Electric Field Induced in Retina and Brain at Threshold Magnetic Flux Density Causing Magnetophosphenes

摘要

In the international safety guidelines/standards, the rationale for human protection from extremely-low-frequency magnetic field exposure is to prevent the magneto-stimulation. The threshold in-situ electric field for magnetophosphenes has been experimentally derived as 8.14 mT at 20 Hz. However, the corresponding in-situ electric field in the central nervous system has not been well investigated. The present study derived computationally in-situ electric field in the brain and retina in different anatomically-based numeric human models exposed to uniform magnetic fields at extremely low frequencies. A quasi-static finite-difference time-domain method was applied to analyze this problem. First, the computational uncertainty caused by using a stair-casing model is investigated by comparing the electric field induced in a three-layer sphere obtained by the computational method to an analytical solution. In addition, the electric fields induced in anatomically-based models with different resolutions are also compared. For our computational results, the 99th percentile value of the in-situ electric field is found to be reasonable both for the sphere and anatomically-based models with different resolutions. Then, the 99th percentile values of the electric field in the brain and retina in different models are computed for exposure at the threshold magnetic flux density for magnetophosphenes. When comparing the 99th percentile value in the brain of the anatomically-based model with those derived from an ellipsoid defined in the IEEE standard, the former was 10-70% larger than the latter. The main reason for this difference is attributed to the remaining part of the body in addition to the model inhomogeneity.