Bioeffects

Ultrasound energy can give both desired and undesired thermal and mechanical bioeffects. This is also used in the clinic: thermal effects in equipment for heating up muscles and tendones in physiotherapy, and mechanical effects when crushing kidney and gallbladder stones during ultrasound surgery. Local heating is due to depositing energy. 

In imaging equipment, the thermal energy is biggest when using Doppler and color Doppler as the pulse length is biggest. The mechanical effects are due to sound pressure that are so big the tissue is ripped from each other (cavitation). This occurs easiest when short pulses are used (B- and M-mode), and maybe especially in M-mode as the beam does not move. Usually, the chance for local heating is bigger than the possibility for mechanical effects. Output level is regulated by requirements from different authorities, where the standard is the American Food and Drug Administration. Equipment that has been developed the last few years has either a thermal or mechanical index. The thermal index is an estimate of local heating in degrees of Celsius. It can be found in 3 variants: TIS (Thermal Index Soft tissue) for soft tissue, TIB (Thermal Index Bone) for bone, i.e. foster imaging, and TIC (Thermal Index Cranial) for bone right under the probe when imaging in the head. The mechanical index (MI), also called the cavitation index (CI), indicates the possibility for cavitation. It is based on an estimation of the cavitation threshold: MI = p_/ vf, where p_ is maximal negative pressure amplitude in MPa and f is frequency in MHz.

With the help of these indicators, it is possible for the user to choose exposure dose and evaluate risk. Generally, one expects that the tissue can be exposed to temperature increases up to 1.5°C for unlimited time, but the time must be reduced for higher temperature exposures. Maximum permitted mechanical index is 1.9.