The following is excerpt form the paper: "Assessing the Risks for Modern Diagnostic Ultrasound Imaging" by Dr. William D. O'Brien , Jr., Professor, Bioacoustics Research Laboratory, Department of Electrical and Computer Engineering, University of Illinois.

" ...... In the early to mid 1970s, there was great uncertainty with respect to the safety of ultrasound as discussed above and what authority and role the US Food and Drug Administration (FDA) would take in terms of regulating diagnostic ultrasound. Regulatory control of diagnostic ultrasound in the United States can be traced to passage of the 1976 Medical Device Amendments to the Food, Drug and Cosmetic Act, but for the several years prior to its passage, the FDA coordinated a classification scheme for all medical devices. There was apprehension among the public, patients, physicians, sonographers, basic scientists, manufacturers, and the government. One of the fundamental difficulties was the lack of an accurate and precise procedure to quantify diagnostic ultrasound equipment outputs. Because of these difficulties, the output levels from diagnostic ultrasound equipment were not well characterized, and human exposure levels could not be compared to results from laboratory experimental studies.

In response to this uncertainty, and the lack of a suitable measurement scheme, the American Institute of Ultrasound in Medicine (AIUM), a medical and scientific professional society, and the National Electrical Manufacturers Association (NEMA), a trade organization that represented many of the ultrasound manufacturers, joined efforts in 1976 to develop a voluntary standard that would assure that sufficient information on the characteristics of diagnostic ultrasound equipment was supplied to allow medical personnel to make informed judgments regarding the application of this equipment to patients. The Safety Standard for Diagnostic Ultrasound Equipment138) was developed over a five-year period and set forth precise definitions, output measurement procedures and labeling requirements related to those characteristics of ultrasound equipment that were believed at that time to pertain to patient exposure and safety. The voluntary standard's labeling requirements called for manufacturers to make available (publicize) to the ultrasound community the maximum values of the following ultrasonic quantities: ultrasonic power; spatial peak, temporal average intensity (ISPTA); and spatial peak, pulse average intensity (ISPPA). The labeling requirements were based on the philosophy that there was a potential risk from diagnostic ultrasound exposure and included those quantities whose magnitudes were known or believed to be related to actual damage or to risk of damage to biological tissues; they were the quantities most often reported in the basic science literature to relate the strength of ultrasound to the production of biological effects in laboratory experimental studies. The voluntary standard did not specify upper limits.

In the early 1980s, the assessment of diagnostic ultrasound risk was addressed by two major activities. One of the activities was sponsored by the US National Institutes of Health consensus development conference processes139) by convening an expert panel of physicians, basic scientists, epidemiologists, nurses, educators, and sonographers to provide answers to specific questions related to safety and efficacy of diagnostic ultrasound in obstetric practice. The document indicated that the increasing use of ultrasound during pregnancy is safe and effective for 28 medical conditions. This was, perhaps, the first time that the issue of diagnostic ultrasound efficacy was critically reviewed. Also, this process recommended against routine scanning of the embryo and fetus. Further, it was suggested that while diagnostic ultrasound does not appear to be associated with any known hazards, investigators should continue to evaluate risks.

The other activity was conducted under the auspices of the National Council on Radiation Protection and Measurements.140) The document rigorously covered the basic physics of ultrasound with an emphasis on medical ultrasound fields and on the quantification of various ultrasonic field quantities to which humans were exposed during the course of an ultrasound examination. Also included were mechanisms by which ultrasound interacts with biological material and effects caused by ultrasound on biological materials such as plants, animals, and in vitro systems. Finally, this document set forth a number of recommendations that fell into the categories of research needs, industrial practices, education, and exposure criteria. It is interesting that a number of the recommendations were consistent with those put forth by the Workshop on the Interaction of Ultrasound and Biological Tissues104) a decade earlier.

When the FDA initiated the regulation of diagnostic ultrasound equipment in the mid 1980s,141) it set application-specific intensity limits in their 1985 "510(k) premarket notification" which manufactures could not exceed (see Table I). This notification is used by the FDA to determine if the new devices are substantially equivalent, in safety and effectiveness, to diagnostic ultrasound devices on the market prior to May 28,1976, the date when the Medical Device Amendments were enacted. The exposure quantities required by FDA were, in part, similar to those in the Safety Standard for Diagnostic Ultrasound Equipment voluntary standard's labeling requirements. 138) However, the limits were not based on safety considerations but rather on the maximum output limits of known diagnostic ultrasound equipment at the time when the Medical Devices Amendments were enacted, in May, 1976; hence the term pre-amendments levels.

To emphasize the date-base regulation approach as opposed to the safety- and efficacy-base regulation approach of the FDA, the American Institute of Ultrasound in Medicine notified142) the FDA in mid 1986 that there existed prior to May 28, 1976 at least two diagnostic ultrasound devices (pre-enactment ultrasound devices) with intensity levels greater than those listed in Table I. In early 1987, the FDA updated their diagnostic ultrasound guidance to higher intensity levels143) to those listed in Table II. The date- based regulation approach has been criticized by technical, scientific, and medical professionals, as well as by the diagnostic ultrasound industry144) because of the implication that these arbitrary limits are safety based, and because they could limit future clinical benefits by preventing the development of more advanced diagnostic ultrasound systems, and hence greater clinical benefit, that may require higher output levels. Further, it must be recognized that limited diagnostic ultrasound capabilities may, in fact, be responsible for greater risk to the patient due to either an inadequate diagnosis, or to the use of an additional diagnostic procedure with a defined risk ......."

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