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 tendons in physiotherapy, and mechanical effects when crushing kidney and gallbladder stones during ultrasound surgery. Local heating is due to deposition of energy. 
<p>
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 is so big that the tissue is torn apart (cavitation). This occurs most frequently when short pulses are used (B- and M-mode), and particularly in M-mode as the beam does not move. Usually, the chance for local heating is bigger than the possibility for mechanical effects. Output levels are regulated by requirements from different authorities, most notably the American Food and Drug Administration. Equipment that has been developed the last few years either has 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 bones, i.e. fetal 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 = <i>p_ / &radic;f</i>, where <i>p_</i> is maximal negative pressure amplitude in MPa and <i>f</i> is frequency in MHz.
<p>
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.