Technical Note
Magnetic Field Strength and Reproducibility of Neodymium Magnets Useful for Transcranial Static Magnetic Field Stimulation of the Human Cortex

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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.

Section snippets

INTRODUCTION

Noninvasive neuromodulation techniques have gained a strategic position in cognitive neuroscience and have reshaped the way brain–behavior relations are investigated. Moreover, these techniques have been proposed as a treatment for neuropsychiatric disorders (1,2). Repetitive transcranial magnetic stimulation and transcranial direct current stimulation are commonly used as noninvasive neuromodulation techniques in humans and animals.

Recently we described that the application of transcranial

Magnets

We measured the strength of the static magnetic field produced by 10 magnets and two inert steel cylinders (useful for sham stimulation). The characteristics of the magnets and of the steel cylinders are summarized in Table 1.

We used three different sizes of cylindrical magnets, which will be referred to as magnets 30, 45, and 60 (M30, M45 and M60). The number indicates the diameter (in millimeters). We also tested a 60-mm magnet with a quasi-ring shape. We will refer to this magnet as M60qr

RESULTS

Figure 1 (b–d) shows the magnetic field strength of the different magnets tested at different distances from the magnet base surface. The reproducibility of the results was very high for all magnet sizes (Fig. 1b,d): Specifically, the standard deviation for all measurements performed on the four M45 magnets at different spatial points (Fig. 1b) was <10 mT. As expected, at distances that are relevant for cortical stimulation in humans (i.e., 2–3 cm from the magnet surface), the magnetic field

DISCUSSION

We believe that tSMS can play a role in the development of future strategies for noninvasive neuromodulation. Herein, we measured the magnetic field strength of permanent magnets of different size and shape at different distances from the magnet base surface. The most important result is that there is good reproducibility of the magnetic field strength when similar magnets are compared. Furthermore, at 2–3 cm from the magnet surface the magnetic field strength is in a range between 120 and 200

Acknowledgements

We would like to thank Francisco Rivadulla (CIQUS, University of Santiago) for useful technical help during the measurements. This work was partially supported by the Ministerio de Educacion y Ciencia (MEC) (BFU2009-08169).

Authorship Statement

Drs. Casto Rivadulla, Guglielmo Foffani, and Antonio Oliviero designed and conducted the study, including data collection and data analysis. Dr. Antonio Oliviero prepared the manuscript draft with important intellectual input from Drs. Casto Rivadulla and Guglielmo Foffani.

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