Bioeffects in echocardiography.

Two mechanisms have been identified through which ultrasound as it is used clinically could produce biologically significant effects. One is heating that results from the absorption of ultrasonic energy by tissues. The other is cavitation, the ultrasonic activation of gas bodies including the potentially violent collapse of small gas bodies in or near tissue that is sometimes called transient or inertial cavitation. The heart, itself, is well perfused and the likelihood of significant heating of the heart tissues in the most extreme conditions known today is negligible. Lung also appears to be relatively immune to heating under diagnostic exposure conditions. In normal echocardiographic procedures, the only tissues that need serious consideration are the ribs. Under extreme conditions, ultrasonic heating of the bone might be as great as 6 degrees C. Nonthermal action of ultrasound has been demonstrated to cause lung hemorrhage at pressure levels on the order of 1 MPa. Although many diagnostic devices produce focal pressures greater than this amount, it appears unlikely that hemorrhage will occur in normal echocardiographic applications. Under certain conditions, pulsed ultrasound can either stimulate or modify the contraction of the heart but the exposures required are not used in normal echocardiographic applications. Since specific devices have been identified who's outputs approach levels required to produce thermal and nonthermal effects, the user should be aware of potential biological effects, particularly in pediatric or obstetric applications, as output levels increase.