A numerical evaluation of SAR distribution and temperature changes around a metallic plate in the head of a RF exposed worker

Abstract

The 1998 International Commission for Non-Ionising Radiation (ICNIRP) Guidelines for human exposure to radiofrequency (RF) fields contain a recommendation to assess the potential impact of metallic implants in workers exposed up to the allowable occupational field limits. This study provides an example of how numerical electromagnetic (EM) and thermal modelling can be used to determine whether scattered RF fields around metallic implants in workers exposed to allowable occupational ambient field limits will comply with the recommendations of relevant standards and guidelines. A case study is performed for plane wave exposures of a 50 mm diameter titanium cranioplasty plate, implanted around 5-6 mm under the surface of the forehead. The level of exposures was set to the ambient power flux density limits for occupational exposures specified in the 1998 ICNIRP guidelines and the current 1999 IEEE C95.1 standard over the frequency range 100-3000 MHz. Two distinct peak responses were observed. There was a resonant response for the whole implant at 200-300 MHz where the maximum dimension of the implant is around a third of the wavelength of the RF exposure. This, however, resulted in relatively low peak specific energy absorption rate (SAR) levels around the implant at the exposure limits. Between 2100-2800 MHz, a second SAR concentrating mechanism of constructive interference of the wave reflected back and forth between the air-scalp interface and the scalp-plate interface resulted in higher peak SARs that were within the allowable limits for the ICNIRP exposures, but not for the IEEE C95.1 exposures. Moreover, the IEEE peak SAR limits were also exceeded, to a lesser degree, even when the implant was not present. However, thermal modelling indicated that the peak SAR concentrations around the implant did not result in any peak temperature rise above 1 degrees C for occupational exposures recommended in the ICNIRP guidelines, and hence would not pose any significant health risk.

Interaction of radio frequency electromagnetic fields and passive metallic implants—A brief review

“…In certain (plane wave) cases, the distance between the implant and the skin surface may match the wavelength in tissue so that constructive interference occurs in the surface layers (e.g., distance ¼ l T /4) [McIntosh et al, 2002McIntosh and McKenzie, 2003]. This may cause elevated SAR in the surface [McIntosh et al, 2002McIntosh and McKenzie, 2003;Anderson et al, 2004].…”

“…In the context of RF dosimetry, analyses of near fields apply to devices used in the proximity of body such as mobile and radio phones, and far field analyses can be regularly applied to the exposure to base stations (except near them). Most of the reviewed implant studies [Johnson and Guy, 1972;Hocking et al, 1991;Fleming et al, 1992;McIntosh et al, 2002McIntosh et al, , 2004McIntosh and McKenzie, 2003;Anderson et al, 2004] concern the far field exposure, but a few [Cooper and Hombach, 1996;Tuppurainen, 2001;Virtanen et al, 2005] have also concentrated on implants exposed in the near fields. The motivation for the far field studies comes partly from the occupational exposure of RF workers, who may encounter high exposure levels at work.…”

“…The Visible Human phantom has also been used in a study of a cranioplasty plate in far fields of 200-3000 MHz [McIntosh et al, 2002McIntosh and McKenzie, 2003]. The plate was placed in the front of the phantom, and a plane wave with the E field parallel to the long axis of a body was applied from the front.…”

Effect of 2100 MHz mobile phone radiation on healing of mandibular fractures: an experimental study in rabbits

“…Numerical SAR simulations of human head with metallic plate were performed before, [40] and metal plate dimensions were one third of the applied RF wavelength, which resulted in resonance peaks at the absorbed electric field. In our study, the dimensions of the implant were much smaller than the RF exposure wavelength, which, in turn, did not result in resonance peaks in electric field distribution.…”

Enhancement of Apoptosis by Titanium Alloy Internal Fixations During Microwave Treatments for Fractures: An Animal Study

“…Previous in vitro studies have shown that tissue ambustion can be caused by the temperature of a metal plate at frequencies near 900 MHz and 27 MHz [7]. However, in vitro studies of radio frequency (RF) electromagnetic fields showed that the metal implants caused little risk using 1800 MHz [8] and 2450 MHz [9,10] microwave radiation. Furthermore, shortwave diathermy, also a high frequency electrotherapy, was clinically applied by some doctors and therapist to the cure and rehabilitation in injuries bone with surgical implanted metal [11,12].…”