Evaluation of an Isosceles-Triangle-Coil Phantom for Magnetoencephalography

Magnetoencephalography (MEG) is one of the most promising applications of biomagnetics. In order to expand and establish MEG applications, an evaluation standard for MEG systems is necessary. We have proposed and fabricated a practical dry phantom that is composed of two isosceles-triangle coils based on Ilmoniemi's model. The uncertainty level of the phantom itself has not always been stated in the literature. In this study, we evaluated the phantom by two methods: a 3-D measurement of the coils with X-ray computed tomography, and a magnetic field measurement with an MEG system. In the former, the maximum displacement of the estimated position of the equivalent current dipole was -0.59± 0.09 mm. In the latter, the displacement was 0.68 ± 0.19 mm. The uncertainty level was found and less than 0.1 mm, or less than 0.2 mm; therefore, we can assess the MEG system itself with an uncertainty level of 0.2 mm. The displacement in the magnetic measurement larger than the uncertainty level of 0.2 mm is estimated due to the systematic error of the MEG system. Hence, dimensional and magnetic measurements are effective for evaluation of the phantom and the MEG.