Guidelines and recommendations for safe use of Doppler ultrasound in perinatal applications

Abstract

Technological development has led to significant improvements in ultrasonographic capabilities in recent years, and this has been accompanied by increases in acoustic output. Meanwhile, there is a developing trend to use ultrasound at early stages of pregnancy when the developing embryo is known to be highly sensitive to damage by physical agents. The advent of pulsed spectral Doppler and color flow imaging has revolutionized perinatal applications. Doppler ultrasound has become widely accepted as a valuable diagnostic tool in obstetric medicine, where it has particular benefits for high-risk pregnancies. The benefits of Doppler screening are less well established. United States Food and Drug Administration (FDA) regulations now provide an option whereby equipment that provides a form of output display can be used to apply substantially higher acoustic output to the embryo or fetus than equipment approved for use under application-specific intensity limits. The Output Display Standard recently adopted by the FDA, in the USA, encourages self-regulation of acoustic exposure by the ultrasound user, on the basis of assumed knowledge of the implications of biophysical interactions. When modern sophisticated equipment is used at maximum operating settings for Doppler examinations, the acoustic outputs are sufficient to produce obvious biological effects, e.g. significant temperature increase in tissue or visible motion of particles due to radiation pressure streaming effects. The risk of inducing thermal effects is greater in the second and third trimesters, when fetal bone is intercepted by the ultrasound beam and significant temperature increase can occur in the fetal brain. Non-thermal bioeffects may be more significant in early gestation, when the relatively loosely tethered embryonic tissues are exposed to an ultrasound beam in a liquid path. The likelihood of producing cavitation-type non-thermal effects is enhanced by the presence in the sound-field of gas-encapsulated echo-contrast media. To ensure the continued safe use of ultrasound in obstetrics, it is important that international ultrasound organizations, such as the International Perinatal Doppler Society, issue advice to members to allow sensible assessment of risk : benefit and the practical implementation of the ALARA (as low as reasonably achievable) principle.