1999.01. Final Report Biological Effects in the Mm Wave Range

7/31/2019 1999.01. Final Report Biological Effects in the Mm Wave Range

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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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Biological Effects in the cm/mm Wave Range, Part II/III 2

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


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


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]


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]


T = 37C.

0,1 1 10 100

Frequency [GHz]

0

20

40

60

80Tissue: fat, T=27C



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]


T = 37C.

0,1 1 10 100

Frequency [GHz]

0

20

40

60

80

100

120

Tissue: muscle, T=27C



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]


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.


25/54


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.


26/54


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


27/54


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.


28/54


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.


29/54


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


30/54


( ) ( )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.


31/54


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].


32/54


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.


33/54


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.


34/54


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.


35/54


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.


36/54


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]


Fig. 42: Distribution of the specific absorption rate in thexz-plane of the eye.


37/54


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]


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


38/54


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.


39/54


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


40/54


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.


41/54


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.


42/54


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.


43/54


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


44/54


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.


45/54


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.


50/54


7 References

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zenberg, Mnchen, 1971.

[ANSI 1991] ANSI C95.1: IEEE Standard for Safety Levels with Respect to Human Exposure to Radio

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[Duck 1990] F.A. Duck: Physical properties of tissue, Academic Press, San Diego, 1990.

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Documents

1999.01. Final Report Biological Effects in the Mm Wave Range