Simplified Magnetic Flux Density Measurement for Local Resolution Analysis of Transcranial Magnetic Stimulation

The detailed energy dynamics of transcranial magnetic stimulation of the cerebral cortex remain unclear. The present study therefore examined the energy distribution of transcranial magnetic stimulation. Magnetic energy was assessed using the magnetic flux density, and the locality of transcranial magnetic stimulation was estimated. The magnetic flux density measuring device comprised a pickup probe and simple electronic circuits (e.g., an integrator and buffer amplifier). The pickup probe comprised a sensor coil (with an inner diameter of 8 mm), twisted pair line, and lead-out cable. Magnetic stimulation was applied using a Rapid2 stimulator and a figure-eight flat coil (with a diameter of 70 mm). A simple brain model made from an acrylic plate was based on a subject with a 62%-63% motor threshold and a scalp-to-primary motor cortex distance (i.e., distance from the scalp to the center of the sensor coil) of 15 mm. The highest magnetic flux density after the removal of background noise was approximately 180 mT at the center of the stimulus coil, whereas the resolution of the transcranial magnetic stimulation was approximately 6 mm (i.e., from the center to within +/- 3 mm). The stimulation area estimated from the measurements was 12 mm(2) around the location of highest magnetic flux density. With the magnetic flux density estimated at the center of the stimulation coil, the stimulus area in which the magnetic flux density decreased by less than 1% was approximately 50.27 mm(2) and that in which the magnetic flux density decreased by less than 5% was 254.47 mm(2). It is suggested that the transcranial magnetic stimulation of the cerebral cortex affects areas around the target cerebral cortex. This study showed that the distribution of the magnetic stimulation energy supplied to the target cortex may be evaluated using a simple measurement device and brain model.