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Final Report
Biological Effects in the cm/mm Wave Range
Part II/III
Determination of Material Parameters and
Analysis of Field Strengths in Human Tissue
by
Institute of Mobile and Satellite
Communication Techniques GmbH,
Germany
Dr.-Ing. Frank Gustrau
Dr.-Ing. Achim Bahr
14. January 1999
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Table of Contents
1 SUBJECT OF INVESTIGATION ........................................................................................................ 4
2 RADIOFREQUENCY FIELD EXPOSURE STANDARDS................................................................. 4
2.1 LEGAL CLASSIFICATION OFSTANDARDS ................................................................................................... 4
2.2 S TANDARDSASSOCIATIONS.....................................................................................................................5
2.3 DISTINCTIONBETWEENAREAS, TIME OFEXPOSURE AND FREQUENCIES......................................................5
2.4 BASICRESTRICTIONS ANDDERIVEDREFERENCELEVELS............................................................................6
2.5 M AXIMUM PERMISSIBLE EXPOSURE FOR HIGH FREQUENCY DEVICES FROM3 GHZ TO 100 GHZ......................7
3 DETERMINATION OF THE RELEVANT MATERIAL PARAMETERS........................................8
3.1 LITERATURESURVEY ANDMEASUREMENTTECHNIQUES.............................................................................8
3.2 M EASUREMENTRESULTS IN THEFREQUENCYRANGE75 100 GHZ........................................................ 12
3.2.1 M EASUREMENT OFDIELECTRICPROPERTIES OFSKINTISSUE.......................................................... 13
3.2.2 M EASUREMENT OFDIELECTRICPROPERTIES OFEYETISSUE........................................................... 17
3.3 M EASUREMENTRESULTS IN THEFREQUENCYRANGE200 MHZ20 GHZ............................................... 20
3.3.1 M EASUREMENT OFDIELECTRICPROPERTIES OFSKINTISSUE.......................................................... 21
3.3.2 M EASUREMENT OFDIELECTRICPROPERTIES OFEYETISSUE........................................................... 24
4 ANALYSIS OF THE FIELD STRENGTHS IN HUMAN TISSUE...................................................26
4.1 M ODELS OF THEOBJECTS UNDERINVESTIGATION................................................................................... 26
4.1.1 LAYEREDMODEL OF THEHUMANSKIN......................................................................................... 26
4.1.2 M ODEL OF THEHUMANEYE........................................................................................................ 27
4.2 S IMULATIONMETHODS........................................................................................................................ 29
4.2.1 ANALYTICALMETHOD................................................................................................................. 29
4.2.2 T HEFINITEDIFFERENCETIMEDOMAINMETHOD.......................................................................... 30
4.3 S IMULATION OF THEFIELDDISTRIBUTION INSIDE THEHUMANEYE AND SKIN............................................ 31
4.3.1 F IELDDISTRIBUTION IN THEHUMANSKIN..................................................................................... 31
4.3.2 F IELDDISTRIBUTION IN THEHUMANEYE...................................................................................... 35
5 INVESTIGATION OF THERMAL EFFECTS..................................................................................39
5.1 I NFRARED THERMOGRAPHY.................................................................................................................. 39
5.2 H UMANSKIN....................................................................................................................................... 39
5.2.1 E XPERIMENTAL SETUP................................................................................................................. 39
5.2.2 M EASUREMENT RESULTS............................................................................................................. 40
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5.2.3 DISCUSSION.............................................................................................................................. 42
5.3 PORCINEEYE...................................................................................................................................... 43
5.3.1 E XPERIMENTAL SETUP................................................................................................................. 43
5.3.2 M EASUREMENT RESULTS............................................................................................................. 43
5.3.3 DISCUSSION............................................................................................................................... 45
5.4 S IMULATION OFTHERMALEFFECTS....................................................................................................... 45
5.4.1 M ATHEMATICALMODEL OFBIO-HEAT-TRANSFER ......................................................................... 45
5.4.2 LAYEREDMODEL OFSKIN........................................................................................................... 46
5.4.3 RESULTS.................................................................................................................................... 47
5.4.4 DISCUSSION............................................................................................................................... 48
6 SUMMARY..........................................................................................................................................48
7 REFERENCES .................................................................................................................................... 50
8 APPENDIX .......................................................................................................................................... 52
8.1 C OMPARISON OFMEASUREDDIELECTRICPROPERTIES ANDDATA FROMLITERATURE................................. 52
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1 Subject of Investigation
On behalf of the FGF the IMST has carried out an investigation referring to biological effects
in the cm/mm wave range. The technical part of the project dealt with the analysis of field
strengths inside human tissue. The investigation was made with respect to the electric and
magnetic fields inside the human eye and the skin on the back. This restriction was made be-
cause of the low vascularity and bad thermoregulation of the human eye. A similar statement
can be made for the skin on the back because of the relative bad vascularity and the low den-
sity of sweat glands.
Part II of the project contained the determination of the dielectric parameters of human tissue
in the frequency range from 200 MHz up to 100 GHz. The tissues under investigation included
muscle, fat, skin (dermis), cornea, retina, lens, sclera, vitreous body, and liquid from camera
anterior. First of all a literature study was made concerning the dielectric parameters of human
tissue. In a next step the unknown dielectric parameters were measured with a material meas-
urement system in the frequency range of interest.
In part III the field strengths in the human eye and skin were simulated. The exposure realized
was a linearly polarized plane wave. For the human skin a layered model was used. Besides
epidermis and dermis, fat and muscle tissue were distinguished. The human eye was modeled
as a quasi-ellipsoid with the tissues mentioned above.
2 Radiofrequency Field Exposure Standards
In nearly any country the protection of human beings against harmful influences is a task of
government. Many different organizations are trying to get a work-out of national and interna-
tional rules and standards to get technical conditions to put this political aim into practice.
2.1 Legal Classification of Standards
To classify standards, rules and regulations it is important to distinguish between the following
terms:
National and international standards Laws (for example national laws of protection against immission like the German
Bundes-Immissionsschutzgesetz [BImSchV 1996])
Recommendations (for example presented by the German national radiation protectioncommission Strahlenschutzkommission [SSK 1993])
Voluntary consumer protection standards (for example standard for low radiation com-puter screens, the MPR II standard)
From the legislators point of view a careful distinction between the terms must be made. In
the Federal Republic of Germany the standard DIN 0848 part 2 (similar to ANSI C95.1 [ANSI
1991]) dealing with the protection of human beings from electromagnetic fields, has not beenput into asserted German right so far. A German law, called Bundes-Immissionsschutz-
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gesetz, is just valid for wireless non mobile installations (indeed there is currently an at-
tempt to extend this law in respect to the DIN 0848). The recommendation of the national
German radiation protection association has no legally binding function as well as the voluntary
consumer protection standards.
2.2 Standards Associations
The approved institution by the German government for preparing standards is the Deutsches
Institut fr Normung (DIN), which promotes the harmonization of the standards for Europe.
In the field of high frequency electromagnetic fields the reference levels of the current
CENELEC (Comit Europen de Normalisation Electrotechnique) prestandard [ENV 50166]
are very close to the most recent draft of the DIN/VDE standard 0848 [DIN 0848 91]. The
corresponding guideline ANSI C95.1 published by the American National Standards Institute
[ANSI 1991] is not only used in the USA, but also by many other countries (for example Aus-
tralia).
The US Federal Communications Commission (FCC) issued a report and order on the 1st
of
August 1996 [FCC 1996], which requires routine dosimetric assessment of mobile telecommu-
nications devices, either by laboratory measurement techniques or by computational modeling,
prior to equipment authorization or use.
One of the most important organizations deals with the international development of standards
is the INIRC of the IRPA (INIRC: International Non-Ionizing Radiation Comitee; IRPA: In-
ternational Radiation Protection Association). Its publications [IRPA 1988], [IRPA 1991] take
a special place as they represent a summary of the Environmental Health Criteria published in
the WHO (World Health Organization) [WHO 1993]. If they have not established nationalstandards themselves, some countries, for example Norway, directly use the IRPA values
(CENELEC survey [CENELEC 1995]).
In all exposure limits safety factors have already been introduced, which partly explains the
differences in the reference levels of the existing worldwide radiofrequency field exposure
standards.
2.3 Distinction Between Areas, Time of Exposure and Frequencies
In nearly all standards a distinction between exposure areas and exposure times is made. There
is a general distinction between two different areas (with different names, but very closemeaning), which are called exposure area number 1 and 2 in the DIN standard and controlled
and uncontrolled environment in the ANSI document.
Controlled environments are locations where there is exposure that may be incurred by persons
who are aware of the potential for exposure as a concomitant of employment, by other cogni-
zant persons. Concerning these areas, the maximum permissible exposure is defined in respect
to human safety. These areas contain:
Controlled areas, for example manufacturing plants
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General accessible areas where it is secured that the exposure is only at short times dueto the operation of equipment or due to the time of stay. Short time means due to DIN
regulation up to 6 hours a day.
The reference levels in uncontrolled environment have been fixed under consideration of addi-
tional safety precautions. These areas include long-term exposure and locations with exposureof individuals who have no knowledge or control of their exposure:
Areas with residential and social buildings Facilities for sports, leisure and relaxation Working places where an electromagnetic field is unexpected.The limits for uncontrolled environment are lower or equal to those for controlled environ-
ment.
In addition to the introduction of different exposure areas a distinction between exposure times
is made. An international limit is made at 6 minutes exposure time ([ANSI 1991]: frequencydependent from 30 minutes at 3 GHz down to 0.62 minutes at 100 GHz). For short-term ex-
posure, higher field strengths are admissible, because it takes a certain time until the human
body warms up.
Due to the influence of frequency on important parameters, as the penetration depth of the
electromagnetic fields into the human body and the absorption capability of different tissues,
the limits in general are frequency dependent.
2.4 Basic Restrictions and Derived Reference Levels
There is a distinction between basic restrictions and derived reference levels concerning allnormative regulations. Basic restrictions are defined for
the specific absorption (SA, dimension: energy/mass), the specific absorption rate (SAR, dimension: power/mass) the electrical current density in the body and the current through the bodybecause they can be referred directly to thermal based biological effects. It has been pointed
out that in the high frequency range especially the specific absorption rate (SAR) is a useful
and a biologically relevant quantity to describe the effect of the electromagnetic field. It is ameasure of the power absorbed per unit mass. The unit of specific absorption rate is watt per
kilogram (W/kg). The SAR may be spatially averaged over the total mass of an exposed body
or its parts, and may be time-averaged over a given time of exposure or even a single pulse or
modulation period of the radiation. A limitation of the specific absorption rate prevents an ex-
cessive heating of the human body by electromagnetic radiation.
As it is difficult to determine these basic quantities directly by measurement, the standards
specify a set of more-readily-measurable reference levels in terms of external electric and mag-
netic field strength and power density, derived from the basic restrictions. These limits have
been fixed so that even under worst case conditions, the basic limits are not exceeded. It mustbe noted that already precaution factors have been introduced into the basic restrictions, which
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are different from each other due to deviations in the rating of the potential of danger of
electromagnetic fields. Thus there exist different values in the limits although all standards con-
sider the latest scientific knowledge.
The most general claim in every standard is: Compliance is established when the basic
limits are not exceeded.
At frequencies between 100 kHz and 6 GHz the limits for the electromagnetic field strengths
may be exceeded if the exposure condition can be shown by appropriate techniques to produce
SARs below the corresponding limits [ANSI 1991].
2.5 Maximum permissible exposure for high frequency devices from 3 GHz to 100 GHz
Having in mind a worst case consideration, all limits listed in the following count for the un-
controlled environment and for long time exposure.
Table 1 contains the relevant basic limits for the specific absorption rate in the frequency range
from 3 GHz to 100 GHz. Because the IRPA standard [IRPA 1991] only considers absorption
rates related to the whole body (0.08 W/kg) this standard is omitted in Table 1.
Standard Status f
[GHz]
Averaging SAR limit
[W/kg]
Reference
DIN VDE 0848
Teil 2, 1991draft 3 - 100 10 g mass 2.0 [DIN 0848 91]
CENELEC
ENV 50166-2,
1995
draft 3 - 100 10 g mass 2.0 [ENV 50166]
ANSI C95.1-
1991
in
force 6
> 6
1 g mass
-
1.6
-
[ANSI 1991]
Table 1: Relevant basic limits for the specific absorption rate (SAR), valid for high frequency
devices in the frequency range of interest from 3 GHz to 100 GHz.
In Table 2 the derived reference levels are listed. In contrast to the European and German
prestandard the ANSI standard defines the exposure to radiofrequency electromagnetic fields
above 6 GHz as quasi-optical. Therefore no SAR limit is valid. On the other hand the equiva-
lent power density is limited as shown in Table 2. The rationale of the ANSI standard is to
define the frequency region from 6 GHz up to 300 GHz as a transition area between the com-
plex field behaviour at the lower frequencies up to 6 GHz and the simple surface heating proc-
ess induced for electromagnetic waves in the optical frequency range. Therefore the ANSI
standard defines a maximum permissible power density for partial body exposure of all parts of
the body except the eyes and testes and device distances no closer than 20 cm, which starts
from 40 W/m2 (3 GHz - 6 GHz) up to 200 W/m2 (30 GHz -100 GHz) and a linear increasing
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function from 6 GHz up to 30 GHz. For comparison: the power density of the sun light
hitting the earth is in the order of 1 kW/m2
[Bohrmann 1993]. The maximum permissible
exposure listed in Table 2 according to the ANSI standard is valid for the eyes and the testes in
order to take into account the worst case.
Standard Status f
[GHz]
Eeff[V/m] Heff[A/m] S
[W/m]
Reference
DIN VDE 0848 Teil 2,
1991
draft 3 - 100 61.4 0.16 10 [DIN 0848
91]
CENELEC
ENV 50166-2, 1995
draft 3 - 100 61.4 0.163 10 [ENV
50166]
ANSI C95.1 in force 3 - 15
15 - 100
50.11f0.5
194.1
0.133f0.5
0.515
f/ 0.15
100
[ANSI 1991]
IRPA in force 3 - 100 61 0.16 10 [IRPA 1991]
Table 2: Relevant derived reference levels for the electromagnetic field valid for high fre-
quency devices in the frequency range of interest from 3 GHz to 100 GHz.
3 Determination of the Relevant Material Parameters
3.1 Literature Survey and Measurement Techniques
Investigations of the dielectric properties of human tissues are presented in the literature since
more than 40 years (e.g. [Schwan 57][Schwan 80][Gabriel 96a]). The results are obtained by
measurements of animal and human tissue in the frequency range up to 90 GHz [Edrich 1976].
Depending on the frequency range of interest three classes of measurement systems have to be
distinguished.
Impedance measurement systems in liquid cells with a typical frequency range from 10 Hzup to 30 MHz. An LCR meter or impedance analyzer is required to measure the relative
permittivity r and the conductivity .
Measurement of the reflection coefficient of an open-ended line, which is immersed in theliquid under test or attached to the solid under test (Fig. 1). The magnitude and phase of
the signal reflected at the open-end depends on the dielectric properties of the material un-
der test. A vector network analyzer measures the reflection coefficient of the sample. A
measurement software converts the measured data into r and . The typical frequency
range is 200 MHz to 20 GHz.
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Measurement of the reflection and transmission coefficients of a line, which is partly filledwith a sample of material. This measurement requires a vector network analyzer too. The
lowest measurement frequency is restricted to about 100 MHz. The upper frequency limit
is not restricted. The measurement system used in this investigation is a W-band system
operating in the frequency range 75 - 100 GHz with rectangular waveguides.
The IMST has material measurement systems at its disposal, which work according to the
both last-mentioned principles. With this systems dielectric properties can be measured in the
frequency range 200 MHz - 20 GHz and from 75 GHz - 100 GHz.
Gabriel [Gabriel 96b] introduced a parametric model, which describes the frequency depend-
ency of the dielectric properties of 17 different human tissues from 10 Hz to 100 GHz. The
model is based on measurements [Gabriel 96a] and on experimental data reported in the
literature [Gabriel 96c]. The frequency range from 10 Hz to 100 GHz is divided into four main
dispersions with different properties. The frequency dependency of each dispersion is described
by a Cole-Cole relation according to equation 1. This enables a closed representation which
can be directly implemented into existing simulation models of the human body. The dielectric
properties of the following tissues are available: blood, bone (cancellous), bone (cortical), brain
(gray matter), brain (white matter), fat (average infiltrated), fat (not infiltrated), heart, kidney
(cortex), lens (cortex), liver, lung (inflated), muscle, skin (dry), skin (wet), spleen, tendon).
In the following Figs. the frequency dependencies evaluated by
( )( )
( )
= +
++
=
n
nn jjn1
11
4
0
(1)
are shown for the relevant tissues.
Fig. 1: Material measurement system II of the IMST. Measurement of the reflection coeffi-
cient of an open-ended coaxial line in the frequency range 200 MHz - 20 GHz.
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101
102
103
104
105
106
107
108
109
1010
1011
Frequency [Hz]
10-4
10-3
10-2
10-1
100
101
102
103
104
105
Tissue: skin (wet)rel. permittivityconductivity [S/m]
Fig. 2: Dielectric properties of skin (wet) as a function of frequency.
101
102
103
104
105
106
107
108
109
1010
1011
Frequency [Hz]
10-4
10-3
10-2
10-1
100
101
102
103
104
105
Tissue: skin (dry)rel. permittivityconductivity [S/m]
Fig. 3: Dielectric properties of skin (dry) as a function of frequency.
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101
102
103
104
105
106
107
108
109
1010
1011
Frequency [Hz]
10-2
10-1
100
101
102
103
104
105
106
107
Tissue: fat (average infiltrated)rel. permittivityconductivity [S/m]
Fig. 4: Dielectric properties of fat (average infiltrated) as a function of frequency.
101
102
103
104
105
106
107
108
109
1010
1011
Frequency [Hz]
10-2
10-1
100
101
102
103
104
105
106
107
Tissue: fat (not infiltrated)rel. permittivityconductivity [S/m]
Fig. 5: Dielectric properties of fat (not infiltrated) as a function of frequency.
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101
102
103
104
105
106
107
108
109
1010
1011
Frequency [Hz]
10-2
10-1
100
101
102
103
10410
510
610
710
8
Tissue: musclerel. permittivityconductivity [S/m]
Fig. 6: Dielectric properties of muscle as a function of frequency.
3.2 Measurement Results in the Frequency Range 75 100 GHz
The experimental investigation of dielectric properties of different human tissues in the fre-
quency range 75 100 GHz was carried out using the above mentioned W-band measurement
system fromDamaskos, Inc.. A rectangular waveguide is partially filled with a small sample of
the biological tissue. The relative permittivity and the conductivity are calculated from themeasured transmission coefficients. Fig. 7 shows the arrangement of the sample in the W-band
measurement system.
The sample has dimensions of 2.54 mm 1.27 mm 1.5 mm (width height length) for
non-liquid biological tissues. For liquid material one end of the sample is attached to a foil, on
the other end the exact shape of the sample is unknown because of the surface tension of the
liquid material (as illustrated in Fig. 7). A first study has shown that this phenomenon results in
a reduction of the effective length of the sample with respect to its transmission characteristics
and therefore has to be considered for the calculation of the dielectric properties. However, in
practice this effective length cannot be determined. In order to minimize the effect of surfacetension, we soaked a piece of cotton wool with the liquid material. The cotton wool has no
significant effect on the calculated dielectric properties. Fig. 8 visualizes a comparison of the
measurement results for water using this procedure and theoretical data from literature
[Gabriel 96c][Duck 1990]. In this case the original length of the sample was used for the cal-
culation of the dielectric properties. The temperature of the sample was T= 27C. This com-
parison shows a reasonable agreement between measurement and theory (Fig. 8). The maxi-
mum deviation from the theoretical model amounts to 30 50%.
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tissue sample (liquid)rectangular waveguide foil
1.2
7mm
2.54
mm
1.50 mm
Fig. 7: Rectangular waveguide partially filled with a liquid sample of biological tissue.
75 8080 8585 9090 9595 100100
Frequency [GHz]
0
20
40
60
80
100
Tissue: water, T=27Ctheoretical rel. permittivity
theoretical conductivity [S/m]measured conductivity [S/m]
measured rel. permittivity
Fig. 8: Theoretical and measured dielectric properties of water. The temperature of the sam-
ple was T= 27C.
3.2.1 Measurement of Dielectric Properties of Skin Tissue
Fig. 9 14 present the measurement results ofin vitro porcine skin, fat and muscle tissue at
temperatures ofT= 27C and T= 37C. The dielectric characteristic of porcine tissue is ex-
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pected to differ not significantly from human tissue, because there is insufficient evidence
to identify consistent variation between species [Duck 1990]. Species-specific variations are
probably masked by other sources of variability (local tissue inhomogeneities, changes follow-
ing death, age). The only tissue showing clear species-dependent variation is skin [Duck 1990].
Our measurements show similar results for porcine skin tissuea and data from human skin tis-sue reported in literature.
In the frequency range from 75 GHz up to 100 GHz all tissues show similar dielectric charac-
teristics as water. The permittivity falls monotonically with frequency and the conductivity
shows in a first approximation a constant curve. Skin and fat are showing a lower conduc-
tivity and relative permittivity than muscle. The measured values of the relative permittivity and
electric conductivity of skin and muscle correspond well with results reported in literature
[Gabriel 1996c] and with the parametric curves from [Gabriel 1996b]. Only fat tissue shows a
significant deviation from the parametric model. This result may be explained by the large
variations for adipose tissue and bone marrow reported in literature. These variations are
caused by the wide range of water content in these tissues [Duck 1990].
For all three tissues the high temperature (T= 37C) measurements showed no significant tem-
perature coefficient for the conductivity and relative permittivity. The changes measured were
within the measurement uncertainties. From literature [Duck 1990] a temperature coefficient of
1-2% can be expected, which results in a variation of 10-20% for both, conductivity and rela-
tive permittivity. This changes are below the resolution of the 75-100 GHz measurement setup,
as discussed in the next section.
75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
Tissue: skin, T=27Cconductivity [S/m]rel. permittivity
Fig. 9: Measured dielectric properties of skin tissue. The temperature of the sample was
T= 27C.
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75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
Tissue: skin, T=37C
conductivity [S/m]rel. permittivity
Fig. 10: Measured dielectric properties of skin tissue. The temperature of the sample was
T= 37C.
75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
Tissue: fat, T=27Cconductivity [S/m]rel. permittivity
Fig. 11: Measured dielectric properties of fat tissue. The temperature of the sample was
T= 27C.
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75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
Tissue: fat, T=37Cconductivity [S/m]rel. permittivity
Fig. 12: Measured dielectric properties of fat tissue. The temperature of the sample was
T= 37C.
75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
100
120
Tissue: muscle, T=27Cconductivity [S/m]rel. permittivity
Fig. 13: Measured dielectric properties of muscle tissue. The temperature of the sample was
T= 27C.
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75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
100
120
Tissue: muscle, T=37C
rel. permittivityconductivity [S/m]
Fig. 14: Measured dielectric properties of muscle tissue. The temperature of the sample was
T= 37C.
3.2.2 Measurement of Dielectric Properties of Eye Tissue
Fig. 15 to 20 present the measurement results ofin vitro porcine eye tissue.
75 80 85 90 95 100
Frequency [GHz]
0
20
40
60
80
Tissue: vitreous body, T=27Cconductivity [S/m]rel. permittivity
Fig. 15: Measured dielectric properties of vitreous body tissue. The temperature of the sample
was T= 27C.
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75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
Tissue: cornea, T=27C
conductivity [S/m]rel. permittivity
Fig. 16: Measured dielectric properties of cornea tissue. The temperature of the sample was
T = 27C.
75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
Tissue: lens, T=27C
conductivity [S/m]
rel. permittivity
Fig. 17: Measured dielectric properties of lens tissue. The temperature of the sample was
T = 27C.
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75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
Tissue: retina, T=27Cconductivity [S/m]rel. permittivity
Fig. 18: Measured dielectric properties of retina tissue. The temperature of the sample was
T = 27C.
75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
100
Tissue: sclera, T=27Cconductivity [S/m]rel. permittivity
Fig. 19: Measured dielectric properties of sclera tissue. The temperature of the sample was
T = 27C.
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75 80 85 90 95 100Frequency [GHz]
0
20
40
60
80
100
Tissue: liquid from cameraanterior, T=27C
conductivity [S/m]
rel. permittivity
Fig. 20: Measured dielectric properties of liquid from camera anterior. The temperature of the
sample was T = 27C.
As already seen for the skin tissues all eye tissues show similar dielectric characteristic as wa-
ter. The permittivity falls monotonically with frequency and the conductivity shows in a first
approximation a constant curve.
The results contain some experimental uncertainties because of the small dimensions of the
sample. Especially liquid tissues show significant loss in water content due to drying effects
during probe preparation and measurement. Therefore measurements at higher temperatures
(i.e. T= 37C) are extremely difficult for liquid material and dropped in this report.
Due to the following effects:
shape of the small probe, inhomogeneity of tissue material, dynamic of the measurement system,
drying of tissue,the uncertainty of the measurement setup can be estimated by about 20-40%.
However, the dielectric properties of biological tissue itself show a significant variability. There
are several factors, which affect the dielectric properties of tissue: post-mortem changes, local
tissue inhomogeneities, age, animal species, and temperature [Duck 1990] [Edrich 1976].
3.3 Measurement Results in the Frequency Range 200 MHz 20 GHz
The experimental investigation of dielectric properties of different human tissues in the fre-
quency range 200 MHz 20 GHz was carried out using the above mentioned measurement
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system II shown in Fig. 1. The dielectric properties are calculated from the measured re-
flection coefficient of an open-ended line attached to the tissue sample.
3.3.1 Measurement of Dielectric Properties of Skin Tissue
Fig. 21 and Fig. 22 show the measurement results of porcine skin tissue for a tissue tempera-ture of 27C and 37C, respectively. Fig. 23 and Fig. 24 show the measurement results of por-
cine fat tissue for a tissue temperature of 27C and 37C, respectively. Fig. 25 and Fig. 26
show the measurement results of porcine muscle for a tissue temperature of 27C and 37C,
respectively. Figures 21-26 include the results from the 75-100 GHz measurement.
Dielectric properties of porcine skin, muscle and fat tissue show characteristics comparable to
data from the parametric model [Gabriel 96b]. Due to the measurement principle which uses a
greater amount of tissues with a well-defined interface at the open-ended line the dielectric
properties obtained in the frequency range from 200 MHz to 20 GHz are more accurate.
Taking into account the uncertainties of the 75-100 GHz measurement both measurements
show consistent results. Only for fat tissue the 75-100 GHz measurement of the dielectric
properties and the 200 MHz-20 GHz measurement show slightly inconsistent behavior. This
may be caused by the great variety of water content of fat tissue reported in [Duck 90]. In or-
der to use fresh tissue the different measurements were conducted using different (fresh) tis-
sues.
0,1 1 10 100
Frequency [GHz]
0
20
40
60
80
Tissue: skin, T=27Crel. permittivityconductivity [S/m]
Fig. 21: Measured dielectric properties of skin. The temperature of the sample was T = 27C.
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0,1 1 10 100
Frequency [GHz]
0
20
40
60
80
Tissue: skin, T=37Crel. permittivityconductivity [S/m]
Fig. 22: Measured dielectric properties of skin tissue. The temperature of the sample was
T = 37C.
0,1 1 10 100
Frequency [GHz]
0
20
40
60
80Tissue: fat, T=27C
rel. permittivityconductivity [S/m]
Fig. 23: Measured dielectric properties of fat tissue. The temperature of the sample was
T = 27C.
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0,1 1 10 100
Frequency [GHz]
0
20
40
60
80
Tissue: fat, T=37Crel. permittivityconductivity [S/m]
Fig. 24: Measured dielectric properties of fat tissue. The temperature of the sample was
T = 37C.
0,1 1 10 100
Frequency [GHz]
0
20
40
60
80
100
120
Tissue: muscle, T=27C
rel. permittivityconductivity [S/m]
Fig. 25: Measured dielectric properties of muscle tissue. The temperature of the sample was
T = 27C.
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0,1 1 10 100
Frequency [GHz]
0
20
40
60
80
100
120
Tissue: muscle, T=37Crel. permittivityconductivity [S/m]
Fig. 26: Measured dielectric properties of muscle tissue. The temperature of the sample was
T = 37C.
3.3.2 Measurement of Dielectric Properties of Eye Tissue
0,1 1 10 100
Frequency [GHz]
0
20
40
60
80
Tissue: vitreous body, T=27Crel. permittivityconductivity [S/m]
Fig. 27: Measured dielectric properties of vitreous body. The temperature of the sample was
T = 27C.
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0,1 1 10 100
Frequency [GHz]
0
20
40
60
80
Tissue: vitreous body, T=37Crel. permittivityconductivity [S/m]
Fig. 28: Measured dielectric properties of vitreous body. The temperature of the sample was
T = 37C.
Fig. 27 and Fig. 28 shows the measurement results for a tissue temperature of 27C and 37C,
respectively. For a sample temperature of 27C the results for the frequency range of 75-
100 GHz are included.As seen in the previous section, the results for both frequency ranges are consistent within
measurement uncertainties and tissue variability. Taking into account this variability it can be
stated that reliable data of the dielectric properties of the tissues under investigation has been
collected for the following analysis of electromagnetic fields in the human skin and eye.
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4 Analysis of the Field Strengths in Human Tissue
4.1 Models of the Objects under Investigation
4.1.1 Layered Model of the Human Skin
The skin of an adult human being covers about 2 m2, and has a thickness between 1.5 mm and
4 mm. Underneath a fat layer (tela subcutanea) is situated, which is located above muscle tis-
sue. The schematic structure of the human skin including the fat layer is shown in Fig. 29. The
epidermis is the outer part of the skin. It is cornified, without vessels, multilayered and nor-
mally dry. The dermis, which represents the rest of the human skin except the fat layer, is the
wet skin region.
In the cm/mm wave range the penetration depth of electromagnetic fields in the human body is
very small. A major part of the electromagnetic energy is absorbed in the surface of the human
body. Because of the high frequency the electromagnetic field can be locally described by a
planar wave. Therefore the field theoretical problem is reduced to a one dimensional investiga-
tion of the field distribution in a layered medium schematically shown in Fig. 30. For this in-
vestigation the field distribution in the human skin on the back is analyzed. This restriction is
made because of the relative bad vascularity and the low density of sweat glands of the human
skin on the back. The typical thickness of the different layers is taken from the literature [FSZ
1985][Lippert 1990]. These values are summarized in Table 3 together with the mass density
of the layers [Dimbylow 1988][Dimbylow 1991].
Fig. 29: Structure of the human skin. The fat layer begins in region 8.
1 mm
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Tissue Mass density
[103
kg/m3]
Thickness [mm]
epidermis 1.1 0.15
dermis 1.1 3.85
fat 0.92 10
muscle 1.04 > 10
Table 3: Thickness and mass density of the different layers of the human skin on the back
including typical values for fat and muscle (40 mm assumed in the simulation) tissue.
For the problem shown in Fig. 30, the oblique incidence of a planar wave on a layered medium,
an analytical solution exists in the literature (e.g. [Balanis 1989]). This algorithm was imple-
mented and a post-processing was made to determine the SAR values according to the ANSI
standard, the DIN and the ENV prestandard respectively.
epidermis
dermis
fatmuscle
Fig. 30: A planar wave hitting the human skin on the back.
4.1.2 Model of the Human Eye
The schematic structure of the human eye is shown in Fig. 31. The tissues of the eye are: retina
(1), choroidea (2), sclera (3), cornea (4), tunica conjunctiva (5), iris (6), corpus ciliare (7), lens
(8), camera anterior (9), camera posterior (10), pupilla (11), vitreous body (12), macula (13),
discus nervi optici (14), and nervus opticus (15).
For the numerical investigation of the field distribution a spherical voxel model of the human
eye has been built up. The eye has a diameter ofd= 20.8 mm and is embedded in muscle and
skin tissue. Fig. 32 shows a cut-plane through this voxel model.
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Fig. 31: Structure of the human eye [Lippert 1990].
cornea
camera
anterior
skin
lens
nervus
opticus
iris
vitreous
body
sclerachoroidea
retina
x
y
z
muscle
4 mm
Fig. 32: Horizontal cut-plane through the voxel model of the human eye.
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4.2 Simulation Methods
4.2.1 Analytical Method
The analytical method used for the calculation of the electromagnetic field in layered media is
taken from [Balanis 1989]. For normal incidence of a plane wave the following terms providethe reflection coefficients
rE
E
Z Z
Z Zmnn
r
n
i
m n
m n
= =
+, (2)
and transmission coefficients
tE
E
Z
Z Zmnn
t
n
i
m
m n
= =+
2(3)
of multiple interfaces, as shown in Fig. 33, with the intrinsic impedances
Zmm
m
=
. (4)
r21
1
air
2
epidermis
3
dermis
4
fat
5
muscle
t21
r12
t12
r32
t32
r23
t23
r43
t43
r34
t34
r54
t54
r45
t45
x2 x3 x4 x5
d2 d3 d4 d5
Fig. 33: Reflection and transmission coefficients in the layered model of skin.
Introducing a phase and an attenuation term upon the traveling E-field of waves propagating in
positive coordinate direction
E x E x e em m m ma x j xm m m m+ + = =( ) ( )0 (5)
and waves propagating in negative coordinate direction
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( ) ( )E x E x d e e
m m m m m
a x d j x d m m m m m m = =( ) ( )
(6)
the distribution of the electric field within the layers is calculated via a ray-tracing model, i.e.
by superposition of the different propagating waves in the media.
4.2.2 The Finite Difference Time Domain Method
The calculations of the electromagnetic fields inside the anatomical model of the human eye
have been carried out using the finite difference time domain (FDTD) method. In 1966 Yee
[Yee 1966] introduced this method, which has become one of the most popular numerical
methods, because of the simplicity and stability of the algorithm. The FDTD is a purely nu-
merically oriented method, which directly discretizes Maxwells equations in the time- and
space-domain with second-order accuracy. According to the unit cell shown in Fig. 34 the
electric field components E are positioned on the middle of the edges and the magnetic field
components H are positioned on the middle of the surfaces. Time-stepping is done in a leap-frog way. The magnetic field H at the time (n+1/2)tis determined from the electric field E at
the time nt, and afterwards the electric field E at the time (n+1)t is determined from the
magnetic field H at the time (n+1/2)t.
Ex
Ey
z
x
y
(i,j+ ,k)
(i+ ,j,k)
Ez(i,j+1,k+ )
Ey(i,j+ ,k+1)
Hx(i,j+ ,k+ )
Hy(i- ,j,k+ )
Ez(i,j,k+ )
Hy(i+ ,j,k+ )
Hx(i,j- ,k+ )
H z(i+ ,j+ ,k)
1
2
12
12
12
12
12
12
12
1
2
1
2
12
12
1
212
1
2
Fig. 34: Lattice unit cell of the Yee-algorithm in Cartesian coordinates.
An important aspect using the FDTD, especially for the solution of radiation problems, is the
availability of an appropriate absorbing boundary condition (ABC), because in contrast to
other numerical methods like the method of moments, the problem space of the FDTD is lim-
ited. To simulate free space conditions a special algorithm has to be evaluated at the outer grid
planes of the FDTD mesh. In 1994 Berenger [Berenger 1994] proposed the perfectly matched
layer (PML) absorbing boundary condition with improved performance in orders of magnitude
compared to other ABCs. This absorbing boundary condition can be placed in the extreme
near-field of the structures under investigation. Therefore this ABC is not only very accuratebut also memory efficient.
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4.3 Simulation of the Field Distribution inside the Human Eye and Skin
4.3.1 Field Distribution in the Human Skin
In this section the SAR distribution inside the human skin according to the model introduced in
Table 3 is analyzed in the frequency range from 3 GHz to 100 GHz. The incident electromag-netic field consists of a linearly polarized plane wave with a power density of 1 mW/cm
2. The
power density was chosen according to the derived reference level valid for frequencies higher
than 3 GHz for uncontrolled environment in the European [ENV 50166] and German prestan-
dard [DIN 0848 91]. The material parameters are taken from [Gabriel 96b].
Fig. 35 shows the frequency dependent SAR value in the human skin in comparison to the
European [ENV 50166] and German prestandard [DIN 0848 91]. It can be seen, that the SAR
value below 20 GHz has a relative complex frequency dependence. The reason for this is the
existence of standing waves in the human skin. Above 20 GHz the SAR value is mainly influ-
enced by the decreasing reflection coefficient of the boundary epidermis-air. This reflectioncoefficient is defined by
rr
r
=
+
1
1
(7)
and r the complex relative permittivity of epidermis. The absolute value is a linear decreasing
function of frequency in the whole frequency range of interest (20 GHz: 0.69, 100 GHz: 0.54).
Comparing the simulated SAR values with the basic restriction for the SAR it can be stated
that there exist a 6 times minimum safety margin when the amplitude of the incident plane
wave is chosen according to the derived reference level.
0 20 40 60 80 100
Frequency [GHz]
0,0
0,5
1,0
1,5
2,0
2,5
S
AR
[W/kg] Human skin:
SAR_10g
ENV and DIN standard
Fig. 35: Simulated SAR values resulting from an incident power density of 1 mW/cm2
inside
the model of the human skin according to Table 3 in comparison to the European
[ENV 50166] and German prestandard [DIN 0848 91].
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The SAR value according to the ANSI standard [ANSI 1991] shows a very similar fre-
quency dependence. Because of the lower averaging mass the simulated SAR values are higher
than the SAR according to the European [ENV 50166] and German prestandard [DIN 0848
91]. Like stated before in section 2.5 the ANSI standard defines the specific absorption rate
only for frequencies up to 6 GHz. For these frequencies a 4 times minimum safety margin ex-ists for the SAR in the human skin. For the higher frequency range the incident power density
Sis the relevant reference level. According to Table 2 Sis restricted to 100 W/m2
for frequen-
cies higher than 15 GHz.
0 20 40 60 80 100
Frequency [GHz]
0,0
0,5
1,0
1,5
2,0
2,5
SAR
[W/kg] Human skin:
SAR_1g
ANSI standard
Fig. 36: Simulated SAR values resulting from an incident power density of 1 mW/cm2
inside
the model of the human skin according to Table 3 in comparison to the American
standard [ANSI 1991].
The maximum SAR values in the different layers of the human skin are shown in Fig. 37-38.
Because of the strong increase of the losses in the human skin as a function of frequency the
highest SAR values are found in the outer layers of the human skin. In the fat and muscle re-
gion noticeable SAR values only exist for frequencies up to 20 GHz. On the other hand the
strong losses lead to high SAR values at the surface of the human skin. As shown in Fig. 37 amaximum of 34 W/kg was found for 100 GHz.
Finally the SAR distribution inside the model of the human skin is shown in Fig. 39-54 for five
frequencies. For the two frequencies at the lower end of the frequency range of interest shown
in Fig. 39 a typical standing wave behavior can be observed for the skin and fat region. Be-
cause of the low permittivity and conductivity of the fat layer a sharp discontinuity appears at
the boundaries dermis-fat and fat-muscle. For 3 GHz the maximum SAR value is not found at
the surface of the skin but at the boundary dermis-fat. The SAR values for the three higher
frequencies depicted in Fig. 54 are linearly decreasing. The highest values are found at the
surface and nearly all energy is absorbed in the outer skin region. For 77 GHz a decrease of theSAR of nearly 10 decades is observed from the epidermis layer to the fat layer.
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0 20 40 60 80 100Frequency [GHz]
0
5
10
15
20
25
30
35
SAR
[W/kg]
Human skin:epidermis
dermis
Fig. 37: Simulated maximum of the SAR values resulting from an incident power density of
1 mW/cm2
inside the outer layers of the human skin according to Table 3.
0 20 40 60 80 100
Frequency [GHz]
0,00
0,02
0,04
0,06
0,08
0,10
0,120,14
0,16
0,18
SAR
[W/kg]
Human skin:fatmuscle
Fig. 38: Simulated maximum of the SAR values resulting from an incident power density of
1 mW/cm2
inside the inner layers of the human skin according to Table 3.
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0 10 20 30 40x [mm]
-50
-40
-30
-20
-10
0
SAR
[dBW/kg]
Frequency:3 GHz6 GHz
Fig. 39: SAR distribution inside the model of the human skin according to Table 3 resulting
from an incident power density of 1 mW/cm2.
0 5 10 15 20x [mm]
-100
-80
-60
-40
-20
0
20
SAR
[dBW/kg
]
Frequency:24 GHz
77 GHz100 GHz
Fig. 40: SAR distribution inside the model of the human skin according to Table 3 resulting
from an incident power density of 1 mW/cm2.
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4.3.2 Field Distribution in the Human Eye
The FDTD-model of the human eye is shown in Fig. 32. In Table 4 the relative permittivity and
the electrical conductivity of eye and skin tissue are summarized for a frequency of
f= 77 GHz. For the excitation a plane wave described by the phasor of the electric field
E e= E ezj x
00 (8)
with a power density of S= 1 mW/cm is applied. Fig. 41 shows the distribution of the specific
absorption rate in thexy-plane, Fig. 42 in the xz-plane and Fig. 43 in the yz-plane. Due to the
strong attenuation of the wave inside the eye and skin, most of the power is absorbed in the
superficial tissues. The field distribution in Fig. 41 (xz-plane) and in Fig. 42 (yz-plane) are very
similar. In addition this symmetry is visible in Fig. 43.
In Table 5 and Fig. 44 the maximum values of the specific absorption rate (SAR) are displayed
in the different tissues of the model as well as the 1g - averaged SAR value for all tissues. The
maximum SAR value occurs in the cornea.
Tissue Mass density
[103
kg/m3]
r [S/m]
muscle 1.04 19.84 106.22
sclera 1.1 22.49 76.76
choroidea 1.06 19.84 106.22
retina 1.035 8.41 54.60
vitreous body 1.006 10.33 40.9
cornea 1.06 5.82 56.27
camera anterior 1.006 5.08 50.02
iris 1.058 19.84 106.22
lens 1.1 14.25 29.44
nervus opticus 1.035 19.84 106.22
skin 1.1 11.67 55.60
Table 4: Dielectric parameters of model of the human eye for a frequency off= 77 GHz.
The mass density has been taken from [Dimbylow 1988][Dimbylow 1991][Flindt
1995] and [Geigy 1985]. r and are measured by IMST.
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0 2 4 6 8 10 12
x [mm]
4
6
8
10
12
14
16
18
20
y
[mm]
SAR in db W/kg10+-2,5 bis 10-15 bis -2,5-27,5 bis -15-40 bis -27,5-52,5 bis -40-65 bis -52,5-77,5 bis -65-90 bis -77,5
Fig. 41: Distribution of the specific absorption rate in thexy-plane of the eye.
0 2 4 6 8 10 12
x [mm]
-8
-6
-4
-2
0
2
4
6
8
z
[mm]
SAR in db W/kg10+-2,5 bis 10-15 bis -2,5-27,5 bis -15-40 bis -27,5-52,5 bis -40-65 bis -52,5-77,5 bis -65-90 bis -77,5
Fig. 42: Distribution of the specific absorption rate in thexz-plane of the eye.
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2 4 6 8 10 12 14 16 18 20 22
y [mm]
-10
-8
-6
-4
-2
0
2
4
6
8
10
z
[mm]
SAR in db W/kg10+-2,5 bis 10-15 bis -2,5-27,5 bis -15-40 bis -27,5-52,5 bis -40-65 bis -52,5-77,5 bis -65-90 bis -77,5
Fig. 43: Distribution of the specific absorption rate in theyz-plane (10 mm into the body from
the surface of the skin).
muscl
escl
era
choro
idearet
ina
vitreo
usbo
dycorne
a
amera
anter
ior irislen
s
nervu
sopti
cus
skin
1E-005
0,0001
0,001
0,01
0,1
1
10
100
SA
R[W/kg]
Fig. 44:. Maximum local SAR values in the different tissues of the human eye model
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The calculations show similar results for the layered skin model and for the three dimensional
model of the human eye for a frequency of f= 77 GHz. In both cases the absorption is domi-
nant in superficial tissues and the 1g-averaged SAR values are comparable: for the layered skin
model the 1g-averaged SAR value is 0.6 W/kg and for the model of the human eye the local
SAR value amounts to 0.6588 W/kg. The same applies to the maximum local SAR values: for
the layered model of skin the maximum local SAR value is 27 W/kg and for the model of the
human eye the local SAR value amounts to 45 W/kg in cornea tissue. Small differences in the
calculations result from different material parameters which were used in the different models:
the calculations in the layered model of skin are based upon dielectric parameters taken from
[Gabriel 96c], because it provides data in a wide frequency range. The simulations of the field
distribution in the human eye are based on measured dielectric parameters from IMST. There-
fore, the maximum local SAR value in skin tissue is slightly higher and amounts to 32.24
W/kg.
Due to the high absorption of the electromagnetic fields in superficial tissue the averaged val-
ues for higher frequencies are determined mainly by the parameters of the first layer of the
model including the reflection coefficient of the interface air-skin.
Tissue SAR [W/kg]
muscle 2.31810-5
sclera 32.49
choroidea 1.714
retina 4.95810-2
vitreous body 1.09710-2
cornea 45.11
camera anterior 0.3657
iris 0.921
lens 1.5810-3
nervus opticus < 110-5
skin 32.24
all eye tissue SAR_1g = 0.6588
Table 5: Maximum local SAR values in the different tissues of the eye model for plane wave
excitation (normal incidence) with 1 mW/cm.
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5 Investigation of Thermal Effects
5.1 Infrared Thermography
For the investigation of thermal effects in the human skin and in the porcine eye a high-speed
thermal image system (Thermo Tracer TH 2111, NEC San-ei Instruments, Ltd.) is used. This
system is a non-contact type infrared thermometer.
All objects above the absolute zero (-273C) continuously radiate infrared energy. Therefore,
infrared rays are closely related to the temperature of physical bodies. The detector unit of the
thermal image system scans the surface of an object and collects the infrared energy by an in-
frared objective lens. After chopping this infrared energy with a reference temperature source it
is converted to an electrical signal using an infrared HgCdTe-detector. The infrared detector
remains cooled to -196C by liquid nitrogen and is capable of converting infrared energy with
high sensitivity. For more information about IR thermography see [Cho 1992].
The main performance specifications of the system are as follows:
Temperature resolution: 0.1C (for blackbody at 30C), 0.02C (in S/N improvement mode,for blackbody at 30C).
Frame time: 20 frames / s. Detector unit: HgCdTe (liquid nitrogen cooling type), measurement wavelength: 8-13 m
(half value width).
5.2 Human Skin
5.2.1 Experimental setup
A generator based on a Gunn diode oscillator with output powers up to 38 mW was used as a
source of millimeter electromagnetic irradiation. The frequency of the continuous wave (CW)
signal is f = 77 GHz and the aperture of the rectangular horn antenna amounts to an area of
1.5 cm 1.1 cm. The gain of the horn antenna is G = 20 dBi. An estimation of the power den-
sity yields S= 1 mW/cm in a distance ofd= 17.3 cm in front of the aperture and a power den-
sity of 10 mW/cm in a distance of d= 5.5 cm. (S= 10 mW/cm represents the maximum per-
missible exposure due to the ANSI Standard [ANSI 1991] and S= 1 mW/cm for the DIN-
VDE prestandard [DIN 0848 91]).
The antenna is aimed at the forearm of a volunteer and the time-dependent temperature field of
the region of interest is recorded by the thermal image system. The schematics of the experi-
mental setup are shown in Fig. 45. All experiments were conducted in an anechoic chamber.
Before the measurement the volunteers had a time of rest in order to wait for thermal equilib-
rium in the human forearm. The thermophysiological response of the skin in-vivo were deter-
mined for different distances dbetween skin and aperture. The power was set to the maximum
ofP = 38 mW for all measurements. The ambient temperature during the different measure-
ments was between 21C and 22C, but was constant (0.1C) for each measurement. The
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relative humidity of the air was between 50% and 60%. There was essentially no air mo-
tion in the laboratory.
Due to (a) small motion artifacts, (b) the resolution of the thermal image system and (c) slight
changes in the ambient conditions the accuracy of the measurement is about 0.1-0.2C in the
region of interest.
skin
horn antenna
generator
IR camera
d
Fig. 45: Block diagram of the experimental setup.
5.2.2 Measurement results
The measured temperature changes in the skin of two volunteers are summarized in Fig. 46.
The measurements show temperature changes up to 2.2C for a distance of d= 18 mm and a
strong decay in temperature rise for greater distances. For d> 10 cm no significant tempera-
ture change occurred.
Fig. 47 (A) and (B) show, as an example, the temperature field of the human forearm beforeand after seven minutes of irradiation for the female subject and a distance ofd= 2 cm. Sub-
traction images of (A) and (B) are shown in Fig. 47 (C) and (D). To make the spatial extension
of the temperature rise more clear subplot (D) contains four isothermal lines. The correspond-
ing time course of the cursor temperature ( = region with maximum temperature rise) is de-
picted in Fig. 48. During irradiation the temperature shows an exponential increase and reaches
steady state conditions after four to seven minutes of exposure. After irradiation the tempera-
tures return to the equilibrium temperature.
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10 20 30 40 50 60 70 80
Distance d[mm]
0
0,5
1
1,5
2
2,5
T
[C]
Maximum temperature changein the human skin (P = 38 mW)
volunteer 1 (male)volunteer 2 (female)
d
Fig. 46: Temperature changes in the human skin caused by RF irradiation.
(A) before irradiation (B) after 7 minutes of irradiation
(C) subtraction image (D) subtraction image with isolines
35.3
34.3
33.3
32.3
31.3
2.0
1.5
1.0
0.5
0.0
T [C]
T [C]
35.3
34.3
33.3
32.3
31.3
T [C]
T [C]
2.0
1.0
0.0
-1.0
-2.0
Fig. 47: Temperature field (A) before and (B) after seven minutes of irradiation. (C) Subtrac-
tion image of (B) and (A). (D) Subtraction image with isothermal lines.
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OFF ON OFF
0 3 6 9 12 15
Time [min]
32,5
33
33,5
34
34,5
Temperature[C]
d = 2 cm, skin
Fig. 48: Time course of cursor temperature for d= 20 mm and P = 38 mW.
5.2.3 Discussion
The measurements revealed local changes in skin temperature up to 2.2C for irradiation in a
distance ofd= 18 mm from the radiation source. In order to assess the measured temperaturechanges some explanatory notes on skin temperature have to be made. The temperature of the
human skin is determined by thermophysiological concerns of the human body, which depend
on several factors, especially ambient conditions and clothing. For a naked subject an increase
of the ambient temperature from 20C to 30C causes a linearly rise of the mean skin tem-
perature from 30C to 34C. In the extremities this dependency is more obvious: For a naked
subject an increase of the ambient temperature from 20C to 30C causes a linearly rise of the
feet skin temperature from 23C to 33C [Aschoff 1971].
Despite of this dependency there is a shift in skin and core temperature due to diurnal changes
of body temperature [Werner 1984][Aschoff 1971]. The amplitude of core temperature varia-tion is up to 2C.
In our study the measured changes in skin temperature were below the threshold for warmth
sensation of the two volunteers.
Considering the above mentioned facts, no adverse effects in the human skin are expected from
the thermal point of view for distances d 2 cm. For comparison: the maximum permissible
exposure due to [DIN 0848 91] and [ANSI 1991] is exceeded for d< 17.3 cm and d< 5.5 cm,
respectively.
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5.3 Porcine Eye
5.3.1 Experimental setup
The experimental setup for the thermal investigation of porcine eyes is similar to that in the
previous section. The schematics of the experimental setup are shown in Fig. 49. The eye isinserted into a polystyrene layer and positioned over a warm water bath ( Twater = 37C). The
experiment was repeated without the warm water bath in order to determine the influence of
the physiological temperature range on thermal performance.
The power of the radiation source was set to the maximum power ofP = 38 mW for all meas-
urements. The ambient temperature during the different measurements was between 21C and
22C, but was constant (0.1C) for each measurement. The relative humidity of the air was
between 50% and 60%. Using this setup the maximum temperature rise for three different dis-
tances was investigated.
polystyrene
horn antenna
generator
IR camera
d
eye
warm water bath
(optional)
Fig. 49: Block diagram of the experimental setup.
5.3.2 Measurement results
Fig. 50 illustrates a temperature rise of up to 1.9C for a distance ofd= 1.1 cm. With increas-
ing distance the temperature rise decreases strongly. For d> 10 cm no significant temperature
rise occurred. The curves for the eye with and without water bath show a similar behavior. The
warm water did not affect the thermal performance of the temperature dynamic. Fig. 51 showsIR images of the porcine eye without warm water bath: (A) before irradiation, (B) after seven
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minutes of irradiation. (C) and (D) show subtraction images of (A) and (B) and demon-
strate the local heating effect.
10 20 30 40 50 60 70 80
Distance d[mm]
0
0,5
1
1,5
2
T
[C]
Maximum temperature changein a porcine eye (P = 38 mW)
eye (without warm water bath)eye (with warm water bath)
d
Fig. 50: Temperature changes in a porcine eye caused by RF irradiation.
(A) before irradiation (B) after 7 minutes of irradiation
(C) subtraction image (D) subtraction image
22.5
20.5
18.5
16.5
14.5
2.87
1.88
0.87
-0.12
-1.13
T [C]
T [C]
22.5
20.5
18.5
16.5
14.5
T [C]
T [C]
4.0
2.0
0.0
-2.0
-4.0
Fig. 51: Temperature field (A) before and (B) after seven minutes of irradiation. (C) Subtrac-
tion image of (B) and (A). (D) Subtraction image with reduced temperature range.
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OFF ON OFF
0 3 6 9 12 15 18
Time [min]
15
15,5
16
16,5
17
17,5
Temperature[C]
d = 11 mm
Fig. 52: Time course of cursor temperature for d= 11 mm and P = 38 mW.
Fig. 52 shows the time plot of the cursor temperature during exposure of the eye without
warm water bath.
5.3.3 Discussion
It is well known that microwave radiation can cause injury to the eye. In the past several ex-
perimental investigations have been carried out to determine time and power thresholds for
cataractogenesis [Rosenthal 1976].
Our measurements show no significant temperature rise for distances d between radiation
source and eye greater than 10 cm. Therefore, this distances are uncritical. Distances
d< 10 cm need further investigation from the biological point of view. The maximum permis-
sible exposure due to [DIN 0848 91] and [ANSI 1991] is exceeded for d< 17.3 cm and
d< 5.5 cm, respectively.
5.4 Simulation of Thermal Effects
5.4.1 Mathematical Model of Bio-Heat-Transfer
In 1948 H. Pennes proposed a mathematical model for heat transfer processes in blood per-
fused tissue. Although more complex models for heat transfer processes have been developed
Pennes approach has been refined and is still being used today [Cho 1992]. Pennes model de-
scribes the effect of blood flow on tissue temperature on a continuum basis. Therefore a heat
source/sink term is introduced in the heat equation:
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( ) ( )
c
T
tT MR SAR c T T p b b a= + + + .
(9)
The parameters of this equation are: mass density of tissue, cp specific heat capacity of tissue,
thermal conductivity, MR heat generation rate according to metabolic processes( = metabolic rate), perfusion rate, b mass density of blood, cb specific heat capacity of tis-
sue, Ta arterial temperature, and SAR the specific absorption rate.
On the surface of the body Cauchy boundary conditions are applied in order to account for the
heat loss to the environment:
( )n qT uT T = , (10)
with the heat transfer coefficient , the heat flow q , the ambient temperature Tu, and the out-
ward unit normal vector n. The heat transfer coefficient contains the four heat loss mecha-
nisms: radiation, convection, conduction, and evaporation. The differential equation with initial
and boundary conditions is solved using Finite-Element method (FEM) [Gustrau 1997].
5.4.2 Layered Model of Skin
Fig. 53 shows a Finite-Element model for the one dimensional analysis of the heat transfer pro-
cesses in the skin. The thickness of the layers is chosen according to Table 3. The specific ab-
sorption rate SAR displayed in Fig. 38 is scaled to an incident power density ofS= 10 mW/cm
(according to [ANSI 1991]) and applied to the thermal model.
Table 6 shows the thermal parameters of the three different tissues. The high perfusion rate of
muscle identifies the isothermal core region of the body. The additional parameters are:Ta = 36C, cb = 3350 J/(kgK), and b = 1000 kg/m.
Thermoregulatory effector mechanisms, i.e. changes in physiological parameters like perfusion
rate caused by the thermal impact, are not considered.
x
y
z
muscle
fat
dermisepidermis
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Fig. 53: Layered Finite-Element model of the human skin for the one-dimensional numeri-
cal investigation.
Table 6: Thermal properties of skin tissue.
5.4.3 Results
Fig. 54 shows the time plot of the superficial temperature change in the layered model.
-5 0 5 10 15 20 25
t[min]
-0,1
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
T[C]
Skin temperature changewithout fat layerwith 1 cm fat layer
Start of irradiation
Fig. 54: Time plot of the superficial skin temperature change for the layered model of skin.
Simulation was carried out with and without fat layer.
Epidermis dermis fat muscle
cp [J/(kgK)] 3350 3350 3350 3350
[W/(mK)] 0.5 0.5 0.5 0.5
[kg/m] 1000 1000 1000 1000
MR [W/m] 0 200 200 200
[m/(sm)] 0 0.0002425 0.0002425 10000
[W/(mK)] 12 0 0 0
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For comparison the fat layer has been omitted in a second calculation. It turns out that the
isolation of the fat layer influences both, the amplitude and the dynamic of the heat balance
process. For the first calculation a steady-state temperature change ofT= 0.84C is achieved
after 15 minutes of exposure, for the second calculation a temperature rise ofT= 0.4C is
achieved after 3 minutes.
5.4.4 Discussion
This simulation results for a power density ofS= 10 mW/cm have to be compared with the
measurement of skin temperature changes for a distance d= 5.5 cm. The interpolation of the
measurement data (see Fig. 46) shows a temperature change of T 0.7C and the thermal
simulations predict a value between 0.4C and 0.84C depending on the consideration of a fat
layer. In view of the natural span of measurement data due to inter- and intraindividual differ-
ences of the volunteers and in view of the simplicity of the model which considers only one-
dimensional heat transfer the results of the thermal measurements and simulations provide con-
sistent results for the assessment of thermal effects of mm-wave irradiation.
6 Summary
This investigation, which was carried out on behalf of the FGF at IMST, quantified the specific
absorption rate in skin and eye tissue and the resultant thermal effects of cm/mm wave radia-
tion. Additionally, it contains the determination of the dielectric parameters of human tissue in
the frequency range from 200 MHz up to 20 GHz and from 75 GHz up to 100 GHz.
The tissues under investigation included muscle, fat, skin (dermis), cornea, retina, lens, sclera,vitreous body, and liquid from camera anterior. First of all a literature study was made con-
cerning the dielectric parameters of human tissue. In a next step the unknown dielectric pa-
rameters were measured using porcine tissue with material measurement systems in the fre-
quency range of interest. The measured material parameters and data from human tissue re-
ported in literature showed a good agreement.
The measured material parameters and additional data from literature were applied to the
analysis of field strengths in the human eye and skin. The exposure that was realized was a
linearly polarized plane wave. For the human skin an analytical method was used for the cal-
culation of the electromagnetic field in a layered model of skin in the frequency range 3 -
100 GHz. Besides epidermis and dermis, fat and muscle tissue were distinguished. The highest
values of the specific absorption rate were found in the outer layers of the human skin because
of the strong increase of the losses in the human skin as a function of frequency. For 77 GHz a
decrease of the SAR of nearly 10 decades was observed from the epidermis layer to the fat
layer. For a power density of 1 mW/cm2
the strong losses led to a maximum SAR of 34 W/kg
at 100 GHz and 27 W/kg at 77 GHz. The 10g-averaged SAR-values were about 0.27 W/kg
and therefore more than 7 times below the maximum permissible SAR value of 2 W/kg
[DIN 0848 91]. Due to the strong absorption of the electromagnetic fields in superficial tissue
the averaged values for higher frequencies were determined mainly by the parameters of the
first layer of the model including the reflection coefficient of the interface air-skin.
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The investigation of the human skin has been carried out using material parameters from
the parametric model of Gabriel [Gabriel96b] because it provides a closed form over the whole
frequency range of interest. Of course, the choice of dielectric properties influences the com-
putational results. This effect can be estimated by the transmission coefficient of the interface
air-skin. The simulations show, that the highest SAR value occurs at the surface of the skin fora frequency of 100 GHz. For this frequency increasing the complex permittivity by 50 % leads
to a 11 % higher maximum SAR value. Decreasing the complex permittivity by 50 % leads to a
16 % lower maximum SAR value.
The human eye was modeled as a rotary body with the tissues mentioned above. The calcula-
tion was carried out using the FDTD method at a frequency of 77 GHz and plane wave expo-
sure with a power density of 1 mW/cm2. At this frequency the calculations of the three dimen-
sional model of the human eye and the layered skin model showed similar results. In both cases
the absorption was dominant in superficial tissues and the 1g-averaged SAR values were com-
parable: for the layered skin model the 1g-averaged SAR value was 0.6 W/kg and for themodel of the human eye this value amounted to 0.6588 W/kg. The same applied to the maxi-
mum local SAR values: for the layered model of skin the maximum local SAR value was
27 W/kg and for the model of the human eye the local SAR value amounted to 45 W/kg in
cornea tissue. This allows the conclusion that at high frequencies f > 30 GHz it is not neces-
sary to model the whole eye in great detail but it is sufficient to consider its surface region.
For the investigation of thermal effects an infrared thermography system was used. A radiation
source with a horn antenna was aimed at the skin of the forearm of two volunteers and at a
porcine eye in vitro. For this radiation source a power density of 1 mW/cm2
[DIN 0848 91]
corresponds to a distance of 17.3 cm and a power density of 10 mW/cm
2
to a distance of5.5 cm. The measured temperature changes in the skin showed temperature changes up to
2.2C for a distance ofd= 18 mm between aperture and skin and a strong decay in tempera-
ture rise for greater distances. The temperature changes were beneath the warmth sensation
thresholds of the volunteers. The measured temperature changes in the eye showed tempera-
ture changes up to 1.9C for a distance ofd= 11 mm between aperture and eye and a strong
decay in temperature rise for greater distances. For d> 10 cm no significant temperature
change occurred for skin and eye.
Finally the calculated distribution of the specific absorption rate was introduced in a thermal
model. The thermal calculations were based on the Finite Element Method. Two skin models
were analyzed: the layered model presented before and a model without fat layer. The optional
isolating fat layer influenced the dynamic and amplitude of the heat balance process. The
simulations showed temperature changes in the same order of magnitude as the measurement:
for a power density of 10 mW/cm2
[ANSI 1991] the temperature rise was about 0.7C in the
measurement and between 0.4C and 0.84C in the simulation.
The investigation of thermal effects showed a temperature rise in the order of 0.7C for a
power density of 10 mW/cm2
[ANSI 1991]. For the more restrictive German and European
standards a permissible power density of 1 mW/cm2
showed temperature rises in the order of
0.1C. In real world applications the typical power density is up to 60 W/cm2
(e.g. automo-
tive radar), which is far below the amplitudes mentioned above.
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7 References
[Aschoff 1971] J. Aschoff, B. Gnther, K. Kramer: Energiehaushalt und Temperaturregulation, in Gauer,
Kramer, Jung (Hrsg.), Physiologies des Menschen, Vol. 2, pp. 43-116, Urban und Schwar-
zenberg, Mnchen, 1971.
[ANSI 1991] ANSI C95.1: IEEE Standard for Safety Levels with Respect to Human Exposure to Radio
Frequency Electromagnetic Fields, 3 kHz to 300 GHz, Inst. of Electrical and Electronics
Engineers, Inc., 1991.
[Balanis 1989] C. A. Balanis: Advanced Engineering Electromagnetics, Chapter 5, New York, John Wiley
& Sons, pp. 180-253, 1989.
[Berenger 1994] J.-P. Berenger: A Perfectly Matched Layer for the Absorption of Electromagnetic Waves, J.
Comput. Phys., Vol. 114, pp. 185-200, 1994.
[BImSchV 1996] Sechsundzwanzigste Verordnung zur Durchfhrung des Bundes-Immissionsschutzgesetzes:
Verordnung ber elektromagnetische Felder - 26. BImSchV, 1996.
[Bohrmann 1993] S. Bohrmann, R. Pitka, H. Stcker, G. Terlecki: Physik fr Ingenieure, Verlag Harri
Deutsch, Frankfurt am Main, 1993.
[Cho 1992] Y.I. Cho, J.P. Hartnett, T.F. Irvine (Eds): Bioengineering Heat Transfer, Advances in Heat
Transfer, Vol. 22, Academic Press, San Diego, 1992.
[Dimbylow 1988] P. J. Dimbylow: The calc1ulation of induced currents and absorbed power in a realistic,
heterogeneous model of the lower leg for applied electric fields from 60 Hz to 30 MHz,
Phys. Med. Biol., Vol. 33, pp. 1453-1468, 1988.
[Dimbylow 1991] P. J. Dimbylow and O. P. Gandhi: Finite-difference time-domain calculations of SAR in a
realistic heterogeneous model of the head for plane-wave exposure form 600 MHz to 3 GHz,
Phys. Med. Biol., Vol. 36, pp. 1075-1089, 1991.
[DIN 0848 91] Deutsche Norm (Entwurf): Sicherheit in elektromagnetischen Feldern, Schutz von Personen
im Frequenzbereich von 30 kHz bis 300 GHz, DIN VDE 0848 Teil 2, Oktober 1991.
[Duck 1990] F.A. Duck: Physical properties of tissue, Academic Press, San Diego, 1990.
[Edrich 1976] J. Edrich and P. C. Hardee: Complex permittivity and penetration depth of muscle and fat
tissues between 40 and 90 GHz. IEEE Trans. Microwave Theory Tech., MTT-24, pp. 273-
275, 1976.
[ENV 50166] European Prestandard ENV 50166-2: Human exposure to electromagnetic fields - High
frequency (10 kHz to 300 GHz), CENELEC, January 1995.
[FCC 1996] Federal Communications Commission: Report and order: Guidelines for evaluating the
environmental effects of radiofrequency radiaton, Tech. Rep. FCC 96-326, FCC, 1996.
[Flindt 95] R. Flindt: Biologie in Zahlen, Fischer Verlag, Stuttgart, 4. ed., 1995.
[FSZ 1985] K. Fleischhauer, J. Staubesand and W. Zenker: Benninghoff Anatomie 3, Chapter 23 and
27, Urban+Schwarzenberg, Mnchen, pp. 485-571, 1985.
[Gabriel 96a] S. Gabriel and R. W. Lau and C. Gabriel: The dielectric properties of biological tissue: II.
Measurements in the frequency range 10 Hz to 20 GHz. Phys. Med. Biol., Vol. 41, pp.
2251-2269, 1996.
[Gabriel 96b] S. Gabriel and R. W. Lau and C. Gabriel: The dielectric properties of biological tissue: III.
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