What is the role of magnetic stimulation in motor evoked potentials (MEPs)?

Updated: Aug 20, 2019
  • Author: Jasvinder Chawla, MD, MBA; Chief Editor: Selim R Benbadis, MD  more...
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Magnetic stimulation of the nervous system can occur only in the setting of a rapidly changing magnetic field. Subjects exposed to a constant field strength (eg, during magnetic resonance imaging [MRI]) do not experience stimulation of nervous tissue. The intensity of the secondarily produced electrical field in nervous tissue (and of the stimulation) is related to the speed of change in magnetic field strength.

Formation of the magnetic pulse starts in the main unit of a magnetic stimulator, which contains a large bank of electrically charged heavy-duty capacitors. When triggered, these capacitors rapidly discharge through a cable into the hand-held coil, producing a brief burst of high current (up to 4000V or several thousand amperes [A]). The current that moves through the hand-held coil produces a large magnetic field (1-3T) that lasts only 50-200 milliseconds.

The stimulating coil consists of tightly wound and well-insulated copper coil. As a result of the brief magnetic field induced from the coil, a secondary electrical field that circulates in the opposite direction to the magnetic field is produced. The strength of the electrical field is related in part to the first derivative of the magnetic flux over time: the more rapid the change in the magnetic field, the stronger the intensity of the secondary electrical field and of nervous stimulation.

Most commercially available stimulators can produce stimulations at a rate as high as 5Hz, although some can produce repetitive stimulations as high as 50Hz. A big advantage of magnetic stimulation over electrical stimulation is magnetic stimulation's ability to penetrate tissues regardless of electrical resistance. The drop-off is essentially the same for air, bone, fat, muscle, and saline.

The magnitude, waveform, and rise time of the magnetic field are important parameters of the stimulation. The diameter, shape, and thickness of the coil are also important. Because of these multiple variables, the measurement of intensity of stimulation usually is expressed as a percentage of the maximal output of the stimulator.

In choosing coils, the tradeoff is between strength and focality of stimulation. Coil diameter may vary between 5cm and 15cm. Large-diameter coils stimulate over a wider area but are less focal than small-diameter coils. With the round coils, the highest intensity electric field is measured at the edges of the coil, with lower intensities in the center.

To obtain more focality, the use of a butterfly coil (also called "figure of 8" coil) is recommended. The focality of these coils makes them particularly suitable for use in mapping out the upper limb and hand musculature.

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