Introduction
This topic covers how alternating current and electromagnetic fields act on living tissue, and how they are used in medicine: high-frequency surgery, diathermy, magnetic stimulation and magnetic resonance imaging. The central ideas are that tissue properties depend on frequency, that alternating current produces heat in tissue, and that the energy absorbed from a field determines its biological effect.
Frequency and the response of tissue
At low frequency, current flows mainly through the extracellular fluid, because the cell membranes act as insulators. As frequency rises, the membranes, which behave as capacitors, pass more current, so the impedance of tissue falls and current also flows through the cells. At low frequencies, up to a few kilohertz, the dominant effect is stimulation of nerves and muscles. At radiofrequency and above, the dominant effect is heating.
Heating by alternating current
Power is dissipated in tissue as heat in proportion to its electrical conductivity and to the square of the field strength. The power absorbed per unit volume is:
where \sigma is the conductivity and E the effective field strength. The specific absorption rate (SAR) is the power absorbed per unit mass, and it is used to set safety limits for radiation exposure:
Here \rho is the tissue density. Tissues with high water and electrolyte content, such as muscle and blood, absorb more energy than fat.