Sami Ilvonen

SAR variation study from 300 to 5000 MHz for 15 voxel models including different postures

An extensive study on specific absorption rate (SAR) covering 720 simulations and 15 voxel models (18-105 kg) has been performed by applying the parallel finite-difference time-domain method. High-resolution whole-body models have been irradiated with plane waves from 300 MHz to 5 GHz by applying various incoming directions and polarizations. Detailed results of whole-body SAR and peak 10 g SAR are reported, and SAR variation in the dB scale is examined. For an adult, the effect of incoming direction on whole-body SAR is larger in the GHz range than at around 300-450 MHz, and the effect is stronger with vertical polarization. For a child (height approximately 1.2 m), the effect of incoming direction is similar as for an adult, except at 300 MHz for horizontal polarization. The effect of the phantom (18-105 kg) on whole-body SAR is larger at around 2-5 GHz and at vertical 300 MHz (proximity of whole-body resonance for the child) than at around horizontal 300-900 MHz. Body posture has little effect on whole-body SAR in the GHz range, but at around 300-450 MHz, one may even expect a 2 dB rise in whole-body SAR if posture is changed from the standing position. Posture affects peak 10 g SAR much more than whole-body SAR. The polarization of the incident electric field may have an effect of several dB on whole-body SAR. Between 2 and 5 GHz for adults, whole-body SAR is higher for horizontal than for vertical polarization, if the incoming direction is in the azimuth plane. In the GHz range, horizontal polarization gives higher whole-body SAR, especially for irradiation from the lateral direction. A comparison between homogeneous and heterogeneous models was done. A homogenized model underestimates whole-body SAR, especially at approximately 2 GHz. The basic restriction of whole-body SAR, set by ICNIRP, is exceeded in the smallest models ( approximately 20 kg) at the reference level of exposure, but also some adult phantoms are close to the limit. The peak 10 g SAR limits were never exceeded in the studied cases. The present ICNIRP guidelines should be revised by lowering the reference levels, especially at around 2-5 GHz.

Computational estimation of magnetically induced electric fields in a rotating head

Change in a magnetic field, or similarly, movement in a strong static magnetic field induces electric fields in human tissues, which could potentially cause harmful effects. In this paper, the fields induced by different rotational movements of a head in a strong homogeneous magnetic field are computed numerically. Average field magnitudes near the retinas and inner ears are studied in order to gain insight into the causes of phosphenes and vertigo-like effects, which are associated with extremely low-frequency (ELF) magnetic fields. The induced electric fields are calculated in four different anatomically realistic head models using an efficient finite-element method (FEM) solver. The results are compared with basic restriction limits by IEEE and ICNIRP. Under rotational movement of the head, with a magnetic flux rate of change of 1 T s(-1), the maximum IEEE-averaged electric field and maximum ICNIRP-averaged current density were 337 mV m(-1) and 8.84 mA m(-2), respectively. The limits by IEEE seem significantly stricter than those by ICNIRP. The results show that a magnetic flux rate of change of 1 T s(-1) may induce electric field in the range of 50 mV m(-1) near retinas, and possibly even larger values near the inner ears. These results provide information for approximating the threshold electric field values of phosphenes and vertigo-like effects.

Performance of convolutional PML absorbing boundary conditions in finite-difference time-domain SAR calculations

The performance of perfectly matched layer (PML) absorbing boundary conditions is studied for finite-difference time-domain (FDTD) specific absorption rate (SAR) assessment, using convolutional PML (CPML) implementation of PML. This is done by investigating the variation of SAR values when the amount of free-space layers between the studied object and PML boundary is varied. Plane-wave exposures of spherical and rectangular objects and a realistic human body model are considered for testing the performance. Also, some results for dipole excitation are included. Results show that no additional free-space layers are needed between the numerical phantom and properly implemented CPML absorbing boundary, and that the numerical uncertainties due to CPML can be made negligibly small.

On the Estimation of SAR and Compliance Distance Related to RF Exposure From Mobile Communication Base Station Antennas

In this paper, maximum specific absorption rate (SAR) estimation formulas for RF main beam exposure from mobile communication base station antennas are proposed. The formulas, given for both whole-body SAR and localized SAR, are heuristic in nature and valid for a class of common base station antennas. The formulas were developed based on a number of physical observations and are supported by results from an extensive literature survey together with supplementary measurements and numerical simulations of typical exposure situations. Using exposure limits, the proposed SAR estimation formulas can be converted to formulas for estimating compliance distance.

The effect of finite-difference time-domain resolution and power-loss computation method on SAR values in plane-wave exposure of Zubal phantom

In this paper, the anatomically realistic body model Zubal is exposed to a plane wave. A finite-difference time-domain (FDTD) method is used to obtain field data for specific-absorption-rate (SAR) computation. It is investigated how the FDTD resolution, power-loss computation method and positioning of the material voxels in the FDTD grid affect the SAR results. The results enable one to estimate the effects due to certain fundamental choices made in the SAR simulation.

Magnetic-Field-Induced ELF Currents in a Human Body by the Use of a GSM Phone

A finite element method (FEM) with nonuniform mesh is employed for the calculation of extremely low frequency (ELF) currents induced in a human body by a global system for mobile communications (GSM) phone. The magnetic field of the phone is measured, and an equivalent source with magnetic dipoles is constructed for the numerical simulation. A cell of variable size is used in the simulation to accurately model the most important areas of the body model, namely areas close to the source and parts of the central nervous system. Three different mobile phone positions are considered: normal operation on the side of the head, breast pocket, and the small of the back where the spinal cord is close to the phone. Obtained results are compared with the guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP).