The biological effects of ultrasound refer to the potential adverse effects the imaging modality has on human tissue. These are primarily via two main mechanisms: thermal and mechanical. Despite this, ultrasound has a remarkable record for patient safety with no significant adverse bioeffects reported in the literature.

Thermal effects

Due to the law of the conservation of energy, all of the sound energy attenuated by tissues must be converted to other forms of energy. The majority of this is turned into heat. As such, it is possible for ultrasound to raise tissue temperature by up to 1.5°C. For sensitive tissues (eg. fetal) this rise in temperature may have deleterious effects if present for an extended period of time.

The thermal index (TI), is the ratio of the power produced by the transducer to the power required to raise a tissue in the beam by 1°C. 

Mechanical effects

The mechanical bioeffect of ultrasound refers to damage caused by the actual oscillation of the sound wave on tissue. The most common is referred to as cavitation and is caused by the oscillation of small gas bubbles within the ultrasound field. In certain circumstances, these bubbles may grow in size or collapse generating very high energies to adjacent tissue. This can increase tissue temperature by more than 1000°C.

The mechanical index (MI) is a value that attempts to quantify the likelihood of cavitation by an ultrasound beam. This value is readily displayed on the ultrasound screen and it is recommended that it is kept below 1.9 to remain safe.