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Ultrasound scans – is there a difference between 3D and 4D scans?

By Liji Thomas, MD

Ultrasound technology

Ultrasound scanning is an important clinical tool in providing images of internal fetal anatomy. It 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, to make sure 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.
  • Doesn’t use ionizing radiation, which has been associated with toxic effects on the embryo.
  • Interactive, enabling the operator to capture different viewing planes by moving the probe.

2D ultrasound

Traditional ultrasound scanning is 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 so far has been done using 2D ultrasound.

3D ultrasound

Further development of ultrasound technology led to the acquisition of volume data, i.e., slightly differing 2D images caused by reflected waves which are at slightly different angles to each other. These are then integrated by high-speed computing software. This provides a 3-dimensional image. The technology behind 3D ultrasound thus has to deal with image volume data acquisition, volume data analysis and finally 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 uses one single sweep to acquire a lot of data. This incorporates a whole series of 2D frames taken in succession. Data analysis then provides a 3-D image. The operator can then extract any view or plane of interest. This helps to visualize the structures in terms of their morphology, size and relationship with each other.  

Data can be displayed using either multiplanar format or rendering of images, which is a computerized process filling 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 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

  • The use of virtual planes helps in better visualization of fetal heart structures by allowing views not attainable by 2D imaging, possibly adding a 6% chance of detecting defects.
  • Diagnosis of fetal face defects like cleft lip.
  • Diagnosis of fetal skeletal or neural tube defects.
  • Less time for standard plane visualization.
  • Less dependent on operator skill and experience for diagnosis of common fetal anomalies.
  • The recorded volume data can be made available for remote expert review for better diagnosis.

3D ultrasound may help to identify structural congenital anomalies of the fetus during the scheduled 18-20 week scan.

4D ultrasound

3D imaging allows fetal structures and internal anatomy to be visualized as static 3D images. However, 4D ultrasound allows us to add live streaming video of the images, showing the motion of the fetal heart wall or valves, or blood flow in various vessels. It is thus 3D ultrasound in live motion. It uses either a 2D transducer which rapidly acquires 20-30 volumes or a matrix array 3D transducer is used.

4D ultrasound has the same advantages as 3D, while also allowing us to study the motion of various moving organs of the body. Its clinical applications 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 unregulated centers offer it as entertainment ultrasounds. Such use violates the ALARA (As Low As Reasonably Achievable) principle governing the medical use of diagnostic imaging.

Disadvantages of non-medical use are:

  • The machines may use higher-than-usual levels of ultrasound energy with 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

Ultrasound at diagnostic levels has the potential to cause cavitation or small pockets of gas in the tissues. It also produces slight heating of tissue. While no significant health consequences have been traced over 20 years of ultrasound use, the use of unregulated ultrasound for other than medical indications 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 as 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 especially of the face, heart, limbs, neural tube and skeleton.
  • In addition it shares 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 bond 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 the numerous benefits, the potential hazards of prolonged fetal exposure to ultrasound energy by using 3D/4D scanning for non-medical and unnecessary ‘entertainment’ purposes is inappropriate. Parents should discuss the issue with their health-care providers before undergoing this purely elective procedure at present.

Further Reading

Last Updated: Mar 11, 2015

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