An extended commentary on "Models and regulatory considerations for transient temperature rise during diagnostic ultrasound pulses" by Herman and Harris (2002)

Arrhenius thermodynamics and birth defects: chemical teratogen synergy. Untested, testable, and projected relevance

This article addresses the issue of hyperthermia-induced birth defects with an accompanying additional teratogen, be it a chemical or a physical agent (i.e., a simultaneous "combinational" exposure to two teratogens, one of which is hyperthermia). Hyperthermia per se is a recognized human and animal teratogen. An excellent example of such combinational exposures is an epileptic woman who becomes pregnant while taking valproic acid (VPA) to control seizures. VPA is a recognized chemical teratogen, and fever (hyperthermia) is not an uncommon event during pregnancy. While VPA also may occasionally induce fever as a side effect, we are concerned here with fevers arising from other, unrelated causes. There is a small but internally consistent literature on these combinational-teratogen exposures involving hyperthermia plus a chemical teratogen; in each instance, the effect level has been observed to be synergistically elevated above levels induced by the separate teratogenic components. The data were empirical. The observed synergy is, however, consistent with Arrhenius thermodynamics, a well-known chemical rate equation. The need for information about combinational teratogen exposures is acute; fever is a common occurrence during pregnancy; and there are many instances whereby there is also the simultaneous presence of some other teratogen(s). Given that the rate of autism spectrum disorders in the United States was recently presented as 1 in 88 births, it seems reasonable to suspect that such combinational regimens are much more prevalent than previously thought. Our hypothesis is that synergistic birth defect levels from combinational regimens are consistent with Arrhenius thermodynamics.

The risk of exposure to diagnostic ultrasound in postnatal subjects: thermal effects

This review evaluates the thermal mechanism for ultrasound-induced biological effects in postnatal subjects. The focus is the evaluation of damage versus temperature increase. A view of ultrasound-induced temperature increase is presented, based on thermodynamic Arrhenius analyses. The hyperthermia and other literature revealed data that allowed for an estimate of a temperature increase threshold of tissue damage for very short exposure times. This evaluation yielded an exposure time extension of the 1997 American Institute of Ultrasound in Medicine Conclusions Regarding Heat statement (American Institute of Ultrasound in Medicine, Laurel, MD) to 0.1 second for nonfetal tissue, where, at this exposure time, the temperature increase threshold of tissue damage was estimated to be about 18 degrees C. The output display standard was also evaluated for soft tissue and bone cases, and it was concluded that the current thermal indices could be improved to reduce the deviations and scatter of computed maximum temperature rises.

Hemorrhage near fetal rat bone exposed to pulsed ultrasound

Ultrasound-induced hemorrhage near the fetal rat skull was investigated to determine if the damage could be correlated with temporal-average intensity. A 0.92-MHz f/1 spherically focused transducer (5.1-cm focal length) was used to expose the skull of 18- to 19-day gestation exteriorized Sprague-Dawley rat fetuses (n = 197). There were four ultrasound-exposed groups (n = 36 each), one sham exposed group (n = 36) and one cage control group (n = 17). Three of the ultrasound-exposed groups had the same peak compressional (10 MPa)/peak rarefactional (6.7 MPa) pressure but different spatial-peak temporal-average intensities (I(TA)) of 1.9, 4.7 and 9.4 W/cm(2); the pulse repetition frequency (PRF) was varied (100, 250 and 500 Hz, respectively). The fourth ultrasound-exposed group had a peak compressional (6.7 MPa)/peak rarefactional (5.0 MPa) pressure and corresponding I(TA) of 4.6 W/cm(2); PRF was 500 Hz. Hemorrhage occurrence increased slightly with increasing I(TA), as well as peak rarefactional pressure and PRF, but the hemorrhage area did not correlate with any of the exposure parameters.

Ultrasound-biophysics mechanisms

Ultrasonic biophysics is the study of mechanisms responsible for how ultrasound and biological materials interact. Ultrasound-induced bioeffect or risk studies focus on issues related to the effects of ultrasound on biological materials. On the other hand, when biological materials affect the ultrasonic wave, this can be viewed as the basis for diagnostic ultrasound. Thus, an understanding of the interaction of ultrasound with tissue provides the scientific basis for image production and risk assessment. Relative to the bioeffect or risk studies, that is, the biophysical mechanisms by which ultrasound affects biological materials, ultrasound-induced bioeffects are generally separated into thermal and non-thermal mechanisms. Ultrasonic dosimetry is concerned with the quantitative determination of ultrasonic energy interaction with biological materials. Whenever ultrasonic energy is propagated into an attenuating material such as tissue, the amplitude of the wave decreases with distance. This attenuation is due to either absorption or scattering. Absorption is a mechanism that represents that portion of ultrasonic wave that is converted into heat, and scattering can be thought of as that portion of the wave, which changes direction. Because the medium can absorb energy to produce heat, a temperature rise may occur as long as the rate of heat production is greater than the rate of heat removal. Current interest with thermally mediated ultrasound-induced bioeffects has focused on the thermal isoeffect concept. The non-thermal mechanism that has received the most attention is acoustically generated cavitation wherein ultrasonic energy by cavitation bubbles is concentrated. Acoustic cavitation, in a broad sense, refers to ultrasonically induced bubble activity occurring in a biological material that contains pre-existing gaseous inclusions. Cavitation-related mechanisms include radiation force, microstreaming, shock waves, free radicals, microjets and strain. It is more challenging to deduce the causes of mechanical effects in tissues that do not contain gas bodies. These ultrasonic biophysics mechanisms will be discussed in the context of diagnostic ultrasound exposure risk concerns.