on the right.
How are the Images Acquired for 3D/4D Ultrasound.
Unlike 2D ultrasound in which a single beam is directed to the area of interest, the acquisition of the 3D/4D image involves steering the ultrasound beam across the area of interest (fetus) to acquire the thousands of images used to construct the 3D/4D volume dataset.
The 2D image is acquired by directing a the ultrasound beam over the area of interest.
When 3D/4D imaging occurs, the ultrasound beam is swept rapidly (3D acquisition) or slowly (4D STIC acquisition) over the area of interest to collect thousands of images for reconstruction of the volume dataset.
How is 3D/4D Ultrasound Used to Examine the Fetal Heart?
Earlier in these lessons the heart was illustrated as a 3D structure, having the appearance of an actual heart if one were examining it in real life. As the result of 3D/4D technology the ultrasound data that is acquired consists of thousands of 2D images (single slices of bread) that are arranged together in a volume dataset (loaf of bread). This volume dataset can be examined with computer technology to do the following:
1. Recreate 2D slices from any angle within the 3D volume. Using the loaf of bread as an example, the physician can cut the loaf at any angle to obtain a slice (2D). This gives the examiner an unlimited number of 2D images to evaluate.
2. Reconstruct the surface anatomy of the heart by examining thousands of 2D slices at one time.
3. Examine multiple slices of the 2D image simultaneously.
4. Examine the 3D/4D volume datasets using different types of shading.
The following sections will illustrate several of these techniques used to evaluate the heart, especially when there is suspected abnormalities.
3D/4D Multiplaner Imaging
This technology enables the examiner to view an unlimited number of 2D images from the volume dataset. This is useful for identifying the 4-chamber, 5-chamber, and outflow tracts of the heart. While the 3D image is useful, it only represents a static, or non-moving image of the heart. When 4D imaging is used, the 2D images are displayed as a single cardiac cycle. The benefit of this is that cardiac structures are easier to examine when the heart is beating vs when it is a still image.
This is a 3D static acquisition. The image first displays the live 2D scan. The 3D acquisition is activated, followed by the multiplaner display. Notice that the heart images are static, not demonstrating any movement.
This is a 4D multiplaner image display. This was acquired using STIC technology, first described by Dr. DeVore and colleagues in 2003.
From this dataset the images can be displayed as three simultaneous images, perpendicular to each other, or as a single display. Using the single display the image can be manipulated around three axis planes to examine an unlimited number of images. The following example illustrates the "SPIN" technique described by Dr. DeVore and colleagues in 2004.
Using STIC technology, the volume dataset can be manipulated to view the heart from an unlimited number of planes. This illustrates this technique to identify the four-chamber, five-chamber, and outflow tracts of the fetal heart. RA=right atrium, LA=left atrium, RV=right ventricle, LV=left ventricle, AA=aortic arch, MPA=main pulmonary artery, LPA=left pulmonary artery, RPA=right pulmonary artery, SVC=superior vena cava.
3D/4D Tomographic Ultrasound Imaging (TUI)
Tomographic Ultrasound Imaging, (TUI) is a technique in which the volume dataset is divided into multiple slices, simultaneously displayed on the ultrasound screen. This is equivalent to selecting specific slices of bread from a loaf to examine. TUI was first described by Dr. DeVore in 2005. The following are examples of this technology.
Tomographic Ultrasound Imaging is a technique in which the volume dataset is selectively "sliced" to display images that are behind each other. This loaf of bread similar to the volume dataset in that it contains multiple slices (2D ultrasound images). By selecting specific slices at intervals (arrows), specific areas within the loaf of bread are examined.
This illustrates multiple levels that would 2D images from the heart. Using TUI, each level is displayed simultaneously on the ultrasound screen.
This is a normal heart demonstrating the four-chamber view, five-chamber view, 3-vessel view, and the tracheal view. RA=right atrium, LA=left atrium, RV=right ventricle, LV=left ventricle, MPA=main pulmonary artery, Asc Ao=ascending aorta, SVC=superior vena cava, Trans Ao=thoracic aorta, DA=ductus arteriosus, Ao=aorta.
This is a fetus with transposition of the great arteries with a ventricular septal defect (VSD). LV=left ventricle, A=aorta exiting the right ventricle, PA=pulmonary artery exiting the left ventricle, RV=right ventricle.
This illustrates multiple slices through the four-chamber view of the heart in which a ventricular septal defect is present (VSD). Blood can be observed crossing the ventricular septum. RV=right ventricle, LV=left ventricle.
This illustrates multiple slices through the four-chamber view in which there is a dilated right atrium and abnormal insertion of the tricuspid valve in the right ventricle. This is consistent with Ebstein's anomaly. A=aorta, TV=tricuspid valve, RA=right atrium, RV=right ventricle, LV=left ventricle, LA=left atrium.
This illustrates multiple slices through the four-chamber and five chamber view. The fetus had an atrial (ASD) and ventricular (VSD) septal defect. This fetus had Down syndrome. RV=right ventricle, LV=left ventricle, RA=right atrium, LA=left atrium.
3D/4D Rendered Imaging of the Heart
Creating a 3D/4D rendered image means that the image will have the appearance of depth. This type of imaging was first described by Dr. DeVore in 2003. There are a number of image displays that can be used to examine the heart in this manner. The following illustrates examples of 3D/4D rendering of the volume dataset.
This is a rendered image from a 3D volume dataset. If you examine the image carefully you will see the four-chamber view. The image has "depth", and you can see the back walls of the heart chambers.
This is a similar image to the one on the left except that it is 4D. Notice the depth of the image and the back walls of the heart chambers.
3D/4D Rendered Imaging of the Surface Anatomy of the Heart
Another method that can be used to examine the fetal heart is to render the surface anatomy. This is performed using an INVERT filter than allows the examiner to reconstruct the surface anatomy, similar to what is observed if one were looking at a heart specimen. The following are examples of this technology.
This is illustrates the surface anatomy of the fetal heart.
This is a rendered surface image of the heart. The main pulmonary artery is observed anterior to the ascending aorta.
This is a 4D model of the heart demonstrating the normal relationship of the main pulmonary artery crossing over, and perpendicular to the ascending aorta.
This is a diagram illustrating Transposition of the Great Arteries in which the main pulmonary artery and the ascending aorta are not perpendicular as they exit their respective ventricles. In addition, the main pulmonary artery originates from the left ventricle and the aorta from the right ventricle. The outcome for the baby is improved when this brith defect is diagnosed before birth and the fetus delivered at a hospital equipped to care for newborns with this condition.
This 4D model of the heart demonstrates the parallel outflow tracts exiting the incorrect ventricles.
