Electromagnetic radiation: Shortwave diathermy in rehabilitative medicine

In physiotherapy settings, low-frequency electromagnetic (EM) waves are used for diagnosis and therapy in diathermy, which is the use of electricity or electromagnetic energy to produce heat. One of the major types of diathermy is shortwave, which is considered a non-ionizing form of radiation. These waves can speed up the healing of tissues by providing deep heat to a large area. On the microscopic level, three basic mechanisms underlie the interactions: the displacement of electrons and ions, orientation of existing dipoles, and polarization of atoms and molecules.

Figure 1. Electric dipole (p) in an uniform electric field.

The two modalities, the condenser and the inductive coil methods, are depicted in Figure 2. For the former, the patient becomes part of the circuit and is placed between the two electrodes, which act like the plates of a capacitor. Thus, the tissue that provide the most resistance to current flow develops the most heat. For the latter, the patient is circumferentially wrapped around with a coil or has an electrode with an insulated coiled within placed onto the target tissue. Heating is due to the eddy currents (small circular electric fields) generated by the alternating electric current through the coil. In this case, the patient is not part of the circuit but a “parallel” circuit where the current will flow through the tissues that provide the least amount of resistance.

Figure 2. Circuit depiction of the condenser (left) and the inductive coil (right) method in shortwave diathermy

When an EM wave interacts with matter, there is absorption, and each type of EM wave has its own characteristic interaction. Ionizing radiations like X and gamma rays are dangerous because such high-energy photons cause an electron to be ejected from its orbital. However, concerns raised about exposure to non-ionizing radiation have prompted governments to set guidelines for exposure, which is quantified by the specific absorption rate (SAR), the rate at which energy is imparted to a given mass of tissue measured in watts per kilogram:

Equation 1. Specific Absorption Rate (SAR)

where σ is tissue conductivity, ρ is tissue density, t is time, T is temperature, E is electric field, and c is specific heat.