Ultrasonic exposures localized to regions smaller than a wavelength minimize the thermal mechanism, but promote nonthermal mechanisms of ultrasonic bioeffects. Microsonation experiments with vibrating needles or wires exploit this situation for the study of nonthermal mechanisms such as acoustic microstreaming flow. Shear stress in microstreaming flow, which is in excess of critical values for biomembranes, leads to cell lysis. Plane wave exposure of small bodies of gas also yields localized exposure, with further amplification of nonthermal mechanisms by resonance activation of oscillation. Gas body activation in vitro causes gathering of suspended cells by radiation forces, aggregation, cellular effects and lysis by microstreaming. When suitable gas bodies are present, these effects may occur at levels below the threshold for ultrasonic cavitation. In vivo, gas body activation generates intracellular microstreaming in Elodea leaves and disrupts the cells for super-critical shear stress levels. Similar phenomena seem to account for cell death, growth and mitotic index reductions in other plant tissues, and developmental abnormalities and delayed death in fruit flies. Only fragmentary and equivocal evidence presently exists on the medically relevant question of whether such subthreshold cavitation-like activity and bioeffects occur in vertebrates.

A review of the ultrasonic bioeffects of microsonation, gas-body activation, and related cavitation-like phenomena.

The larvae of Drosophila melanogaster were exposed to repetitive, microsecond length pulses of 2 MHz ultrasound. The observed effects included killing of larvae and delayed lethality in the pupal stage. The effects depended strongly upon the temporal peak intensity with killing observed at spatial. temporal peak intensities above 10 W/cm 2 . In fact, the observations suggest that there may be a threshold for these effects at approximately this intensity. Temporal average intensity appears to be a poor predictor for these biological effects. Marked decreases in survival rates were observed at spatial, temporal average intensities as low as 3 mW/cm 2 .

Effects of ultrasound on drosophila: III. Exposure of larvae to low-temporal-average-intensity, pulsed irradation

5 – Effects and Mechanisms

Premature ovulation after ovarian ultrasonography.

Drosophila larvae are damaged by exposures to low temporal average intensity pulsed ultrasound with peak intensities of 10–20 W/cm 2 (2 MHz). Eggs of the same organism are affected by exposures to 3 W/cm 2 c.w. ultrasound. This experiment shows that eggs become sensitive to high peak intensity (50–100 W/cm 2 ) pulsed ultrasound only shortly before hatching. At this age the larvae have formed and have taken air into the respiratory system. This observation supports the postulate that the sites of action of the ultrasound are the small stabilized gas bodies within the organisms.

Effects of ultrasound on Drosophila—IV. Pulsed exposures of eggs

Eggs of the fruit fly, Drosophila melanogaster (wild type), were exposed to 1 MHz traveling and standing wave ultrasound at intensities up to 5 W/cm 2 . Traveling waves at 3 W/cm 2 and “1 W/cm 2 standing waves” killed about one third of the exposed population. Death occurred at the egg stage of development. This study was unable to confirm earlier reports of delayed lethality as a result of exposure to ultrasound in that fraction of the population which survived beyond the egg stage.

Effects of ultrasound on drosophila—I killing of eggs exposed to traveling and standing wave fields

Hamster fibrosarcomas were exposed to 1.06 MHz ultrasound at intensities of 90 W/cm 2 in a pulse mode (0.1 sec on, 0.9 sec off) to avoid excessive heating. Tumor temperatures ranged from 19 to 30°C during these exposures. The treatment had no apparent effect on the rate of growth of the primary tumor or on the rate of metastasis.

K. Smachlo

Papers

Effects of ultrasound on drosophila—I killing of eggs exposed to traveling and standing wave fields

Ultrasonic treatment of tumors-III high intensity, low frequency exposures