Technical and practical aspects of magnetic nerve stimulation.

Magnetic stimulators consist of a high-voltage (400 V to more than 3 kV), high-current (4 kA to more than 20 kA) discharge system producing a brief pulse of magnetic field (1-10 T lasting up to a few milliseconds). In order to ensure safety and reliability, care has to be taken in the construction of magnetic stimulators and especially their stimulating coils, which may come into contact with the patient, by the use of adequate mechanical and electrical insulation and the incorporation of sensors to monitor temperature, voltage, etc. Where possible, equipment should be manufactured to comply with safety standards such as UL504 and IEC601. The effectiveness of a magnetic stimulator depends on many parameters such as the maximization of the peak coil energy, fast magnetic field rise times, and good coil design. It is also important to choose a coil based on its intended clinical application and not simply on account of its high magnetic field strength. A small coil, producing a high surface magnetic field, is suitable in the stimulation of superficial nerves, whereas a larger coil, with a low magnetic field intensity, may well be more suitable for the stimulation of deep nerves. Double, figure-eight, or butterfly coils produce more localised induced currents allowing for more selective stimulation. The exposure of the brain to high magnetic field strengths can also be reduced by using larger coils.

Magnetic stimulation of the human brain and peripheral nervous system: an introduction and the results of an initial clinical evaluation.

This report describes a novel method of stimulating the motor cortex and deep peripheral nerves in humans. The technique, developed in the Department of Medical Physics of Sheffield University, uses a large pulse of magnetic field to induce currents within the body and is painless. The basic principles of magnetic stimulation are described, and the technique is compared with conventional electrical stimulation. Safety aspects are discussed with reference to established clinical electrical and magnetic procedures. The results of the first clinical study using magnetic stimulation are described and show clear central motor pathway slowing in multiple sclerosis patients.