Although ultrasonic energy is relatively thin at the molecular level, it can cause irreversible change to biological tissue after it is accumulated as heat. Thermal treatment using high-intensity focused ultrasound (HIFU) is such an application. Biological tissue converts ultrasonic energy to heat due to its shear viscosity. Because its conversion ratio, i.e., absorption, is approximately proportional to the ultrasonic frequency, the frequency has to be chosen according to the propagation distance in tissue to the target tissue to be treated. By choosing a HIFU exposure duration within the time in which the heat conduction has not taken place significantly, e.g., within a few seconds for focused ultrasound at a few MHz with an f-number of 1, the spatial selectivity of focused ultrasound can be utilized for the treatment. To suppress overheating outside the target tissue, a long cooling time between each consecutive HIFU exposure, in the order of the time constant of blood perfusion ranging from 1 to several minutes, is necessary. This significantly lowers the throughput of HIFU treatment. These are explained in this paper based on the principles of ultrasound and heat transfer. The imaging methods for aiming HIFU and monitoring the treatment as well as research to enhance the treatment throughput are also explained. They are explained without relying too much on equations, which are only used as a supplement.
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