Guidelines for the RF exposure assessment of metallic implants

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November 2006

Abstract

This chapter of the EMF Dosimetry Handbook provides guidance for assessing whether persons bearing metallic implants inside their bodies should be restricted from exposure to the upper tier limits of the radiofrequency (RF) safety guidelines (1998) published by the International Commission for Non-ionising Radiation Protection (ICNIRP) and the C95.1 (2005) standard of the Institute of Electrical and Electronic Engineers (IEEE). The recommendations presented here are based on original research by the authors, investigations of specific implant cases by the Telstra Research Laboratories in Melbourne, Australia and various publications in the scientific literature. Wherever possible, rules-of-thumb have been developed to provide simple and practical ways for assessing implants and for some external body worn metallic objects. Nonetheless, there remain some assessment scenarios that will still require detailed analysis.

Document history

Release number	Date issued	Details of revision
R1	21 Nov 2006	First released issue from Anderson and McIntosh, reviewed by the ACRBR

Acknowledgements

This material is based on research sponsored by the Air Force Research Laboratory, under agreement number FA4869-06-1-0115. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon.

The authors also acknowledge the following organisations for their sponsorship and support of the research and literature review that was required for the drafting this chapter:

• Telstra Corporation Ltd, Australia

• Kordia Pty Ltd, Australia ( http://www.kordiasolutions.com/ )

Lastly, the authors thank Mr Raymond McKenzie for his helpful reviews of this chapter on behalf of the Australian Centre for Radiofrequency Bioeffects Research ( www.acrbr.org.au ).

Index

1Background

1.1Types of metal implants in the body

Many people carry pieces of metal implanted within their bodies, which vary in their origin. These metal implants can for example be unwanted remnants of shrapnel, or more commonly, screws, rods, wires, pins or plates that are implanted by orthopaedic surgeons to repair broken bones and worn joints. Other metallic implant types include arterial stents and implanted electronic devices such as cardiac pacemakers and cochlear implants. External body-worn metallic objects, such as spectacles, jewellery, and the outer components of the cochlear implant system, are also considered in this chapter.

1.2RF heating and human exposure limits

Tissue heating is a well established effect of exposure to electromagnetic radiofrequency (RF) fields due to the absorption of RF power from fields induced inside the body. The common metric for RF heating is the Specific energy Absorption Rate, or SAR in W/kg, which is simply related to the internal RF electric field at any point by:

where E_int is the magnitude of the internal electric field (V/m),  is the electrical conductivity of the tissue (S/m), and  is the mass density of the tissue (kg/m³).

Metallic implants can sometimes concentrate the RF heating effect around them by the way they scatter the incident RF field. This possibility has been recognised in various RF safety guidelines and standards (ICNIRP, 1998, IEEE, 2005, ARPANSA, 2002) which caution that the potential for exceeding allowable exposure limits for localised SAR around metal implants should be assessed for persons exposed up to upper tier limits, i.e. the occupational limits in the ICNIRP Guidelines for electromagnetic exposures (1998) and the controlled environment limits in the IEEE C95.1 RF safety Standard (2006). The lower tier limits prescribed for general public exposures in these documents incorporate substantial additional safety margins that are generally regarded as providing sufficient protection for implant RF field enhancements.

The localised SAR limits in the ICNIRP Guidelines (1998) and the IEEE C95.1 standard (2006) are assessed by averaging the point SAR over a mass of 10 g, usually in the shape of a cube, in recognition of the thermal diffusion properties of tissues. Different upper tier limits apply to different parts of the body as indicated in Table 1 below:

Table 1 Upper tier limits for localised SAR in the ICNIRP Guidelines (1998) and the IEEE C95.1 (2006) standard for human exposure to RF fields.

The upper tier RF limits for exposure to ambient electric (E) and magnetic (H) fields in both the ICNIRP Guidelines (1998) and the IEEE C95.1 Standard (2006) have been primarily formulated to restrict whole body average (WBA) SAR absorption to less than 0.4 W/kg for standing children and adults exposed to uniform plane wave fields. There is a general presumption that these E and H-field limits will also ensure that the localised 10/20 W/kg SAR limits are not exceeded for all circumstances.

1.3Safety targets for RF tissue temperature increases

Tissue temperature rise is a more fundamental indicator of RF heating hazard than localised SAR as it includes the effect of the body’s capacity to dissipate RF heating. A conservative safety target is to restrict RF tissue heating to no more than 1°C in the head and torso (ICNIRP 1998). For other parts of the body that are more tolerant of temperature increases and have less critical functions (i.e., where the higher 20 W/kg SAR limit of the ICNIRP Guidelines or the IEEE C95.1 standard is applied), then a target temperature rise of 2°C would seem appropriate.

1.4RF safety assessments of metal implants

Detailed assessments of SAR concentrations and temperature rises around metal implants generally require complex analyses and specialised skills that are beyond the reasonable capabilities and resources of the great majority of affected persons and organisations. Furthermore, due to enormous diversity in the size, shape and location of metal implants, as well as the many different possible scenarios for RF exposure, it has been difficult to make generalizations from one particular implant assessment to another.

As a result, and despite warnings from RF safety standards and guidelines, most persons bearing personal metal objects and working in high RF fields have not been assessed for the potentially adverse RF heating of those metal objects. To address this lack, this chapter offers practical and accessible guidelines, or rules-of-thumb, for making such assessments for as many implant scenarios as possible.

In devising these rules-of-thumb, the authors have drawn upon many sources including their own research, previous assessments conducted by the Telstra Research Laboratories in Melbourne, Australia, as well as published scientific papers on this topic, and have assumed that:

• The rules-of-thumb only apply to persons who are not exposed above the upper tier limits of the ICNIRP Guidelines (1998) or the IEEE C95.1 standard (2006).

• A metallic implant in the head or torso can be considered safe if the localised SAR (averaged over a 10 g cube) in tissue around the metal object does not exceed 10 W/kg or if the RF induced temperature rise in tissue around the metal object does not exceed 1°C.

• A metallic implant in the limbs and pinna can be considered safe if the localised SAR (averaged over a 10 g cube) in tissue around the metal object does not exceed 20 W/kg or if the RF induced temperature rise in tissue around the metal object does not exceed 2°C.

• The rules should hold for all orientations of the metal objects with respect to the incident field since an individual will generally move about in the field.

• There is no RF heating of the implant itself, i.e. the implant is assumed to be a perfect electrical conductor and the only RF heating occurs in the tissue around the implant.

As this topic is still in a relatively early stage of development, it should be expected that at least some of the rules-of-thumb offered in this chapter will require further amendment as more research is accumulated. Nonetheless, it is hoped that the publication of even imperfect rules-of-thumb will at least be a useful starting point and impetus for the development of better guidelines, and preferable to making no assessments at all.

2RF and thermal factors affecting implant assessments

2.1RF factors

2.1.1RF absorption in the body

The factors influencing the electromagnetic interaction between metal implants and the RF exposure field external to the body are varied and complex. Firstly, one must consider the general interaction of the body with the RF exposure field, as this affects the local incident field exposure around the implant. The main factors that affect the efficiency and distribution of RF absorption in the body are:

1. The frequency of the RF source

2. The polarisation of the incident RF field with respect to the body and its parts

3. The position of the RF source to the body which may lead to partial body exposures and near field coupling effects

4. The size, shape and grounding of the body

5. The dielectric properties (permittivity and conductivity) of body tissues

The dependence of SAR distribution on the RF exposure frequency offers a number of avenues for developing rules-of-thumb. For certain frequency ranges, the internal SAR may be too low to exceed peak allowable limits in particular parts of the body, even with RF field concentrations around the implants. Thus, it would be useful to identify those frequency ranges where metallic implant assessments are not necessary in all or parts of the body. Conversely, in certain frequency ranges, there may be parts of the body where localised SAR levels are relatively high, and where the additional SAR enhancement effect of the implant is more critical. Frequency ranges that are particularly worth noting include:

1. Frequencies above ~4-6 GHz
   At frequencies above this range the small skin depth of absorption, , can provide effective RF shielding of implants buried in the body. At one skin depth, the point SAR will diminish by a factor of 0.14 relative to point SAR at the surface.

2. Frequencies up to the MF band (300 kHz – 3 MHz)
   In this range RF coupling to the body is weak, and E-field limits in the ICNIRP Guidelines (1998) and the IEEE C95.1 Standard (2006) are predicated on the more stringent requirements of protecting against external shock and burns arising from contact with passively charged conductors.

3. Frequencies in the HF (3–30 MHz) and VHF (30–300 MHz) bands
   In this range, whole and partial body resonances occur. Induced RF currents in the ankle and neck are of particular interest due to the concentration of RF current flows in these narrowed conduction areas.

2.1.2RF absorption around the metal implant

Having established a base level of RF exposure in the body, the next step is to determine how the metal object perturbs and possibly concentrates the SAR around it. This should include a consideration of the following factors:

1. The size of the metal object

2. The shape of the metal object

3. Any gaps in the metal object

4. Location of the metal object within the body

5. Dielectric values of tissues around implanted metal objects

6. Whether the implant traverses local tissue boundaries

7. Orientation of the implant with respect to the local induced in vivo fields.

8. Distribution of the in vivo field around the implant (more important for large implants)

It should be noted that passive metal objects cannot of themselves generate any additional RF energy in accordance with the thermodynamic law for conservation of energy. However, due to RF field scattering, they can redistribute the incident RF energy around them, leading to SAR concentrations at some points and corresponding SAR reductions in other areas.

There are at least four basic mechanisms of SAR enhancement around implants as displayed in Figure 1:

1. SAR enhancement at the ends of implants, particularly when the long axis is parallel to the in situ electric field

2. SAR enhancement in gaps of linear implants

3. SAR enhancement in the gaps of broken loops that are cut by changing magnetic flux density (B)

4. Constructive interference in surface layers with underlying metallic plates

Figure 1 Four basic mechanisms of SAR enhancement around metallic implants

2.2Thermal factors

Thermal factors that can influence the local temperature rise around RF exposed implants include:

1. The thermal conductivity and physical structure of the implant which influences the ability of the implant to redistribute temperature variations around it through internal heat transfer.

2. The size and specific heat capacity of the implant which alters the thermal mass of the implant, and affects the transient response to heating.

3. The heat transfer environment around the implant including: the thermal conductivity of surrounding tissues; the micro blood perfusion of surrounding tissues, and; the proximity of implant to large blood vessels.

4. The proximity of the implant to the body surface, where heat transfer from the skin to the ambient environment becomes important.

To a good first approximation, heat transfer inside the body can be numerically modeled using the classic bioheat equation (Pennes, 1948):

where T is the tissue temperature (°C), c is the specific heat capacity (J/kg°C), K is the thermal conductivity (W/m°C), A₀ is the metabolic heat production (W/m³), b is the heat-sink strength from each tissue volume by blood perfusion (W/m³ °C), and T_b is the temperature of the perfusing blood. The desired solution for T is obtained when the system reaches steady-state thermal equilibrium.

Heat transfer at the surface of the body can be modelled as a convective boundary:

where h is the convection coefficient (W/m²°C), T_a is the ambient temperature (°C), q_e is the evaporative heat loss (W/m²) and n is the direction of the unit normal to the surface. The convection coefficient may include a linearised component for heat radiation.

A favoured method for computational analysis of the bioheat equation in human bodies is the finite difference (FD) technique whereby complex heterogeneous models of the human body and implants can be represented by voxels in a regular rectangular mesh. A particular advantage of this approach is that the finite difference mesh can be made to coincide with the voxel mesh of an RF model based on the Finite Difference Time Domain (FDTD) technique, thereby allowing easy transfer of the calculated RF SAR data to the FD thermal analysis. For a detailed example of this approach see, for example, McIntosh et al. (2005).

3Canonical studies of implant rods and infinite plates

3.1Introduction

This chapter section is drawn from a project report by the authors (Anderson and McIntosh, 2004) for a study on metallic implants that was sponsored by the Asian Office for Aerospace Research and Development (AOARD) of the United States Air Force Office of Scientific Research (AFOSR). It also includes observations gathered from earlier implant modelling for specific assessments that were conducted at the Telstra Research Laboratories in Melbourne, Australia.

Results and conclusions about implants are provided from canonical studies of the following areas:

• Calculations of SAR attenuation in planar multilayer skin/muscle/bone/metal models exposed to a plane wave; and;

• A canonical assessment of rod implants exposed to a plane wave in infinite medium that investigated the influence of size, tip shape, rod orientation and the dielectric properties of the surrounding tissue medium.

3.2Canonical modelling of plane waves travelling through layered tissues

Radios waves are attenuated as they travel through lossy materials such as human tissues. At high frequencies, above 4-6 GHz, the rate of attenuation with depth is so pronounced for human exposures that most of the RF power is absorbed at the body surface. At these frequencies, metallic objects located deeper in the body may be effectively shielded from RF exposures.

The extent of this shielding can be gauged by examining a simple canonical model of a plane wave travelling through multiple planar layers representing skin, muscle and bone, as represented in Figure 2 below. This scenario has been modelled in a commercial RF analysis package, FEKO v4.1 (EMSS, 2003), using Greens functions for planar multilayered substrates. The final bone layer extends infinitely in the z direction.

Figure 2 Model setup for examination of a plane wave incident on infinite multilayers of skin, muscle and bone.

Skin and muscle thickness over bone varies in different locations of the body. In some places, there is effectively no muscle layer at all, e.g. on the forehead and shins. To cover these different scenarios, the following scenarios were analysed:

• 3 mm skin layer, infinite bone

• 5 mm skin layer, infinite bone

• 7 mm skin layer, infinite bone

• 5 mm skin layer, 10 mm muscle layer, infinite bone

• 5 mm skin layer, 30 mm muscle layer, infinite bone

The models were examined in the frequency range of 1-10 GHz using dielectric values for dry skin, skeletal muscle and cortical bone from Gabriel (1996) as shown in Table 3.

Table 2 Tissue dielectric values for multilayer models

Results of the multi tissue layer model analyses are shown in Figures 3 to 7. The curves for each frequency have been normalised so that the point SAR = 1 at the air skin surface.

Figure 3 Normalised point SAR in a multilayer tissue model (3 mm skin, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

Figure 4 Normalised point SAR in a multilayer tissue model (5 mm skin, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

Figure 5 Normalised point SAR in a multilayer tissue model (7 mm skin, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

Figure 6 Normalised point SAR in a multilayer tissue model (5 mm skin, 10 mm muscle, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

Figure 7 Normalised point SAR in a multilayer tissue model (5 mm skin, 30 mm muscle, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

A number of general trends are evident from these results:

1. SAR decays more rapidly with depth as the frequency of exposure increases. For all cases studied, the point SAR at a depth of 10 mm had diminished by at least a factor of 10 for exposures above 6 GHz. This observation lends support to the treatment of RF exposures above 6 GHz as a surface heating phenomenon.

2. The large disparity in dielectric values between bone and skin or muscle causes a reflected wave from the bone interface. This can lead to a significant standing wave pattern in the skin and/or muscle exhibiting constructive or destructive interference depending on the layer thickness and the exposure wavelength, , which is dependent on frequency. A constructive interference pattern occurs when the skin/muscle layer is approximately a quarter wavelength thick, resulting in enhanced SAR. This phenomenon was most pronounced at 1-2 GHz for the models studied.

3. At the depth of the bone layer, the point SAR is substantially diminished compared to SAR at the surface in all of the studied cases.

3.3Canonical modelling of plane waves reflected off metallic planar boundaries

An obvious thread to follow up from the observation of standing waves described in the preceding section is the constructive interference patterns that can result from RF waves reflected off a planar metal surface underneath the skin. Classical transmission line theory indicates that the maximal constructive interference occurs when the thickness of the skin between the air and plate is a quarter of the RF wavelength, λ, in the skin. Using a FEKO model as indicated in Figure 8, the calculated field pattern in a 3 mm layer of skin in front of a metal boundary is shown in Figure 9. It shows the maximal SAR levels occur at 4 GHz where the wavelength in skin is approximately 12 mm, in accordance with the λ/4 expectation.

Figure 8 Model setup for examination of a plane wave incident on an infinite layers of skin overlaying a perfect electrical conducting (PEC) plane. The averaging cube for calculating 1 g or 10 g SAR is positioned against the air/skin surface and extends behind the metal plane where SAR equals zero.

Figure 9 Point SAR distribution for a 1 mW/cm² plane wave normally incident on a 3 mm thick layer of skin overlaying a metallic plane. Results were calculated in a similar FEKO model as described in the previous section.

For other skin thicknesses, the frequency at which maximal quarter wave enhancement occurs is indicated in Table 3 as gauged by the 10 g average SAR over the shape of a cube at the surface. The point SAR in the skin layer for exposures at these frequencies at the upper tier ambient E-field limit in the ICNIRP Guidelines (1998) is shown in Figure 9.

Table 3 10 g average SAR (in the shape of a cube) in a skin layer overlaying a metal plane exposed to a normally incident plane wave at the upper tier limit for ambient E-field exposure ICNIRP Guidelines (1998), as shown in Figure 10.

Figure 10 Point SAR distribution in a layer of skin (3 – 8 mm thick) overlaying a metallic plane. Exposure consisted of a normally incident plane wave of intensity equal to the ICNIRP Guidelines (1998) upper tier (occupational) E or H field reference levels at the quarter wave frequency for each skin thickness. SAR is zero behind the metal plane interface.

Table 3 indicates that the ICNIRP 10 g localized SAR limits (10 W/kg for head and torso, 20 W/kg for the limbs) are not exceeded for ambient exposures at the occupational field limits. The quarter wave enhancement effect appears to monotonically decrease for increasing skin thickness greater than 4 mm (see figure 10).

3.4Canonical modelling of rod implants in infinite medium

3.4.1General approach

As depicted in Figure 1, SAR can be enhanced at the tips of linear metal structures, particularly when the E-field is oriented parallel to the longest dimension of the implant. This phenomenon has been investigated by the authors in a series of canonical models for rods exposed to a plane wave in an infinite dielectric medium, and with particular regard to the following factors:

1. The length of the rod

2. The shape of the rod tip

3. The orientation of the rod with respect to the incident E-field exposure

4. The dielectric medium around the rod

3.4.2Use of VAR instead of SAR

Rather than calculate mass averaged SAR around the rod, it was decided that the Volumetric Absorption Rate (VAR) in W/m³ averaged over a fixed sized cube was a more appropriate metric for comparing the relative RF field enhancements. The VAR at any point is calculated as VAR = |E|², c.f. SAR = |E|²/. The RF power calculated by integrating point VAR over a 10 cm³ cube is equivalent to the RF power obtained by integrating SAR over a cube of 10 g mass if the density of the medium is 1000 kg/m³, as is commonly assumed for most tissue types.

The decision to choose volume averaged VAR as the comparison metric was based on its greater ease of calculation and because it is more directly related to tissue temperature rise. On the first point, the density of metals (steel ~ 8000 kg/m³) is much higher that the density of tissues (~ 1000 kg/m³) which can substantially affect the size of a constant mass averaging cube when it intersects a metal implant and hence greatly complicates the calculation of mass averaged SAR compared to a constant size VAR averaging cube.

Moreover, this variability in the size of a SAR averaging mass also introduces an arbitrary variation in the level of RF power deposited in the cube which makes mass averaged SAR less directly related to temperature rise than volume averaged VAR. The more direct coupling between VAR and tissue temperature can be plainly seen in the bioheat equation for steady state RF heating shown below:

SAR VAR

3.4.3Model setup

The analyses were performed using Method of Moment (MoM) analysis in the FEKO v4.1 software (EMMS, 2003). The 10 cm³ volume averaged VAR was calculated by averaging point VAR in a 24 x 24 x 24 cubic array as depicted in Figure 13. The rod models consisted of a perfect electrically conducting (PEC) round rod exposed to a plane wave in an infinite tissue medium. The volume averaged VAR over a 10 cm³ cube was calculated along the length of the rod as shown in Figure 14.

Figure 11 The 10 cm³ averaging cube for VAR was subdivided into a cubic array of 24 x 24 x 24 cuboids. The 10 cm³ VAR was obtained by averaging the point VAR at the centre of each of the13,824 cuboids.

Figure 12 FEKO model of a PEC rod implant exposed to a plane wave in an infinite tissue medium. The 10 cm³ VAR was evaluated along the length of the rod.

In these canonical analyses, the rod was immersed in an infinite tissue medium of either bone or muscle. The analyses were conducted over a frequency range of 0.1 MHz to 10 GHz with uniform logarithmic spacing of 5 points per decade (1, 1.6, 2.5, 4, 6.3, 10). The tissue dielectric values were obtained from Gabriel et al. (1996) as shown in Table 4.

Table 4 Relative permittivity, _r, and conductivity, , of cortical bone and skeletal muscle used in the canonical analyses of a rod exposed to a plane wave in infinite tissue medium. The plane wave wavelength, λ, and the skin depth, , in the tissues are also shown.