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Introduction
Microwave and shortwave radio frequency radiation is used clinically to heat tissue situated deep in the body and minimize the coincidental rise in skin temperature seen with other forms of therapeutic heating. Microwave radiation is defined as that with a frequency of 300 MHz-300 GHz, which lies on the electromagnetic spectrum between radio frequency and infrared radiation. Microwave diathermy units operate at a frequency higher than that used for shortwave diathermy and this confers significant operational advantages. Microwave radiation is radiated as a beam from an antenna and absorbed by water-rich tissues 7000 times more effectively than shortwave radio frequency energy. The latter must be coupled to the patient by a capacitor or an inductor, whilst microwave energy is radiated as a beam from an antenna. A microwave beam can be narrowed to allow small areas to be treated accurately, is highly directional, and can be aligned rapidly.
Equipment
Microwave diathermy units and domestic microwave ovens usually operate at the internationally agreed frequency of 2450 MHz (2.45 GHz). Some clinical devices, particularly in the United States, emit radiation with a frequency of 915 MHz. Although these are less common in Europe, energy at this lower frequency is absorbed more efficiently and uniformly within the tissues and lies close to 750 MHz, the optimum frequency for therapeutic heating. The power output of therapeutic microwave units can approach 250W although the maximum in practice is limited by the design of the antenna. Small antennae and reflectors are rarely capable of radiating safely more than 25 W and protective circuitry within the generator prevents this limit being exceeded. Domestic microwave ovens often deliver 600-1000 W. Microwave energy is generated by an electronic device known as a magnetron. The magnetron was invented in 1938, but did not find civilian application until 1945 when the classified military role in airborne radar became less important. Some apparatus is designed to emit a pulsed output that causes less heating and should promote non-thermal interactions between electromagnetic energy and the tissues.
Microwave diathermy units and domestic microwave ovens usually operate at the internationally agreed frequency of 2450 MHz (2.45 GHz). Some clinical devices, particularly in the United States, emit radiation with a frequency of 915 MHz. Although these are less common in Europe, energy at this lower frequency is absorbed more efficiently and uniformly within the tissues and lies close to 750 MHz, the optimum frequency for therapeutic heating. The power output of therapeutic microwave units can approach 250W although the maximum in practice is limited by the design of the antenna. Small antennae and reflectors are rarely capable of radiating safely more than 25 W and protective circuitry within the generator prevents this limit being exceeded. Domestic microwave ovens often deliver 600-1000 W. Microwave energy is generated by an electronic device known as a magnetron. The magnetron was invented in 1938, but did not find civilian application until 1945 when the classified military role in airborne radar became less important. Some apparatus is designed to emit a pulsed output that causes less heating and should promote non-thermal interactions between electromagnetic energy and the tissues.
