Abstract
Although ultrasonic energy is relatively thin at the molecular level, it can cause irreversible changes to biological tissue after it accumulates as heat. Thermal treatment using high-intensity focused ultrasound (HIFU) is one of its applications. The scientific and engineering basis of this application is explained in this article. Because the ratio of biological tissue to convert ultrasonic energy to heat, i.e. ultrasonic absorption, is approximately proportional to ultrasonic frequency, this frequency must be chosen in accordance with the propagation distance in tissue to the target tissue to be treated. By choosing the HIFU exposure duration to be within the time in which no significant heat conduction takes place, the spatial selectivity of focused ultrasound can be utilized for medical treatment. To suppress the overheating of the intervening healthy tissue, a long pause between consecutive HIFU exposures is necessary to allow blood perfusion to cool the intervening tissue: this significantly lowers the throughput of HIFU treatment. These are explained in terms of the basic principles whenever possible. Imaging methods for use in HIFU and for monitoring the treatment as well as research to enhance the treatment throughput are also explained.
