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

Abstract

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.

A formula for human average whole-body SARwbunder diffuse fields exposure in the GHz region

“…The authors showed that the worst-case incident plane wave polarization depends on the ratio of the phantom size and the considered wavelength. This result is confirmed in Hirata et al (2009) and Uusitupa et al (2010) wherein it is stated that the polarization of the incident electric field may have an effect of several dB on the SAR wb . The vertical polarization based SAR wb is higher up to the resonant frequency (order of 60-120 MHz), whereas the horizontal polarization result becomes higher in the GHz region (Uusitupa et al 2010).…”

“…This result is confirmed in Hirata et al (2009) and Uusitupa et al (2010) wherein it is stated that the polarization of the incident electric field may have an effect of several dB on the SAR wb . The vertical polarization based SAR wb is higher up to the resonant frequency (order of 60-120 MHz), whereas the horizontal polarization result becomes higher in the GHz region (Uusitupa et al 2010). Obviously, the SAR wb due to a single plane wave illumination depends on its direction of arrival: this is confirmed in Conil et al (2011) where it was shown that the SAR wb varies periodically with the azimuth.…”

“…It is observed that k H (k value for the horizontal polarization) is always -as a function of the azimuth -higher than k V (k value for the vertical polarization). This is an expected result since the SAR wb from a horizontally polarized plane wave is higher than the one from a vertically polarized plane wave in the GHz region (Uusitupa et al 2010). Given the low relative standard deviations, the k-parameter can therefore be considered as phantom and frequency independent.…”

“…Finally, the SAR wb due to a LOS exposure depends on the incident plane wave azimuth, polarization, considered frequency, etc. (ICNIRP 1998, Uusitupa et al 2010. By analyzing (2a) and (2b), it is clear that the resulting ACS los will depend on the parameters mentioned above.…”

“…The human absorption in the GHz region is mainly influenced by the body surface area (BSA). This is due to the absorption occurring at the body surface rather than inside the body volume because of the relative small penetration depth in the GHz region (Hirata et al 2007, Uusitupa et al 2010. Based on those findings, a BSA based formula to determine the SAR wb caused by diffuse fields in the GHz region -from 1.45 GHz up to 5.8 GHz -is proposed.…”