Ultrasound question - Your Letters

I am writing in regard to the article "Weighing the Risks" by Sarah Buckley (September-October 2000). I must admit I do not have a medical background and am neither for nor against the use of ultrasound, but I found the following information from the article to be quite disturbing: "The second effect is cavitation, where the small pockets of gas that exist within mammalian tissue vibrate and then collapse. In this situation' ... temperatures of many thousands of degrees Celsius in the gas create a wide range of chemical products, some of which are potentially toxic.' The significance of cavitation in human tissue is unknown...."

As you should know, to quote the aforementioned reference, cavitation occurs in fluids via a resultant drop in pressure below the vapor pressure of the gas in the fluid, with damage occurring following implosion of the gas bubble. For the layman, cavitation is boiling at, sometimes, room temperature and occurs in fluids. The process mentioned above isn't cavitation. For cavitation to occur in blood or the womb, for example, the fluid pressure ,would have to be so low or velocity of the fluid flow so high that life would not be capable of existing.

I do not mean to belittle the intent of most of the piece, but to either so poorly misquote an original article or to use an article that is so scientifically unsound (whichever the case) is disappointing. People who read this article aren't physicists and shouldn't have to be.

ROBERT STEVENS
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Sarah Buckley, MD, responds:

Thank you for the opportunity to respond to the issue that you have raised. The section of my article that you are questioning is, as referenced in my article, directly taken from the American Institute of Ultrasound Medicine (AIUM) "Bioeffects Report," 1987, as published in the Journal of Ultrasound Medicine 7 (September 1988): S1-S38.

The AIUM committee "continually monitors and evaluates research findings and publications," and this report is the result of "an in-depth review of current knowledge regarding bioeffects...."

In this 38-page report, nine pages are devoted to the discussion of "Cavitation Mechanism," and, not being a physicist myself, I refer you to this section for a fuller explanation of the mechanisms involved.

The section that I quote, and the dramatic language, are those of the report's authors. The quote continues, "These violent processes may be produced by microsecond pulses of the kind which are used in medical diagnosis...." The conclusions regarding cavitation, that were approved by the committee, begin: "Acoustic cavitation may occur with short pulses and has the potential for producing deleterious biological effects."

My interest in this paper, which has, overall, reassuring conclusions about obstetric ultrasound, is that it details the mechanisms by which any significant harmful effects may occur--vis-a-vis heating and cavitation. Since its publication, more evidence of effects in mammals (and babies) has accumulated, as referenced in my article.

As I hope I have conveyed in my article, there is not absolute proof that ultrasound is harmful, but there are a number of studies--and I include this paper--that raise doubts, such that I believe that we should be extremely cautious about exposing our developing babies to this very recent technology.

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