At the beginning of almost every scan, patients or family members ask if we are going to use the ‘new 4D’ to see their growing baby. There is a lot of confusion and misconception about 3D versus 4D, what we can see when we use it, and if it is ‘better’ than the regular black/white scans we do. In this section, you will learn the differences between the different scanning methods and why 3D and 4D are not going to replace the traditional 2D black and white images.
What is 3D ultrasound?
Those beautiful pictures that you see in the magazines and promotional pictures are known as 3D ‘surface rendered’ images. In other words, the ultrasound machine uses sophisticated software to construct an image of the baby as you would see it ‘with the skin on’. The technology is similar to that used to make animated cartoons like Shrek, so that the image you see on the ultrasound monitor is really a computer-constructed image, not a true peek at the baby itself. In order to get a beautiful 3D ultrasound image, the baby’s face must not be covered by hands or touching anything. Just like any picture, if the baby isn’t looking at the ‘camera’, you can’t get a good image. When the baby’s face is clear, the image can be astounding. But, if the hands, the cord, or something else is blocking the view, we can’t get that gorgeous shot like in the promotions. Don’t count on getting a great image every time. See limitations of 3D/4D.
What is 4D ultrasound?
The 4th ‘D’ is TIME. 4D is simply a stream of static 3D images, updated many times per second so that you can see the baby move. It can be really fun to see the baby change expressions, stick out its tongue, and wiggle fingers. But often we just watch as the baby moves out of view or rolls away, so it can also be frustrating. In general, 3D and 4D images can be downright scary sometimes because the fetus is exactly that—a fetus which isn’t necessary adorable just yet.
In the video below, you can see a term baby who is being unusually photogenic for this late in the pregnancy. You can see the foot by the head and face, which would break you or me, but is perfectly comfortable for fetuses.
Why 2D is ‘better’
3D/4D surface-rendered images use software to view the baby with the ‘skin on’. But in order to see if the baby’s anatomy is normal, we need to see through the skin to the structures inside. 2D imaging allows us to take slices of data through the baby’s body to examine the internal organs and bones with great detail. 3D/4D is useful to show the extent of externally-visible defects such as a cleft lip, but it isn’t going to show abnormalities of the heart, brain, and other internal structures.
In the 2D image on the left, you can see the brain behind the bright white line of the forehead. In addition, the heart, stomach, left kidney, ribs and spine towards the bottom of the picture are all visible here to the trained eye. The sonographer can zoom into all of the anatomy to examine each individually and ensure that they have developed normally.