Thermal effects of ultrasound, because tissues or water absorb the ultrasound energy with increase in temperature.
Cavitation is the formation, growth, and dynamic behavior of gas bubbles (e.g. microbubbles used as contrast agents) at high negative pressure. This dissolved gases come out of solution due to local heat caused by sound energy. This has been determined harmful at the level of the medical usage.
The ultrasoundsafety is based on two indices, the mechanical index (MI) and the thermal index (TI). The WFUMB guidelines state that ultrasound that produces temperature rises of less than 1.5°C may be used without reservation. They also state that ultrasonic exposure causing temperature rises of greater than 4°C for over 5 min should be considered potentially hazardous. This leaves a wide range of temperature increases which are within the capability of diagnostic ultrasound equipment to produce and for which no time limits are recommended. However, it has not been determined that medical ultrasound causes any adverse reaction or deleterious effect.
The American Institute of Ultrasound in Medicine states that as of 1982, no independently confirmed significant biologic effects had been observed in mammalian tissue below (medical usage) 100mW/cm2.
See also Ultrasound Regulations and Ultrasound Radiation Force.
The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair.----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- - Douglas Adams