THE SENSITIVITY OF BIOLOGICAL TISSUE TO ULTRASOUND

Mammalian tissues have differing sensitivities to damage by physical a gents such as ultrasound. This article evaluates the scientific data i n terms of known physical mechanisms of interaction and the impact on pre-and postnatal tissues. Actively dividing cells of the embryonic an d fetal central nervous system are most readily disturbed. As a diagno stic ultrasound beam envelopes a small volume of tissue, it is possibl e that the effects of mild disturbance may not be detected unless majo r neural pathways are involved. There is evidence that ultrasound can be detected by the central nervous system; however, this does not nece ssarily imply that the bioeffect is hazardous to the fetus. Biological ly significant temperature increases can occur at or near to bone in t he fetus from the second trimester, if the beam is held stationary for more than 30 s in some pulsed Doppler applications. In this way, sens ory organs that are encased in bone may be susceptible to heating by c onduction. Reports in animals and humans of retarded growth and develo pment following frequent exposures to diagnostic ultrasound, in the ab sence of significant heating, are difficult to explain from the curren t knowledge of ultrasound mechanisms. There is no evidence of cavitati on effects occurring in the soft tissues of the fetus when exposed to diagnostic ultrasound; however, the possibility exists that such effec ts may be enhanced by the introduction of echo-contrast agents. (C) 19 97 World Federation for Ultrasound in Medicine & Biology.