Main Effect Of The Propagation Of Ultrasonic Waves In Human Tissues
The most obvious effect is heating. Since a significant portion of the energy is absorbed and converted into heat during ultrasound propagation, this could potentially lead to a temperature increase. As the wave intensity is increased, temperature rises and if the temperature becomes higher than 38.5″C, adverse biological effects may occur. However, no harmful effects have been observed for temperatures lower than 41″C. Since heating is strictly caused by the wave intensity, a pulsed transmission with a low duty cycle can potentially reduce this effect of a factor proportional to the duration of the duty cycle.
Another important effect caused by ultrasound wave propagation is the so-called cavitation which denotes the behavior of gas bubbles within an acoustic field. Pressure variations of the ultrasound wave cause bubbles present in the propagation medium to contract and expand. For large pressure variations, the bubble size drastically increases, reaching an expansion peak when pressure is minimum and then collapsing when pressure reaches its peak. During this process, the internal pressure and temperature in the bubble can reach high values causing serious biological effects and damaging the objects located in closest proximity. It can be shown that cavitation is a frequency-dependent phenomenon. Since higher frequencies lead to shorter pressure oscillations, the time for bubble expansion is restricted and the cavitation effect tends to disappear. Pulsed transmissions may also reduce the cavitation effects. In fact, during the off period, the bubbles assume again their initial sizes without imploding. However, these destructive effects are not seen for low pulse intensities.
Unfortunately, data collected on bioeffects of ultrasounds are frequently inconsistent and controversial. However, based on the medical ultrasound experiences of the last decades, no dangerous bioeffects have been observed as long as the energy provided to the tissues is less than 50 J/cm2. Therefore, ultrasound communications in tissues at low transmission pressure levels, and consequently low transmission power levels, are not expected to cause any harmful bioeffects. We thus believe that ultrasonic communications can represent a feasible alternative to traditional electromagnetic RF communications.