Ultrasound Scans – Is There a Difference Between 3D and 4D Scans?

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.

Image Credit: Natalia Deriabina / Shutterstock.com

Ultrasound technology

Ultrasound scanning is an important clinical tool in providing images of internal fetal anatomy, as well as a wide range of other organ systems. This imaging technique is also called sonography because it uses high-frequency sound waves to produce images of slices through the body.

A transducer or probe which emits ultrasound waves is placed on the skin after coating it with a thin layer of conductive gel, which ensures that the waves pass smoothly through the skin. The emitted ultrasound waves are reflected by different structures encountered by the waves.

The strength of the reflected waves, and the time they take to return, form the basis for interpreting the information into a visible image. This is performed by computer software.

The advantages of ultrasound imaging over other imaging techniques include:

• Real-time visualization of the fetus or organs.
• Non-invasive.
• Eliminates the use of ionizing radiation, which has been associated with toxic effects on the embryo.
• Interactive, as it enables the operator to capture different viewing planes by moving the probe.

2D ultrasound

Traditional ultrasound scanning is two-dimensional (2D), meaning it sends and receives ultrasound waves in just one plane. The reflected waves then provide a flat, black-and-white image of the fetus through that plane.

Moving the transducer enables numerous planes of viewing, and when the right plane is achieved, as judged by the image on the monitor, a still film can be developed from the recording. Most of the detailed evaluation of fetal anatomy and morphology, to date, has been done using 2D ultrasound.

3D ultrasound

Further development of ultrasound technology led to the acquisition of volume data, which produces slightly differing 2D images caused by reflected waves that are at slightly different angles to each other.

These images are then integrated by high-speed computing software to produce a three-dimensional (3D) image. The technology behind 3D ultrasound thus has to deal with image volume data acquisition, volume data analysis, and volume display.

Volume data is acquired using three techniques:

• Freehand movements of the probe, with or without position sensors to form the images.
• Mechanical sensors built into the probe head.
• Matrix array sensors, which use one single sweep to acquire a considerable amount of data. This incorporates a whole series of 2D frames taken in succession, followed by data analysis that is used to provide a 3D image. The operator can then extract any view or plane of interest, which helps to visualize the structures in terms of their morphology, size, and relationship with each other.  

Data can be displayed using either a multiplanar format or rendering of images, which is a computerized process that fills in the gaps to create a smooth 3D image. There is also a tomographic mode which allows the viewing of numerous parallel slices in the transverse plane from the 3D or four-dimensional (4D) data set.

The multiplanar format allows the operator to evaluate several 2D planes at the same time. Using a reference dot on the screen which represents the point of intersection of three orthogonal planes (X, Y, and Z), it can be freely moved to obtain an image at any plane within the scanned volume.

Thus, for instance, while visualizing the fetal heart, the operator is able to summon any of the classical fetal heart views by moving the reference dot, be it four-chamber, three-vessel, or any other plane of interest. This format can be displayed using gray-scale, color Doppler or power Doppler. The Doppler settings help to display the movement of blood through the various chambers and valves.

Advantages

There are several advantages associated with 3D ultrasound imaging. The use of virtual planes, for example, helps to achieve better visualization of fetal heart structures by allowing views that would not otherwise be attainable by 2D imaging, possibly adding a 6% chance of detecting defects.

Additional advantages include the ability of 3D ultrasounds to diagnose fetal face defects like cleft lip, as well as fetal skeletal or neural tube defects. Taken together, 3D ultrasounds may help to identify structural congenital anomalies of the fetus during the scheduled 18-20 week scan.

3D ultrasound imaging also requires less time for standard plane visualization as compared to 2D ultrasounds. Furthermore, this imaging technique is less dependent on operator skill and experience for the diagnosis of common fetal anomalies. The recorded volume data can also be made available for remote expert review for better diagnosis.

4D ultrasound

3D imaging allows for the visualization of fetal structures and the internal anatomy as static 3D images. Comparatively, 4D ultrasounds allow for a live-streaming video of the images, showing the motion of the fetal heart wall or valves, as well as the current blood that is flowing through various vessels.

In short, 4D ultrasound imaging is a 3D ultrasound in live motion. 4D ultrasounds utilize either a 2D transducer, which rapidly acquires 20-30 volumes or a matrix array, which instead uses a 3D transducer.

4D ultrasound imaging is associated with the same advantages as 3D, while also allowing clinicians to study the motion of various moving organs of the body.

The clinical applications of 4D ultrasound technology are still being studied. At present, it is mostly used to provide fetal keepsake videos, a use which is discouraged by most medical watchdog sites. This is because many unregulated centers will offer these videos as entertainment ultrasounds, which violates the As Low As Reasonably Achievable (ALARA) principle governing the medical use of diagnostic imaging.

Additional disadvantages of this non-medical use of 4D ultrasounds include:

• The machines may use higher-than-usual levels of ultrasound energy, which can have potential side effects on the fetus.
• The ultrasound sessions may be prolonged.
• Uncertified or untrained operators may lead to missed or inadequate diagnosis since they are not required to be certified by law.

Side effects of ultrasound

When used for diagnostic purposes, ultrasound imaging has the potential to cause cavitation or small pockets of gas in the tissues, and can also produce slight heating of the tissue. While no significant health consequences have been traced over 20 years of ultrasound use, the use of unregulated ultrasounds for non-medical purposes is not encouraged.

However, recording videos of the fetal movements is permissible if it occurs during the medically indicated examination performed by trained medical personnel, and without the need for additional fetal exposure to ultrasound energy.

Advantages of 3D/4D ultrasound

• Shorter time for fetal heart screening and diagnosis.
• Volume data storage for screening, expert review, remote diagnosis in remote areas, and teaching.
• Enhanced parental bonding with the baby.
• Healthier behavior during pregnancy as a result of seeing the baby in real-time and in 3D.
• More support by the father after visualizing the baby’s form and movement.
• Possibly more accurate identification of fetal anomalies, particularly those involving the face, heart, limbs, neural tube, and skeleton.
• In addition, these advanced ultrasound techniques share the benefits of 2D ultrasound, namely:
  • Assessment of fetal growth.
  • Evaluation of fetal well-being.
  • Placental localization and assessment.
  • Seeing and hearing the fetal heartbeat.
  • Capturing images of the baby, which bonds the family and friends with the baby before birth.

Disadvantages

• Expensive machinery.
• Longer training required to operate.
• Volume data acquired may be lower-quality in the presence of fetal movements of any kind, which will affect all later planes of viewing.
• If the fetal spine is not at the bottom of the scanned field, sound shadows may hinder the view.

Summary

Even with their numerous benefits, the potential hazards of prolonged fetal exposure to ultrasound energy by both 3D and 4D ultrasound scanning for non-medical and unnecessary ‘entertainment’ purposes should be carefully considered. Parents should discuss the issue with their healthcare providers before undergoing this, presently, purely elective procedure.

Further Reading

• All Ultrasound Content
• What is an Ultrasound?
• Ultrasound Therapeutic Applications
• What is Transvaginal Ultrasound?
• Musculoskeletal Ultrasound Uses