Ultrasoundis a form of vibrational wave. It can transmit harmlessly through livingtissues, and this makes it possible to use an extracorporeal source ofultrasound for therapeutic purposes. If ultrasound beams are focused andsufficient ultrasonic energy is concentrated within volume while they propagatethrough tissues, the temperature in the focal region may be raised to levels atwhich the tumors are cooked, resulting in tissue ablation. This process occurswithout any damage to surrounding or overlying tissues, and the tissue ablationtechnique that employs such beams is known interchangeably as high intensityfocused ultrasound (HIFU).

HIFU is adifferent therapy from conventional hyperthermia, which has been used as anadjuvant to radiotherapy and chemotherapy for cancer treatment since the 1980s.The purpose of hyperthermia to raise the temperature of the tumor from 37℃ to 42-45℃, and tomaintain uniform temperature distributions in a narrow therapeutic range for 60minutes. However, the temperature distributions induced in vivo are usuallynon-uniform because of tissue cooling by blood flow, and it is extremelydifficult to avoid local cold spots that do not reach the necessary therapeutictemperature level. With HIFU, the temperature within the focal zone is rapidlyraised to temperatures between 56℃ to 90℃ and is held for one second. The rapiddeposition of HIFU thermal energy causes a peak temperature rise that isunaffected by blood flow cooling. Therefore, it can avoid the problem ofnon-uniform temperature distribution in hyperthermia, and there is no need forinserting thermocouple probes into the targeted tumor to describe temperaturedistribution during HIFU procedures.