Non-Thermal Effects of Ultrasound on Intact Animal Tissues
Investigations on the effects of ultrasound on intact animal tissues are complicated by mutual interaction between the ultrasound beam and the tissue. Most of the reported biological effects may be ascribed to either heat or cavitation. There remain a number of phenomena that are apparently due to other mechanisms although it is particularly difficult to completely exclude all possibility of cavitation. These effects include destructive changes in the central nervous system resulting in ‘focal’ brain lesions, and spinal cord injury resulting in paraplegia. Pulsed insonation of both liver and spinal cord produces vascular damage and this occurs more easily in the presence of hypoxia. There is evidence of dose accumulation in the production of these changes.
Stimulation of tissue regeneration by low intensity insonation rationalises a therapeutic use of ultrasound. There is evidence that this is a non-thermal effect. Occurrence of red cell stasis has relevance to the use of the energy form for measuring blood velocity since standing waves may easily occur in vivo.
Finally, the effects of ultrasound on developing chick embryos are reported. Continuous insonation with Doppler devices did not affect their subsequent development. When delivery of energy was pulsed, relatively long pulses (20 us) at a high p.r.f. (5000 s−1) produced perverted development if applied during the early stages of organogenesis. An intensity threshold for this effect was found between 10 and 25 W cm−1. Embryos at later stages of development were unaffected even by intensities as high as 100 W cm−2. The results indicate a large margin of safety for the dose-parameters used in current applications.
KeywordsChick Embryo Intact Tissue Standing Wave Field Cavitation Threshold Doppler Device
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