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October 9, 2006

UltraSound Check — Yale Study Rekindles the Fetal Ultrasound Debate
By Beth W. Orenstein
Radiology Today
Vol. 7 No. 20 P. 24

Research in mice—which everyone involved agrees doesn’t translate to humans—nevertheless stirred the long-simmering pot about the appropriate use of ultrasound. No one disputes that benefits outweigh the risk for medical indications, but experts raise the point that research could help fine-tune understanding of ultrasound’s bioeffects.

Ultrasound is generally considered safe and harmless.

So in August, when researchers from the Yale University School of Medicine in New Haven, Conn., published their study in the Proceedings of the National Academy of Sciences that found that ultrasound scans caused disruption in neuronal migration to the cerebral cortex of mice embryos, it caused quite a stir.

The mass media jumped on the research and asked in headlines: “Are our unborn children at risk?” It has become fairly standard practice to perform ultrasounds at least once or twice during pregnancies, even those considered low-risk.

The ultrasound and larger medical communities were quick to point out, however, that the risks of diagnostic medical ultrasound—if there turn out to be any—would outweigh the benefits. Obstetricians and radiologists use sonograms to determine the age of the fetus and its position in the womb and to screen for anatomical abnormalities throughout the pregnancy.

Larry Platt, MD, professor of obstetrics and gynecology at the David Geffen School of Medicine at UCLA and president of the International Society of Obstetrics and Gynecology, says the study by such a prestigious university in such a reputable journal should not be ignored. “It gives us time to pause and time to reflect,” he says. “It shouldn’t be pooh-poohed.”

However, Platt, who is also director of the Center for Fetal Medicine and Women’s Ultrasound in Los Angeles, adds that the study does not change his or the position of the American Institute of Ultrasound in Medicine (AIUM) on fetal ultrasound studies: when medically indicated, the benefits would outweigh the risks.

Indeed, Platt says, the study only serves to reinforce the AIUM’s and the FDA’s position that fetal sonograms should not be performed solely for entertainment or psychosocial purposes because the risks may not yet be known.

AIUM President Lennard Greenbaum, MD, codirector of the Hughes Center for Fetal Diagnostics at the Winnie Palmer Hospital for Women & Babies in Orlando, Fla., agrees: “We know at the present time that ultrasound does not appear to have any harmful effects on the developing fetus. However, that does not mean it should be used indiscriminately. Most of us who are involved in doing obstetric ultrasound feel exposure just for entertainment purposes is unwarranted. The study by the researchers at Yale only proves what we’ve been talking about regarding keepsake videos: ‘If no medical information is to be obtained from it, why do it?’”

The lead researcher of the Yale study, Pasko Rakic, MD, PhD, also says the results in mice do not indicate that the use of ultrasound on human fetuses for diagnostic and medical purposes should be abandoned. However, he, too, says he would caution against unnecessary ultrasound scans until more research is completed.

The issue of nondiagnostic sonograms has been brewing over the past several years as a growing number of boutique ultrasound operations have popped up in shopping centers and strip malls offering expectant mothers ultrasound portraits of their unborn children for approximately $200.

The TomKat Factor
The controversy was further fanned last year when actor Tom Cruise announced that he had bought an ultrasound machine so he and fiancee Katie Holmes could monitor the daily progress of their daughter, Suri, who was born in April. Cruise’s offering to donate the machine to a medical facility when they were finished with it did not help.

Many physicians are also concerned because ultrasound equipment is readily available for a few thousand dollars on eBay and other auction sites. The problem, they say, is that the people who buy and indiscriminately use the equipment may not be trained in its use and could overlook hard-to-detect spinal abnormalities, clubfoot or cleft lip, or other issues that those who are properly trained would likely see.

Expectant parents could be falsely reassured when they should be consulting with their obstetrician, Greenbaum says.

Most physicians were not alarmed by the results of Rakic group’s study because they say the experimental model in the mouse is not a meaningful one for human fetal biology.

Not only are there major differences between the development of mice and human embryos but also the way the scans were done for the research and how fetal ultrasounds are performed.

Of Mice, Not Men
“It is pretty clear that the results of the study are not directly applicable to humans,” Platt says.

“It’s apples and oranges in terms of what they did and how it applies to human fetuses,” Greenbaum agrees.

Rakic’s study involved 335 mice. The mothers of 146 mice were continuously exposed to ultrasound at frequencies of 6.7 megahertz and at a scanning rate of 11 frames per second for various amounts of time ranging from five to 420 minutes. The exposure to ultrasound occurred on each of the final three days of gestation, which in mice is 19 days. During this time, the cells would have completed their final round of division and be migrating.

Ten days after the mice were born, the researchers chemically tagged the brains to compare those exposed to ultrasound with those that had not been exposed. They found nothing remarkable about the general appearance of the brains of either group. The researchers reported no difference in brain size or neocortical cellular composition.

However, they found that in a small but statistically significant number of the mice that had been exposed to ultrasound for more than 30 minutes, a number of brain cells failed to grow in their proper position and remained scattered in incorrect parts of the brain. The longer the mice had been exposed, the greater the number of affected cells, the researchers reported. A relatively small percentage of E16 neurons were located more deeply at all levels of the cerebral wall.

Misplaced Cells
The researchers, who were funded by the National Institute of Neurological Disorders and Stroke, note that while the cells were in abnormal positions, they appeared to retain their class-specific properties. However, it is possible that the misplaced cells could potentially result in brain abnormalities such as mental retardation or seizures. The researchers did not test whether the affected mice had abnormal mental abilities.

Rakic says the study results suggest it is possible that the ultrasound waves somehow disrupt the connections formed between cells as they move to their proper location.

Physicians say the study is not directly applicable to human fetuses for several reasons. One key reason is that while a scan may take as long as 30 minutes, the sonographer is generally moving the probe around the patient to find the optimal image. The sonographer may hold the probe down in one spot for longer periods of time, but it wouldn’t be the entire time.

“During a 30-minute scan, you’re scanning over the entire body of the fetus,” Greenbaum says, “and you may be examining the brain for a very small fraction of that time vs. having the probe there and having exposures to the same spot for up to 420 minutes.”

Also, Greenbaum says, the Yale study was performed on mice in which neuronal migration occurs over a period of only a few days, whereas in humans the migration occurs over several weeks. Therefore, a human fetus’ brain would be exposed to ultrasound for only a tiny fraction of its neuronal migration period.

Jean Lea Spitz, MPH, RDMS, president of the Society of Diagnostic Medical Sonography and a professor at the University of Oklahoma’s Health Sciences Center in Oklahoma City, says the Yale study only confirms the need to have sonograms performed by qualified professionals.

“Credentialed sonographers understand ALARA [as low as reasonably achievable] principles and safeguard the fetus by obtaining images using the minimally necessary energy and by reducing dwell time,” she says.

“We know that ultrasound can heat bone such as the skull in vitro and, in late pregnancy, this theoretically could contribute to secondary effects on nearby neural tissue. Dwelling on the head as in fetal portraiture is unwise,” she adds.

Rakic says more research is needed and that his group has planned further studies.

Spitz agrees that more research into the bioeffects of sonography is important, especially as imaging equipment continues to evolve.

Some earlier studies have found associations between fetal exposure to ultrasound and delayed speech, an increase in non-right handedness, and reduced birth weight, Spitz says. She points out that those studies show associations and do not prove cause.

Energy Levels
“The sonograms that we can create today are phenomenal, but our energy levels have increased in order to give us those excellent pictures,” Spitz says. “The energy we use today is as much as eightfold higher than the energy used 12 years ago. When you consider that most of the studies on bioeffects of ultrasound on the fetus relate to exposures in the ’70s and ’80s, you realize that we may not know all that we need to know about effects with today’s equipment.”

Greenbaum says the AIUM encourages physicians to provide their patients with images obtained during their appropriate obstetric ultrasound examinations and will continue to do so in light of the Yale study.

The physician-provided print or video “may not be pictures with the baby’s name etched in a Lucite frame,” he says, “but it’s not going to cost parents the $100 to $200 the keepsake boutique is charging and it’s not going to expose their fetus to unnecessary ultrasound.”

— Beth W. Orenstein is a freelance medical writer and a regular contributor to Radiology Today. She writes from her home in Northampton, Pa.