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 (R2＝0.9993). 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