nir2016-watanabe and hirata ver3 · 2016. 5. 14. · hirata et al. 2007 hirata 2005 wainwright2007...
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
HF DosimetrySoichi Watanabe (ICNIRP, NICT)
Akimasa Hirata (ICNIRP, Nagoya Institute of Technology)
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Dosimetry in HF region
• Dosimetry has acted the important function in deriving the HF guidelines;• Relationship between the temperature elevation and the BR, or SAR and
power density.• Relationship between the BR and the RL, or electric, magnetic and power
density. The most known outcome of the HF dosimetry is the RL considering the whole-body resonance phenomenon.
Whole-Body Resonance Frequency
Who
le-b
ody
pow
er a
bsor
ptio
n
Whole-Body Resonance of adults
Whole-Body Resonance of children
Frequency
RL
(pow
er d
ensi
ty)
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Electrical properties of biological tissues and organs
Three relaxations, known as α, β, and γ dispersion, generally appear in thefrequency characteristics of the electrical properties of the biological tissues andorgans.
α β γ
Frequency characteristics ofthe relative permittivity andconductivity of muscle tissue(Gabriel, et al., Brooks AFBReport, 1996).
γ dispersion is caused by therotation of water molecules†.
(†) http://commons.wikimedia.org/wiki/File:Water_molecule.svg
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Variability of electrical properties
Electrical properties of biological tissues and organs are significantly variable, whichcan be 100 times or more and depends on the water-content ratio.
Frequency characteristics of therelative permittivity and conductivity ofmuscle as a high-water-content tissueand fat as a low-water-content one(Gabriel, et al., Brooks AFB Report,1996).
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
1996 Gabriel’s database
electrical properties of muscle(Gabriel, et al., Brooks AFB Report, 1996)
typically extrapolated from the measurement
data below 20 GHz
resent researches have reported measurement data over 20 GHz.
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Recent update on the electrical properties in HF region (1/2)
schematic view of skin and eye tissues (ICRP, 2002)
• Skin tissues; from 0.5 GHz to 3 THz• epidermis (in vivo, human); 0.1
THz-3 THz• stratum corneum and the rest of the
epidermis plus dermis (in vivo,human); 37 GHz-74 GHz
• epidermis and dermis (in vitro, pig);0.5 GHz-110 GHz
• Eye tissues; from 0.5 GHz to 100 GHz• cornea, iris, lens, sclera, and
vitreous (in vitro, rabbit)• aqueous humour (in vitro, pig)
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Recent update on the electrical properties in HF region (2/2)
Sasaki et al., PMB, 2015
Sasaki et al., PMB, 2014
Large discrepancy appears in
skin tissues.
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Human modeling in HF region
Various human models or physical phantoms have been used for the HF dosimetry..
• Numerical human models are one of the most important advances in the dosimetry,which have fine spatial resolutions with accurate anatomical structure based on CTand MRI technology.
• Physical human phantoms have relatively poor anatomical structure, which frequentlyconsist of single material such as tissue-equivalent liquid. However the physical humanphantoms have widely been used in the compliance evaluations of radio devices becauseactual radio devices can be used for the evaluation, and the simple phantom can providehighly-reproducible and safety-side evaluation.
Phantom
AntennaIR camera
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Latest numerical human models
From the homepage of IT’IS foundation.
NICT’s voxel human models.
France-Japan research project (FETUS)
New child models with ICRP specifications.Details are shown in the poster by Nagaoka et al.
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Uncertainty in HF dosimetryUncertainty, defined by ISO/IEC Guide 98, is usually evaluated with its expanded valuewhich covers 95% confidential interval of the probability distribution of the evaluatedresult. Uncertainty of EMF dosimetry depends on many factors. 30% or 1 dB may be areasonable target for a specific case with a technique of state of the art in HF regionwhile larger uncertainty has been recognized in LF region, i.e., 3-dB reduction factordue to the uncertainty of the dosimetry is considered in deriving RLs in LF region.
Uncertainty of HF numerical dosimetry and other factors considered in the reduction factor
Red
uctio
n fa
ctor
Uncertainty of numerical techniques;• Approximation of Maxwell’s eq.,• Boundary conditions,• Convergence,• Post processing (local averaging).
Uncertainty of discrete modeling;• Approximation with staircase of
smooth shape,• Spatial resolution vs complex
heterogeneous structure.
Variation of a human body, population;• Size and weight,• Internal structure (fat thickness).
Variation of exposure conditions;• Polarization and direction,• Reflection and grounding,• Other source conditions.
Other factors to be considered.
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Thermal Dosimetry for whole-body exposure: back exposed at 100 MHz
0 2 4 6 8Power Density (mW/cm2)
-0.20
0.40
0
0.20
Esop
hage
al te
mpe
ratu
re c
hang
e [o
C]
Ambient temperature 31 oC
24 oC
28 oC
Whole-body average (WBA) SAR at 0.68 W/kg for duration of 45 min.Measured ΔT = 0.15-0.20 oC (100 MHz) AdairComputed ΔT = 0.177 oC@28 oC (65 MHz) unpublishedComputed ΔT = 0.17 oC@28 oC (100 MHz) by Nelson
Adair et al, Bioelectromagnetics, 2003Hirata et al, Phys. Med. Biol., 2007.Laakso & Hirata, Phys. Med. Biol., 2011.Nelson et al, Phys. Med. Biol., 2014
Exposure duration45 min
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Core temperature elevation normalized by WBA-SAR
0
0.1
0.2
0.3
0.4
0.5
10 100 1000 10000
WBA-SAR 0.4 W/kg
WBA-SAR 4.0 W/kg
WBA-SAR 0.08 W/kg
Simplified eq. (0.08 W/kg )
Simplified eq. (0.4 W/kg)
4.0W/kg Simplified eq.ΔTco
re/ W
BA
-SA
R [o
C·k
g/W
]
102 103
Frequency [MHz]10410
Hirata et al, Phys. Med. Biol., 2013
In the elderly, thermoregulatory response is weaker than that in the young adult due to declined heat sensitivity etc, resulting in higher core temperature.
Ambient temperature28 oC.
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Local temperature elevation in anatomical head models
SAR T
• Frequency=6 GHz• Thermal steady state (~30 min); depending
on the frequency• Temperature elevation distribution in the
human model is smoother than that of SARdue to heat diffusion.
Wang and Fujiwara (IEEE MTT,1999)
Leeuwen et al.(Phys.Med.Biol.1999)
Bernardi et al. (IEEE MTT, 2000)
Wainwright(Phys.Bio.Med.2000)
Gandhi et al. (IEEE MTT, 2001)
Hirata et al. (IEEE EMC, 2003)
Samaras et al. (IEEE EMC, 2007)
McIntosh et al. (BEMS, 2010)
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Effect of averaging mass: Averaged SAR vs Temperature Elevation
1.0 GHz 1.5 GHz2.0 GHz 2.5 GHz3.0 GHz 4.0 GHz6.0 GHz
Averaging Mass [g]
Hea
ting
Fact
or [o
C·k
g/W
]
0.1
0.01
1
0.1 1 10 100
The heating factors converge at the averaging mass of ~10 g.ΔT can be estimated in terms of SAR at different frequencies.
The blood flow may increase for the temperature elevation caused by local exposure.The heating factors without considering the response are conservative.
Alekseev et al, BEMS, 2005
Hirata et al, Phys. Med. Biol., 2009
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Maximum local temperature elevation by local SAR (10 g)
Tissue Heating FactoroCW-1kg
FrequencyGHz
Averaging Exposure data Reference
Brain 0.13 0.180.22 1)0.21
0.12
12.55 0.9-2.45
0.915
cube, 10 g
cube, 10 g
cube, 10 g
Plane wave,upfront,female adult
Near field averages, adult,Near field
Laakso 2009
Fujimoto et al. 2006
Van Leeuwen 1999
Eye 0.1 -0.12 (lens)0.13 (lens)0.11-0.147 (lens)0.22 (lens)0.16-0.17 (lens)
0.18-0.190.21 0.23
1-3 7.50.6-3 50.9-1.9
0.380-1.8 13
cube, 10 g
eye 7.5 g
Eye 11.5 g
eye, 9.4 g cube, 10 g
Plane wave averages, male, Plane wave, adult
Near fields and plane waveNear field Plane wave and nearfields
Laakso 2009
Hirata et al. 2007
Hirata 2005
Wainwright 2007McIntosh et al. 2010
Skin, fat, pinna, muscle
0.2-0.260.28-0.37
1-36
cube,10 g Plane wave and nearfields
McIntosh et al. 2010
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Temperature elevation for millimeter wave exposure
Alekseev et al, Bioelectromagnet., 2005
Foster et al, Health Phys., 2010
Also see poster by Sasaki et al
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Thermal Time Constants for Local and Whole-body Exposures
0
100
200
300
400
500
600
700
800
900
1 10
The
rmal
Tim
e C
onst
ant [
s]
Frequency [GHz]
IEEE
ICNIRP
1D (fat)
3D model
1D (skin)
Foster et al, Bioelectromagnetics, 1998 Hirata et al, Phys. Med. Biol., 2013
WBA-SAR = 0.4 W/kg
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Threshold of perception and pain due to contact current
• Above 100 kHz, heat sensation. Below 100 kHz, sensation of electrostimulation.• Pain threshold in finger may reach the heat perception threshold after 10-20 s. • Pain is due to the stimulation of nociceptors in the cutaneous and subcutaneous tissues. • Skin temperature elevation from 34 to 36 oC at rate of 4oC/s elicits a pain sensation
(Green et al.2010).
Chatterjee et al. 1986
Pain and perception thresholds in fingercontact are close above 100 kHz
finger contact
grasp contact
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Local SAR induced by contact current of 40 mA in finger of an adult
SAR scaled from 16 mA to 40 mA (Table 8)
Frequency(MHz)
SAR10g(W/kg) 1)
0.1 4681 39310 469100 387
40 mA=ICNIRP1998 reference level for occupational exposure
20 W/kg=ICNIRP 1998 basic restriction for occupational exposure
1) IEEE averaging scheme Extremely high SAR in finger
Chan et al., Phys. Med. Biol., 2013
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ICNIRP 8th INTERNATIONAL NIR WORKSHOP
Cape Town, South Africa, 9-11 May 2016
Summary
•The metrics, averaging area/mass and time of SAR and power density, as the limit is revisited in published studies.•Not only novel computation with anatomical model but also physics and/or mathematical formula behind the phenomena should be explained.
Measured data temperature elevation
DosimetryElectromagnetics, Thermodynamics
Measured dataTissue properties
1. Threshold assessment2. Relationship between external and internal physical quantities
Numerical Human body models