Original Article

Development of an integration circuit to measure pulsed magnetic field: evaluation of its usefulness by comparing measured with theoretical magnetic field structure

Shin-Ichi Izumi, MD, PhD, Yutaka Oouchida, PhD, Taishi Okita, PhD, Eizaburo Suzuki, RPT, MS, Toshihiko Abe, PhD, Ryoichi Nagatomi, MD, PhD, Nobukazu Nakasato, MD, PhD, Toshiyuki Takagi, PhD
Jpn J Compr Rehabil Sci 3: 42-50, 2012

Objective: To develop a search coil with a capacityresistor (CR) integration circuit for instantaneous measurement of a pulsed magnetic field, and to compare the magnetic field structure measured by the novel device with the theoretical values.
Methods: The integration circuit consists of a capacitor and a resistor connected in series, and generates a voltage output across the capacitor. For magnetic field measurement, we selected capacitance ~ resistance = 4 ms. Using the search coil and CR integration circuit, we measured the magnetic flux density generated by a circular coil, over a range of magnetic flux densities (0.1 to 1.4 T) and pulse widths (0.1 to 0.6 ms). We also obtained the corresponding magnetic fields by numerical integration of the electromotive force induced in the search coil. Then we measured the distribution of magnetic flux densities generated by a commercial figure-of-eight coil commonly used in transcranial magnetic stimulation (TMS), and compared the results with the magnetic field derived from model calculation.
Results: The values measured by the CR circuit and the values obtained by numerical integration showed good agreement (R20.9993). At 100% output from the stimulator, the peak magnetic flux density of the horizontal and vertical magnetic fields at a distance 10-30 mm from the figure-of-eight coil surface was 0.2 to 0.5 T. The measured magnetic field showed less focalization when compared with the values obtained from model calculation.
Conclusions: We identified the appropriate CR combination for the integration circuit used to measure a pulsed magnetic field. This device would be useful for quantitative analysis of TMS-induced effects on cortical function.

Key words: transcranial magnetic stimulation, magnetic field structure, figure-of-eight coil, integration circuit, teslameter, magnetic flux density

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