Description of Magnetic Stimulation
Magnetic nerve stimulation is a technique where neurons and nerve fibers are activated by electric current induced via time varying magnetic fields.
These devices work by first storing energy in the order of 100s of joules in internal capacitors charged to 1,000 volts and higher. At the time stimulation is needed, the energy stored within is discharged into a stimulating coil. The aim is to discharge all of the energy into the coil in 100µs or less. This discharge generates a pulse of current that peaks at about 5,000 amperes depending on the design of the system. In monophasic machines, the current then returns to zero and the discharge is over. In biphasic machines, the pulse cycles from positive to negative and then stops. Some devices have programmable settings allowing pulse shape to be controlled.
A current passing through a conductor generates a magnetic field. In order to amplify the magnetic field produced stimulating coils use 5-20 turns depending on size and inductance requirement. In the most common stimulating coil, two windings are used side by side to make a focal figure of eight shape. Windings can be circular or a variety of shapes, for example, to have a flat side in the form of the letter D, or have a curve for better coupling.
Owing to the high voltage and currents that pass through the stimulating coil, correct design of the stimulating coil becomes very important to ensure sufficient insulation and mechanical strength. In addition, resistance heating of the stimulating coil causes the stimulating coil to warm up over time. In all cases stimulating coils are equipped with temperature monitoring equipment to ensure the surface temperature of the stimulating coil remains within safe limits. Depending on the applications, some devices have built-in liquid or air cooling.
Time-varying magnetic pulses produced by stimulating coils create electric fields that permeate through space passing through skin and bone. Current is induced in any conductive tissue as a result of this field. Induced current close to the stimulating coil reaches approximately 1mA/cm² and is capable of stimulating neuromuscular tissue or altering its threshold for stimulation. Tiny stimulating coils are more focal but only reach 10mm stimulating depths with larger ones. Large coils are less focal but reach a greater depth of up to 50mm or 2". Small coils may be more focal but they do warm up far faster than larger ones.