Section 8--clinical relevance. American Institute of Ultrasound in Medicine.

Abstract

his final section addresses the clinical implications of the nonthermal (mechanical) bioeffects of diagnostic ultrasound. As previously defined in Section 2, bioeffects are assumed to be mechanical when the temperature rise at the site of a biologic effect is < 1°C. An important source of mechanical bioeffects in biological tissues is believed to be acoustic cavitation. Acoustic cavitation may be defined as the interaction between a stabilized gas pocket (microbubbles) and an applied acoustic field, resulting in either transient (inertial) or stable cavitation. Gas bodies may occur naturally, as in the lung and intestine, may be induced from preexisting nuclei in tissues subjected to an appropriate acoustic pressure (renal effects of lithotripsy), or may be exogenously introduced as contrast agents. CLINICAL RELEVANCE: MECHANICAL BIOEFFECTS IN TISSUES KNOWN TO HAVE NATURALLY OCCURRING GAS BODIESFrom a clinical relevance perspective, the first obvious question is whether diagnostic ultrasound systems can generate specific ultrasound quantities known to reliably produce mechanical bioeffects in biological tissues. A number of investigators have theoretically suggested the generation of transient cavities in liquids by microsecond pulses of clinically relevant ultrasound (Flynn, 1982; Apfel, 1982), whereas others have experimentally demonstrated evidence for ultrasonically induced cavitation both in vitro and in vivo at peak acoustic negative pressures less than those found in some diagnostic ultrasound equipment (ter Haar and Daniels, 1981; Fowlkes and Crum, 1988;Roy et al, 1990;Holland et al, 1996). The second obvious question is whether there are tissues or tissue conditions in the human fetus, neonate, child, or adult that make them susceptible to these known mechanical bioeffects produced by diagnostic ultrasound systems. There are only two organ systems in humans known to have naturally occurring stabilized gas pockets-the lung and the intestine. This is true after birth only because in the human fetal state (as is true for all mammals), the lung and intestine are both fluid filled. Although to date, there are no clinical or experimental mechanical bioeffects data available for human lung or intestine (exclusive of incidental extracorporeal shockwave lithotripsy anecdotal data), a number of investigators have clearly demonstrated the occurrence of lung hemorrhage in a variety of mammalian models, as well as hemorrhage in mouse intestine from diagnostic ultrasound (Child et al, 1990;Zachary and O'Brien, 1995;Holland et al, 1996). Bioeffects in tissues with naturally occurring gas bodies are extensively reviewed in Section 4 together with a discussion of the theoretical basis for their occurrence and available experimental proof. SECTION 8-CLINICAL RELEVANCET Several additional questions can be posed if the answer to the previous two questions is in the affirmative. The first is whether such a mechanical bioeffect always occurs in susceptible tissue(s) when they are exposed to diagnostic ultrasound. Another qu...