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Modeling of RF exposure in children

J. Wiart, A. Hadjem, N. Gadi ,

A Pradier and C. Dale

R&D Division of France Telecom

Joe.wiart@francetelecom.com

38 40 Rue du Général Leclerc

92794 Issy Moulineaux Cedex 9 France.

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Outline

Uncertainties linked to SAR estimation

Specificity of children

Analysis of SAR in children head

Analysis of SAR in foetus

Conclusion

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Specific Absorption Rate Estimation

SAR EstimationHeterogeneous tissues via numerical methodHomogeneous tissues via experimental or numerical tissues

ρσ2

2ESAR =

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Usual SAR calculation

Phantom

Handset

Electric field strength

Num

eric

al m

etho

d FD

TDFD

TD

SAR

http://www.fcc.gov/fcc-bin/dielec.shTissues properties: .

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Example of results:SAR in tissues

Adult head

Tissues abs. Power SAR 10g

Skin 34 %Skull 7 %CSF 4 %

Brain 10 %Muscle 24 %

Skin 46 %Skull 5.7 %CSF 2.4 %

Brain 8 %Muscle 24.5 %

0.39 W/kg

1800MHz

0.67 W/kg

900 MHz

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Uncertainties linked to the Calculation

Handset location

Handset Modelling

Head modelRepresentative ? Conservative ?

+Intercomparisonshowed differences larger than 30%

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Questions linked to children

What are the characteristics of the RF power deposited in children and mainly in the children head?

Are the dosimetric analysis developed for adult valid for children?

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SAR and Children

From 1992 to 2004 more than 10 studies(e.g. : Kuster & al 1992, Shönborn & al 1998 , Gandhi & al 2001, Wang & al 2004) have been done to analyse the energy absorption of RF in the head of children compared to the absorption in adult These studies show that the model of head, the phone model and the position of the handset relative to the head are sensitive parameters

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First approach :homogeneous down scaling

Adult

80%

BUTBUTA children head is not a small adult head and the homogeneous down scaling induces anatomical

errors.

Children MRI are difficult to obtain.Homogeneous down scaling of adult head model has been the first approach

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Body Shape vs age.

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Head chape vs age.

Shape changes

Proportions are different

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Second approach :Non uniform down scaling

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Non Uniform transformation

4 y.o 12 y.o Adult

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Example of resultsCheek positionCheek position

Adult ----- Like Child head 5 years old from VHMax SAR over 10g : 0.67W/kg Max SAR over 10g : 0.92W/kg

Tissues abs. Power SAR10g contiguous abs. Power SAR10g contiguousSkin 34 % 1.20 W/kg 28 % 1.03 W/kgMuscle 24% 0.44W/kg 25 % 0.42 W/kgSkull 7.0 % 0.16W/kg 6.5% 0.15 W/kgCSF 4 % 0.19 W/kg 5 % 0.22W/kg

900MHz

Brain 10 % 0.33 W/kg 17. % 0.2 W/kg

Remarks :

- The positioning is a very sensitive parameter

- The thickness of the ear is important

- These results have to be compared to SAM

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Limits of the non uniform

Adult12 y 4 y

…. Still Visible Human …

Moreover a representative external shape does not mean that internal tissues are well modelled

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Organs have specific growthFrom 5 to 20 the brain volume increase of

about 10% but skull thickness is increase of more than 70%

From 3 to 20 the skin thickness is increase of 60%

Skin thickness (mm) vs age

0

0,5

1

1,5

2

2,5

2 - 3 y.o 11- 13 y.o Adult

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Children head model from MRI are requested

MRI based phantom are required because

> The uniform down scaling is not representative> The non uniform down scaling need to be validated

Compared to MRI based head model The domain of validity must be analysed.

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MRI based Children Head model Models exist or are in development (eg www.tsi.enst.fr/adonis/ is planning child head model, from MRI, at different age)

The representativity is still a question

Examples

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Variability of the morphology

Height In cm

∆ / mean (in σ)

Equi Age In years

Width In cm

∆ mean (in σ)

Equi Age In years

4 y.o 18.5 -0.6 3 18.5 +0.5 8 12 y.o 21 - 1 10 19.5 +0.9 15

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Influence of the head model

SOURCE : Dipole « cheek » position 1.5 cm from tissues

Freq 900 MHz Dipole Head Model 12 y.o from Visible Human 12 y.o from MRI 12 y.o from a 4 y.o MRI based

Tissues Abs Power SAR over 10g Abs Power SAR over 10g Abs Power SAR over 10gSkin 31% 1,93 29% 1,39 27% 2,27 Muscle 27% 1,10 24% 0,92 12% 0,46 Skull 6% 1,24 9% 0,29 5% 0,65 CSF 5% 0,55 11% 0,74 37% 0,95 Brain 17% 0,46 27% 0,48 17% 1,14

Max SAR over 1 g of contiguous tissues

0,001,002,003,004,005,00

SkinMus

cle Skull

CSF

Brain

W/k

g

12 y.o from VH

12 y.o from MRI

12 y.o from a 4 y.o(MRI based)

Max SAR over 10g

0

0,5

1

1,5

12 y.o from VH 12 y.o from MRI 12 y.o from a 4y.o (MRI based)

W/kg

Difference ~ 50%

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Influence of the head model+position

Max SAR over 10g

00,20,40,60,8

11,2

12 y.o from VH 12 y.o from MRI 12 y.o from 4 y.oMRI based

W/k

g

Difference ~ 35%

Freq 900 MHz PatchHead Model 12 y.o from Visible Human 12 y.o from MRI 12 y.o from a 4 y.o MRI based

Tissues Abs Power SAR over 10g Abs Power SAR over 10g Abs Power SAR over 10gSkin 29% 1,14 33% 0.93 26% 0.98Muscle 26% 0.41 29% 0.40 14% 0,31Skull 7% 0.16 9% 0,17 14% 0,32CSF 4% 0,18 8% 0,29 6% 0,29Brain 14% 0.15 21% 0,21 39% 0.54

Handset

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Comparaison of SAR in specific tissues of a 12 y.o Child head model Non Uniform- MRI based

12 y.o from V.H 12 y.o from MRI 12 y.o from 4 y 0.95 W/kg 0.74 W/kg 1 W/kg abs. Power SAR1g

contiguous abs. Power SAR1g contiguous abs. Power SAR1g

contiguous Skin 29 % 3.43 W/kg 33 % 2.92 W/kg 26 % 3.18 W/kg skull 6.6 % 0.30 W/kg 9 % 0.30 W/kg 14 % 0.86 W/kg LCR 4.4 % 0.51 W/kg 8.3 % 0.81 W/kg 5.6 % 1.25 W/kg Brain 14 % 0.24 W/kg 21 % 0.34 W/kg 39 % 1.00 W/kg

Muscle 26 % 0.94 W/kg 29 % 0.88 W/kg 14 % 0.98 W/kg

SAR in specific tissues vs different 12 y.o head child model

012345

Skin skull LCR Brain Muscle

SAR

ove

r 1g

in W

/kg

From VHFrom MRIFrom a 4 y.o

These simulations show the influence of the skull and brain specific growthFrom 4 to 12 the brain volume does not increase in real head but is enlarge using non uniform scalingFrom Adult to 12 the brain volume does not increase in real head but is reduce using a uniform scaling

Frequency=900 Mhz ---- Handset=Patch

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SAR in different children head modelsFrequency=900 Mhz ---- Handset=Patch

SAR over 1 gram of contiguous tissue

0,002,004,006,008,00

SkinMuscl

eSkullCSFBrain

tissues

SAR

ove

r 1g

in

W/k

g

VH12 y.o from MRI4 y.o from MRI

Max SAR over 10g -- Abs.Power/Radiat.Power

0,00

0,20

0,40

0,60

0,80

1,00

1,20

VH 12 y.o fromMRI

4 y.o fromMRI

VH 12 y.o fromMRI

4 y.o fromMRI

SAR over 10g of contiguous tissues

0,000,501,001,502,00

SkinMuscl

eSkullCSFBrain

Tissues

SAR

ove

r 10g

in

W/k

g VH12 y.o from MRI4 y.o from MRI

Freq 900 MHz Head Model Visible Human 12 y.o from MRI 4 y.o from MRI

Tissues Abs Power SAR over 10g Abs Power SAR over 10g Abs Power SAR over 10gSkin 34% 1,20 33% 0,93 27% 1,08 Muscle 24% 0,44 29% 0,40 12% 0,28 Skull 7% 0,16 9% 0,17 13% 0,34 CSF 4% 0,19 8% 0,29 6% 0,31 Brain 10% 0,13 21% 0,21 40% 0,54

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Influence of the position

12 y.o Adult4 y.o

The shape of the head are different The thickness are different….

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Patch vs Dipole in 12 y.o children head (2)

Normalised SAR over 1g in contiguous tissues,

(Normalised to the SAR obtain with the dipole in skin)

Normalised SAR in tissues vs sources

0,000,200,400,600,801,001,201,40

Skin Muscle Skull CSF Brain

SAR

in 1

g no

rmal

ised

DipolePatch

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Influence of the source alimentation

Field in tissues are proportional to current on the antenna

SAR in tissues are proportional to (current ) ²

Input power is proportional to real part of the impedance and to the (current ) ²

Therefore

SAR is proportional to (input power/impedance)

Power or Current constant?According to J. Wang,( Wang & al IEEE Trans MTT, Vol 51, N° 3, March 2003) the difference observed in the litterature should be due to the different calculation conditions.

SAR max over 10g vs P or I const

0

0,2

0,4

0,6

0,8

VH 12 y.o fromMRI

4 y.o fromMRI

W/k

g P constI const

Impedance

VH 12 y.o from

MRI 4 y.o from

MRI 19 – 11.8i 11.4-13.9i 10.4-10.8i

Based on RF Amplifier Technologynormalisation is usually based on

power

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First Intermediate Conclusions

A The morphology is an important parameter to estimate the SAR distribution in tissues.

Head model based on MRI have to be developed.

B In comparison with the multiple sources of uncertainties (source modeling, variability of the morphology, positioning of the handset…) the observed differences between max SAR over 10g are not significant

C Since limited number of head have been studied, theses conclusions have to be confirmed by extensive study on different head models

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Fœtus Specific situation

Exact data are missing.

There is no 3D anthropomorphic modelData are complex to obtain

The uterus thickness increases up to 3cm at 4 month and is between 4 and 10 mm at term

The abdominal muscle are extended so the thickness is small

Strategy

Analyse the influence of thickness in 1DDevelop a simple 3D model

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1D Multilayer approachMulti-layers approach :

Skin, Hypoderm(Fat), Muscle, Uterus, placenta, Amniotic fluid, fœtus (model as muscle).Thickness [ 2 8-70 5 5 10 2 200] mm

Incident field strength 120 v/m (~0.8 W/kg in a head)

Conductivity vs tissues

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Influence of the hypoderm thickness

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Influence of the uterus thickness

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Simple 3D model

Dipole 1,4 cm in front of epiderm

f=900MHz & 1800 MHz

FDTD & PML ABC’s

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Results

900 MHz

1800 MHz

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Plane Wave à 900 MHz

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Second Intermediate conclusion

With frequencies higher than 900 MHz the ratio of Max SAR in fœtus and mother seems to be higher than 6.

These results have to be confirmed with realistic models of tissues and sources

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This research has been done under the umbrella of the French national research network in

telecommunication ( RNRT)

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ADONIS ADONIS www.tsi.enst.fr/adonis

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Thanks