Therapy and treatment

Claims of beneficial effects of static or low-frequency magnetic fields in pain relief and treatment of inflammation are controversial. It is thought that broken bones may set more quickly if they are exposed to a magnetic field. Unfortunately, no plausible explanation of such effects has yet been advanced.

Pulsed magnetic fields can be used to induce electric fields and drive currents in conducting tissue. The effects of transcranial magnetic stimulation of the brain using trains of pulses with dB/dt ≈ 10³−10⁶ T s⁻¹ are under investigation, and there is evidence that it may be beneficial in the treatment of neurological and psychiatric conditions such as Parkinson’s disease and depression. The pulses can induce weak emfs (nV–μV) at a cellular level, but the effects on the scale of an organ are more significant, because the induced electric fields increase in proportion to the dimensions, the magnitude of the induced electric field around a circuit is

where l is the dimension of the circuit. Electric fields in excess of 200 V m⁻¹, needed to induce firing of neurons, are induced in circuits some millimetres or centimetres in size. Gradient coils can be used to help confine the induced electric stimulus to some region of the brain, or some other part of the body. Better established is the use of magnetic microparticles for hypothermia. Here the magnetic material in the bead should be conducting or exhibit hysteresis, so that it can be heated by eddy-current or hysteresis loss in an externally applied high-frequency field. If the beads accumulate in an area of interest, such as a tumour, heat can be applied locally to raise the temperature to about 45^◦C, thereby destroying the tumour.