modeling of rf exposure in children - who · modeling of rf exposure in children j. wiart, a....
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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
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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Thanks