Ultrasound safety and collapsing bubbles.Ultrasound Safety and Collapsing Bubbles
Physicians and medical researchers use high-frequency sound waves to examine organs or a developing fetus or to track the flow of blood. Numerous experiments and years of experience have turned up no clear evidence that diagnostic ultrasound diagnostic ultrasound
Use of ultrasound to obtain images for medical diagnostic purposes. has any harmful effects in human beings (SN: 6/12/82, p.396). However, a recent finding that pulsed ultrasound, with characteristics similar to that used in diagnosis, can kill fruit fly larvae Larvae, in Roman religion
Larvae: see lemures. has again focused attention on the safety of diagnostic ultrasound.
The question is whether the effects seen in larvae also show up in some way in human subjects. "Clearly, flies are far different from human beings,' says Edwin L. Carstensen of the electrical engineering and biophysics biophysics, application of various methods and principles of physical science to the study of biological problems. In physiological biophysics physical mechanisms have been used to explain such biological processes as the transmission of nerve impulses, the muscle departments at the University of Rochester The University of Rochester (UR) is a private, coeducational and nonsectarian research university located in Rochester, New York. The university is one of 62 elected members of the Association of American Universities. (N.Y.). "The only way to make so great a conceptual leap,' he says, "is to learn the basic physical and biological mechanisms [that] are responsible for the effects in the flies and use those concepts in experimental and theoretical evaluation of the human safety question.' Carstensen was one of several speakers addressing the issue at this week's Acoustical Society of America The Acoustical Society of America (ASA) is an international scientific society dedicated to increasing and diffusing the knowledge of acoustics and its practical applications. History
The ASA was instigated by Wallace Waterfall, Floyd Watson, and Vern Oliver Knudsen. meeting in Anaheim, Calif.
In the case of fruit fly larvae, researchers now suspect that the damage is done by a process called acoustic cavitation cavitation
Formation of vapour bubbles within a liquid at low-pressure regions that occur in places where the liquid has been accelerated to high velocities, as in the operation of centrifugal pumps, water turbines, and marine propellers. . When a sound wave passes through a fluid like that within a larva's body, pressure changes in the fluid cause any microscopic gas bubbles present to expand, then contract violently. This collapse creates a tremendous, highly localized shock wave and often raises the gas temperature enough to ionize i·on·ize
To dissociate atoms or molecules into electrically charged atoms or radicals.
ion·iz the gas and produce potentially toxic chemical products.
For acoustic cavitation to occur with the short pulses typically used in diagnostic ultrasound, bubbles about 1 micron across must be present in the fluid. Insect larvae are loaded with such bubbles because oxygen, for example, must be transported through the fluid between cells directly to various types of tissue. Although such microscopic bubbles are also likely to occur in mammals, says Carstensen, little is known about their location and the conditions under which they occur.
At intensities much higher than those used for diagnosis, ultrasound pulses clearly have an effect on human tissue. A machine called a lithotripter lithotripter /litho·trip·ter/ (lith´o-trip?ter) an instrument for crushing calculi in lithotripsy.
n. , for example, generates intense acoustic pulses to destroy kidney stones and gallstones Gallstones Definition
A gallstone is a solid crystal deposit that forms in the gallbladder, which is a pear-shaped organ that stores bile salts until they are needed to help digest fatty foods. (SN: 4/26/86, p.265). Each pulse, says physicist Lawrence A. Crum of the University of Mississippi The University of Mississippi, also known as Ole Miss, is a public, coeducational research university located in Oxford, Mississippi. Founded in 1848, the school is composed of the main campus in Oxford and three branch campuses located in Booneville, Tupelo, and Southaven. in University, probably produces an enormous amount of acoustic cavitation, which may play a role in destroying the stones and perhaps in damaging nearby tissue.
Similarly, experiments done by Leon A. Frizzell of the University of Illinois at Urbana-Champaign Early years: 1867-1880
The Morrill Act of 1862 granted each state in the United States a portion of land on which to establish a major public state university, one which could teach agriculture, mechanic arts, and military training, "without excluding other scientific show that focused, high-intensity ultrasound beams damage liver tissue. The nature of the damage indicates that acoustic cavitation rather than a heating effect is responsible for the destruction.
Stephen Daniels of the Oxford (England) Hyperbaric hyperbaric /hy·per·bar·ic/ (-bar´ik) having greater than normal pressure or weight; said of gases under greater than atmospheric pressure, or of a solution of greater specific gravity than another used as a reference standard. Group and his colleagues have shown that continuous-wave ultrasound also produces cavitation, even at the low intensities used in ultrasound devices designed for treating injuries. Unlike diagnostic ultrasound sources, therapeutic devices usually generate a continous sound wave rather than pulses. The researchers observed bubbles expanding and contracting in the blood of guinea pigs and detected evidence for the formation of reactive chemical species. The bubble oscillations oscillations See Cortical oscillations. may aid healing, says Crum, by promoting the transport of nutrients to cell membranes. These and other results suggest that gas bubbles of the appropriate size are present in mammalian tissue.
"From the little bit that we know about the distribution of gas bubbles in mammal tissue,' Carstensen says, "they're probably rare, and the damage that is done is highly localized.' For many types of diagnosis, the risk would be very small, he says, unless something as sensitive as an embryo is involved (SN: 2/18/84, p.102). However, such localized effects also mean that acoustic cavitation and its effects would be difficult to detect.
Crum is worried that people using diagnostic or therapeutic ultrasound devices are not being careful enough, especially in regulating the intensity and duration of exposures. "There's potential here for causing severe damage that you probably wouldn't see immediately,' he says. "It's incredible how unrestricted the use of ultrasound is.'
Nevertheless, says Carstensen, "it would be premature, at the present time, to set exposure limits on diagnostic ultrasound based on the effects of acoustic cavitation.' Although researchers have shown that acoustic cavitation may occur at the intensity levels generally used for diagnosis or healing, no one has yet connected this phenomenon with harm to human beings.
"We know the cause,' says Crum, "but we don't know the effect.' Much more research is needed, first, to study the mechanisms of bubble formation in tissue and, second, to link bubble expansion and collapse with specific biological effects such as tissue damage.