Assessing the Resolution of Transcranial Magnetic Stimulation by Measuring the Magnetic Flux Density at the Cortical Surface

Transcranial magnetic stimulation (TMS) is a non-invasive and localized stimulation technique. Local stimulation is useful for studying brain functions and treating brain disorders. Previous studies have reported a stimulus resolution of approximately 5 mm or a stimulus area smaller than 50 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>. Stimulus resolution has previously been evaluated using various measurement techniques. In this study, the magnetic flux density measured using a simplified system was analyzed using the quality factor (Q)-value. Magnetic pulses (at 75% intensity) were delivered using a figure-eight-shaped flat coil to a curved-head model. A distance of 15 mm from the stimulus coil to the center of the sensor coil was assumed as the same distance from the scalp to the cortex. The stimulation resolution was evaluated based on the magnetic flux density measured on the surface of the curved-head model, and the size of the stimulation area was evaluated based on the Q-value. The results revealed that in both the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i>- and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i>-axis directions, the stimulation resolution on the cortical surface was less than 2 mm (p < 0.01). The Q-values from the peak to −0.1 and −3.0 dB were estimated at approximately 7.0 and 0.8 and at approximately 4.0 and 0.6 along the x- and y-axes, respectively. These results suggest that the use of a curved-head model enabled the TMS device to stimulate a target in a narrow resolution range and that Q-values can be used to easily evaluate the stimulation area.