Coverage Factors for Efficient Demonstration of Compliance of Low-Frequency Magnetic Near-Field Exposures with Basic Restrictions

Jingtian Xi, Andreas Christ, and Niels Kuster, Physics in Medicine & Biology 2022, online 2 December 2022; doi: 10.1088/1361-6560/aca875

Regulators require that wireless power transfer (WPT) systems and other strong magnetic field sources are compliant with the basic restrictions (BRs), defined as the limits of the fields induced in the human body, i.e., the induced electric field/current density/specific absorption rate limits. This can be achieved by demonstrating compliance with the reference levels (RLs) defined in air without the human body, i.e., the incident electric/magnetic field limits. Local sources, such as WPT transmitters, generate non-uniform fields that can locally exceed RLs, while the induced fields are still well within the BRs. In these cases, robust compliance with BRs can be demonstrated but generally requires a large number of simulations. In this study, we propose an efficient evaluation based on use of a homogeneous phantom and application of a coverage factor to account for the local field enhancements caused by the dielectric contrasts of highly inhomogeneous human tissues. The generally applicable coverage factors were derived from a statistical analysis of the field enhancements observed with four magnetic near-field sources placed at different separation distances (2 – 80 mm) and locations on the backs of 12 anatomical models. The field enhancements were characterized by the ratios between the peak induced fields in the anatomical models and those in the homogeneous half-space phantom (ε_r = 55, σ = 0.75 S/m, ρ = 1,000 kg/m³) at the same distance. The resulting 99th percentile coverage factors range from 1 to 9, depending on the dosimetric quantity. The use of these coverage factors reduces the compliance testing effort from hundreds of simulations to only one and makes experimental testing feasible without the support of simulations. The study also demonstrates that running only a few use-case simulations with anatomical models may lead to underestimation of the exposure by more than 10 dB.

The scientific and technical impact of the study can be summarized as:

• Demonstration that WPT systems operating close to the body are in compliance with safety guidelines is becoming a major challenge for the industry
• The use of simplified models and even one or two complex human models may lead to severe underestimation of exposure, because the induced field – which strongly depends on anatomical features – varies by more than 10 dB, i.e., a large number of simulations would be required for compliance evaluation
• In this study, we propose a novel method, based on a phantom and application of coverage factors, that greatly simplifies compliance testing
• Determination of these coverage factors is based on 3456 simulations for the various BR quantities representing the statistical weighted enhancements of the induced electrical fields due to the dielectric contrasts of the highly inhomogeneous human tissues compared to the those of the homogeneous half-space IEC 63184 phantom