Ultrasound and electromagnetic radiation in hyperthermia--a historical perspective.

An account is given of the historical developments of ultrashortwave, microwave and ultrasonic diathermy techniques, for which bio-physical and bio-engineering principles were developed mainly after the Second World War. Topics included in this survey are: electric and acoustic properties of tissues, penetration depth and focusing ability for ultrasound (US) and electro-magnetic (EM) radiations, heat deposition patterns in complex tissue layers, hot-spot formation, temperature elevations in local areas of elevated heat deposition (SAR), and thermoregulatory responses. The present possibilities of placing spatially defined increases in temperature in tissue by either ultrasound or radio frequency radiation are summarized.