Three and Four Dimensional Ultrasound

Contents

1. Depth and motion come from computing
2. Clinical use is narrower than the publicity
3. Keepsake scanning moved outside the clinic
4. Professional bodies set limits

Three dimensional and four dimensional scans belong to the sonography Candace Cooley has carried out and read, and they are the forms of the test that add depth and then motion to the familiar flat image. A conventional scan shows a thin gray slice through the body. A three dimensional scan presents a surface with depth, and a four dimensional scan is, in the words of the National Institute of Biomedical Imaging and Bioengineering, 3D in motion.⁹ The fourth dimension is simply time.

The idea is older than the souvenir videos that made it famous. A history of the ultrasound machine in the specialty records that research groups in Japan began studying three dimensional imaging in 1984, that the approach resurfaced in 1990 with a machine that showed it had a place in obstetric and gynecologic care, and that by 2000 real time scanners with a 3D and 4D option had become inexpensive and easy to obtain.⁷ The same basic physics runs underneath, and the article on how medical ultrasound works sets it out.

Figure

Flat, deep and moving images

	Two dimensional scan	3D and 4D rendering
What appears on screen	A flat grayscale profile that can be hard to read	A lit face with depth; in 4D, expressions and even a yawn
Place in the examination	Comes first, to assess the baby	Switched on afterward, once the baby is judged healthy
What it depends on	The standard views of the routine scan	A baby lying in a position where the rendering works

Depth and motion come from computing

The probe still sends out sound and listens for echoes. As the NIBIB fact sheet explains, the scanner uses the speed of sound and the time each echo takes to return to calculate how far away each tissue boundary lies, and those distances are the raw material of every picture.⁹ What changes is the processing. When CBS News reported on four dimensional imaging in 2001, it described the technique as high speed computer enhancement of ultrasound scans that could produce a live picture before birth, detailed enough to count fingers and toes and to watch a yawn.⁴ The obstetrician Stuart Campbell told the network that adding a dimension made the surface features of the baby much clearer, and the report added that the method could help with diagnosis and with precise biopsies.⁴

Later refinements went after realism. A 2013 report in Time described rendering with a movable light source that lit the face so that skin tone, lips, nose and eyelids could be picked out, and noted that the feature could be used from the first trimester onward.¹ Even then, a hospital sonographer quoted in the piece said her team first assessed the baby with the ordinary two dimensional grayscale scan, and switched to the rendered view only once they were satisfied the baby was healthy and lying in a position where the imaging would work.¹ That order of work is the clearest guide to the role of these images: they are added to the standard examination, not substituted for it.

Clinical use is narrower than the publicity

In everyday obstetric care, MedlinePlus notes that 3D and 4D images are more commonly made during the third trimester.¹⁰ The measurements that make up most of a pregnancy scan, surveyed under obstetric ultrasound, are still taken from flat images. Outside pregnancy, three dimensional scanning has a quieter use in gynecology. RadiologyInfo explains that it allows the uterus and the ovaries to be evaluated in planes that cannot be imaged directly, and that it gives information about the outer contour of the uterus.⁶ The probe for those pelvic studies is often placed in the vagina, the approach set out under transvaginal ultrasound.

Figure

From the first scanners to 3D and 4D

1. 01
   1958The first two dimensional obstetric scanner makes slow, static images.
2. 02
   1984Researchers in Japan begin studying three dimensional ultrasound imaging.
3. 03
   1990Three dimensional scanning resurfaces, and a new machine shows a place for 3D and 4D imaging within obstetrics and gynecology.
4. 04
   By 2000Real time scanners with a 3D and 4D option are inexpensive and easy to obtain.

Keepsake scanning moved outside the clinic

Once the machines became affordable, the same images were sold directly to expectant parents. In 2004 AuntMinnie reported on a partnership offering 3D keepsake images on a fee for service basis, with an order from a medical professional, delivered on video, disc and print.⁵ That summer CBS News visited a Manhattan studio where parents watched real time 3D images of a baby opening its eyes and sucking its thumb, and noted that many such studios limited the number of visits and said they required a doctor’s approval first.³ Time’s Sanjay Gupta wrote that parents were paying for golden toned images at studios with playful names, and that the machines, which could cost as much as $200,000, were also turning up in the offices of obstetricians who wanted to offer patients what a shopping mall studio could.²

Figure

A medical scan and a keepsake session

	Medical scan	Keepsake session
Purpose	To see and diagnose problems early in pregnancy	Pictures and video for the family
Setting	A doctor's office or hospital	Studios and boutiques open to the public
Safeguards	A doctor explains what the images mean	Many studios cap visits and ask for a doctor's approval
Main worry	Scanning only when there is a medical indication	False reassurance, or alarm with no one to explain

Professional bodies set limits

The objection was never that the images were inaccurate. It was that the scans had no medical purpose. Gupta reported in 2004 that the Food and Drug Administration had issued a firm warning in its consumer magazine: although no case of harm to a fetus had been reported, too little was known about the long term effects of repeated exposure, and laboratory studies had shown that even low levels of ultrasound could cause vibration and warming in tissue. He added that the same kind of sound waves, at higher exposure, are used to break up kidney stones.² The NIBIB fact sheet states the position plainly. The FDA requires diagnostic devices to work within set limits, and the agency and many professional societies discourage casual use such as keepsake videos, recommending scans only when there is a true medical need.⁹ The Office on Women’s Health gives the same advice to the public: ultrasound is considered safe for medical purposes, and doctors advise against it for a keepsake when there is no medical need.⁸

A second worry was practical. Physicians quoted by CBS feared that parents would come away either falsely reassured or needlessly alarmed without a doctor to explain what they had seen, and Time reported concern that some women were skipping standard prenatal visits after a reassuring session at a studio.³²

What these images cannot do is the honest summary. A 3D or 4D view cannot replace the measurements of the standard scan, and it cannot be made at all when the baby lies in a position the rendering cannot work around.¹ Like any ultrasound, it cannot see well through bone or air.⁹ It cannot answer the safety question either, because long term studies of sound waves on a fetus have not been done, which is why the professional advice is to scan only for a medical reason.³ When and whether a pregnancy needs any particular scan is a judgment for each woman to make with her own clinician.