Guidelines For Limiting Radiofrequency Exposure - Short Wave Diathermy - Safety Code 25

4. Typical RF Fields Close to Diathermy Devices

There is some variety in designs of short-wave diathermy devices, and probably more diversified designs will be introduced in the future, however, certain common features can be distinguished. A diathermy device consists of an applicator, RF generator, and control console. The applicator, also called an electrode, applies RF energy to a certain portion of a patient's body. The RF generator is usually housed in the control console, although there are designs where it is incorporated into the applicator. RF power is delivered from the generator to the applicator by two cables forming an unshielded two-wire transmission line.

Two basic types of electrodes (applicators) are in use, the capacitor-type and the inductor-type such as a "pancake" coil or diplode. The heating mechanism and profile are somewhat different for the two types⁽¹¹⁾. In the case of capacitive electrodes, tissue heating is basically due to the RF electric field, while for inductive electrodes (coils), heating occurs by eddy currents induced in tissue by the magnetic field. The other field is also present, but at a much lower intensity.

The stray fields close to the applicator, but outside the treatment area, are relatively strong and highly irregular(3,12). High intensity fields are also produced near the cables. The radiation field close to the short-wave diathermy applicator is of the near-field type, so that the intensities of the electric and magnetic field are not at a constant ratio of 377 as they are for the far-field. There is no simple formula to predict the intensities of stray fields, except that the higher the power setting the stronger they are, if all other parameters (e.g., electrode placement) remain unchanged.

The typical range of intensities of the stray electric and magnetic fields around the diathermy cables is shown in Figure 2.

Figure 2a - The range (shaded area) of the magnetic field intensities around cables of the short-wave diathermy device for various types of electrodes and typical power settings.

Figure 2b - The range (shaded area) of the electric field intensities around cables of the short-wave diathermy device for various types of electrodes and typical power settings.

The intensities of the fields around the electrodes depend on the electrode type and design, power setting, part of patient being treated, and placement of the electrode over the area being treated. The intensity decreases with distance away from the electrodes. The intensities are always highest in the plane of the applicator and are less above and below this plane for a given distance. However, disturbances to the stray field pattern can be introduced by large metal objects, e.g., a metal bed. In most treatment protocols, for properly placed electrodes, the intensities of the fields drop to below an equivalent power density of 1 mW/cm²at a distance of less than 1m(3,12). A significant departure was observed for the treatment configuration shown in Figure 3, where fields with an equivalent power density of 10 mW/cm²extended to 1 m from the electrodes.