VP1-05 Assessing the resolution of transcranial magnetic stimulation by measuring the magnetic flux density at the cortical surface
Creators: Aya Sato
Tetsuya Torii
Publisher: name: IEEE
Year: 2025

Transcranial magnetic stimulation (TMS) is a non-invasive and localized stimulation technique. Local stimulation technologies play an important role in brain function research and treatment. Previously, the TMS resolution was found to be 5 mm when using a figure eight-shaped stimulus coil. The stimulus resolution has been evaluated using various measurement techniques. Here, we explored whether the stimulation area could be evaluated using a simple method and small dataset. In our experiment, a Rapid2 stimulator (Magstim Co.,) was set to deliver pulses at 75% magnetic intensity, and the distance from the scalp to the cortex was assumed to be 15 mm. The magnetic flux density was measured within ±30 mm on the x- and y-axis from the center of the stimulus coil. On a curved model of the surface of the head, this corresponded to a curve with a radius of 70 mm on the x-axis and 100 mm on the y-axis. We used a sensor coil and simple measurement circuit to measure the magnetic flux density. Our results showed that the stimulus resolution on a flat surface was within approximately 5 mm of the center position. In contrast, the stimulus resolution on the curved surface was extremely small, as indicated by a significant difference at each measurement point compared with the central magnetic flux density (p < 0.01, less than 2 mm in diameter). The stimulation area within which the magnetic flux density decreased by less than 1% compared with the full magnetic flux density at the center of the stimulation coil was approximately 37 mm2 on a flat surface and 23 mm2 on a curved surface. The Q (quality factor) value, which indicates the kurtosis of the measured value, was obtained by inputting the measured data into an approximate formula model. The Q value on the flat surface was approximately 0.68 on the x-axis and 0.56 on the y-axis, while the Q value on the curved surface was approximately 0.75 on the x-axis and 0.63 on the y-axis. Therefore, the Q value obtained by measuring of the magnetic flux density may be useful as an index of magnetic stimulation resolution.References: S. Ueno, T. Matsuda et al., “Localized stimulation of the human brain and spinal cord by a pair of opposing pulsed magnetic fields,” J Appl. Phys., vol. 67(9), pp. 5838-5840, 1990. T. Torii, H. Sakamoto, A. Sato, “Simplified Magnetic Flux Density Measurement for Local Resolution Analysis of Transcranial Magnetic Stimulation,” 2023 IEEE International Magnetic Conference (INTERMAG), doi.org/10.1109/INTERMAG50591.2023.10265102, 2023.
Transcranial magnetic stimulation (TMS) is a non-invasive and localized stimulation technique. Local stimulation technologies play an important role in brain function research and treatment. Previously, the TMS resolution was found to be 5 mm when using a figure eight-shaped stimulus coil. The stimulus resolution has been evaluated using various measurement techniques. Here, we explored whether the stimulation area could be evaluated using a simple method and small dataset. In our experiment, a Rapid2 stimulator (Magstim Co.,) was set to deliver pulses at 75% magnetic intensity, and the distance from the scalp to the cortex was assumed to be 15 mm. The magnetic flux density was measured within ±30 mm on the x- and y-axis from the center of the stimulus coil. On a curved model of the surface of the head, this corresponded to a curve with a radius of 70 mm on the x-axis and 100 mm on the y-axis. We used a sensor coil and simple measurement circuit to measure the magnetic flux density. Our results showed that the stimulus resolution on a flat surface was within approximately 5 mm of the center position. In contrast, the stimulus resolution on the curved surface was extremely small, as indicated by a significant difference at each measurement point compared with the central magnetic flux density (p < 0.01, less than 2 mm in diameter). The stimulation area within which the magnetic flux density decreased by less than 1% compared with the full magnetic flux density at the center of the stimulation coil was approximately 37 mm2 on a flat surface and 23 mm2 on a curved surface. The Q (quality factor) value, which indicates the kurtosis of the measured value, was obtained by inputting the measured data into an approximate formula model. The Q value on the flat surface was approximately 0.68 on the x-axis and 0.56 on the y-axis, while the Q value on the curved surface was approximately 0.75 on the x-axis and 0.63 on the y-axis. Therefore, the Q value obtained by measuring of the magnetic flux density may be useful as an index of magnetic stimulation resolution.References: S. Ueno, T. Matsuda et al., “Localized stimulation of the human brain and spinal cord by a pair of opposing pulsed magnetic fields,” J Appl. Phys., vol. 67(9), pp. 5838-5840, 1990. T. Torii, H. Sakamoto, A. Sato, “Simplified Magnetic Flux Density Measurement for Local Resolution Analysis of Transcranial Magnetic Stimulation,” 2023 IEEE International Magnetic Conference (INTERMAG), doi.org/10.1109/INTERMAG50591.2023.10265102, 2023.