Magnetic field strength and reproducibility of neodymium magnets useful for transcranial static magnetic field stimulation of the human cortex.

OBJECTIVE The application of transcranial static magnetic field stimulation (tSMS) in humans reduces the excitability of the motor cortex for a few minutes after the end of stimulation. However, when tSMS is applied in humans, the cortex is at least 2 cm away, so most of the strength of the magnetic field will not reach the target. The main objective of the study was to measure the strength and reproducibility of static magnetic fields produced by commercial neodymium magnets. METHODS We measured the strength and reproducibility of static magnetic fields produced by four different types of neodymium cylindrical magnets using a magnetic field-to-voltage transducer. RESULTS Magnetic field strength depended on magnet size. At distances <1.5 cm, the magnetic field strength was affected by the presence of central holes (potentially useful for recording electroencephalograms). At distances >1.5 cm, the measurements made on the cylinder axis and 1.5 cm off the axis were comparable. The reproducibility of the results (i.e., the consistency of the field strength across magnets of the same size) was very high. CONCLUSIONS These measurements offer a quantitative empirical reference for developing devices useful for tSMS protocols in both humans and animals.