Ultrasound energy can give both desired and undesired thermal and mechanical bioeffects. This is also used in the clinic: thermal effects can be used for heating 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. 
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In imaging equipment, the thermal energy is at its largest when using Doppler and color Doppler due to the long pulse length. The mechanical effects are due to sound pressure that is so large that the tissue is nearly 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 US Food and Drug Administration. Equipment today display either a thermal or a mechanical index. The thermal index is an estimate of local heating in degrees Celsius. It can be found in 3 variants: TIS (Thermal Index Soft tissue) for soft tissue, TIB (Thermal Index Bone) for bone, 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 estimate 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.
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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.