CHARACTERISTICS, DOSIMETRY &

MEASUREMENT OF EMFMasao Taki

Tokyo Metropolitan University

Soichi Watanabe and Kanako WakeCRL

JAPAN

WHO Meeting on EMF Biological Effects & Standards Harmonization in Asia and Oceania

22 - 24 October, 2001, Seoul, KOREA

Roles of Engineering in EMF Health Issue

• Characterize external EMF precisely• Measure the field correctly• Provide insight into the interaction of EMF

with cells, organs, and body (Dosimetry)• Design and install the exposure setups for

experiments on biological effect, and control their quality

• Develop guidelines based on biological data• Assess the compliance with guidelines

Characterization

Characterization of EMF

• EMF– Electric field (E,D) : vector function E(t,r)– Magnetic field (H,B) : vector function H(t,r)

• Waveform– Sinusoidal waves (frequency, amplitude, phase)– Non-sinusoidal waves

(spectrum=amplitude and phase at any frequencies)

• Polarization– Orientation of E and/or H field vectors

Sinusoidal and Non-sinusoidal Waves

Waveforms with 5-harmonic components

Amplitude spectrum

Spectrum of EMF

10151012109106103100

103106 10-3100 10-6

Frequency[Hz]

Wavelength[m]

RF10MHz – 300GHz

IF300Hz-10MHz

ELF- 300Hz

RF100kHz – 300GHz

LF- 100 kHz

Non-ionizing Radiation (wavelength < 100nm)

Radiation of EMF

E-field H-field

Polarization

• Orientation of the field vector• In “far field region” E and H are perpendicular

to each other. Orientation of E-field• Important factor in the coupling between field

and body

Linear and Circular Polarization

Measurement

Measurement

• Small Antennas (broad-band)– Infinitesimal electric dipole = Electrical gap

• Element of E-field sensor

– Infinitesimal magnetic dipole = Small loop• Element of B-field sensor

• Resonant antennas (narrow-band)• Other sensors

– Pockels effect E-field sensor– Hall effect B-field sensor

• Isotropic E-field probe with three orthogonal small dipoles

• Provides rms values with a certain time constant

• Well developed technique• Easy operations, widely used

E-Field MeasurementBroadband technique

• Some types give weighted sum of rms values following frequency dependence of reference levels of specific guidelines

• Diode detector type is not suitable for pulsed field measurement (thermocouple type is recommended)

• up to >10 GHz

H-field MeasurementBroadband Technique

• Isotropic probe with three orthogonal small loops

• Common readout unit with E-field instrument

• Similar features to E-field probe• up to about <1GHz• If plane wave condition is applied, H can

be obtained by H=E/120π

Dosimetry

Dosimetry• Metrology of “dose”• “Dose”

– “the amount of a substance we are exposed to or come in contact with”

• Dose in EMF – Induced current density

• Stimulation effect (< 100 kHz)– Specific Absorption Rate (SAR) [W/kg]

• Thermal effect– Other metric

• Possible non-thermal, non-stimulation effect (?)

Basic Coupling of E and B with body

EB

Models for Dosimetry

• Early works– Analytical approaches– Simple shapes

MRI-Based Models

horizontal section at the height of 50 mm

y

x

vertical sectionx

z

x

Rat ModelHuman Model

Development of numerical methods

- Finite Difference method (LF)

- Finite-Difference Time-Domain (FDTD) Method

Induced Current Distributions in Human

Bx (body axis) By ( left to right) Bz (front to back)

Induced Current Distributions in Rat

Bx (body axis) By ( left to right) Bz ( front to back)

Whole body

Brain

Bone

Muscle

Fat

Heart

　　　　　　 Bx (body axis)　　 By (left to right)　　　Bz (front to back)　　　 Max.　　 Ave.　　　 Max.　　 Ave.　　 Max.　　 Ave.Rat 9.6 0.55 24 0.69 7.7 0.71 Human 66 2.9 70 3.5 92 4.0

Rat 0.61 0.31 1.2 0.41 0.61 0.14Human 2.5 0.81 4.6 1.5 3.7 1.2

Rat 1.1 0.20 1.3 0.25 1.2 0.19 Human 8.5 0.71 7.2 0.82 12 0.98

Rat 5.4 0.95 6.6 1.2 6.2 1.2Human 53 4.6 37 5.8 70 6.8

Rat 1.2 0.19 1.2 0.23 1.3 0.21Human 10 1.1 8.4 1.2 20 1.4

Rat 0.62 0.28 0.88 0.41 0.52 0.15 Human 11 2.3 11 1.9 4.9 1.4

Current Densities in Rat and in Human

• Currents induced in human are larger than currents in rat because of the size difference B: 50 Hz, 1 µT. J [µA/m2]

Recently Developed Human Models

• Japanese male and female – Spatial resolution：2 mm– Identified tissues：> 50

Resonance of Whole Body SAR

GroundedUngrounded

10110-4

10-2

10-3

102 103

10-1

100

frequency , MHz

Ave

rage

SA

R ,

W/k

g

E

kH

大地

E-Polarizatoin

SAR distributions and Effect of Grounding

d = 10 cm d = 50 cmd = 1 cmContact Free space

SAR distribution （Computed at 37 MHz）

Limb Current in 10 - 110 MHz

Current （ｍＡ） Occupational exposure 100

General public 45

Reference levels for current induced in any limb in 10 - 110 MHz

Equivalent Antenna for Limb Current Measurement

SAR Measurement

2ESARρσ=

Realistic Head Phantom(Standard Japanese)

Lossless spacer With ear

Ear affects SAR distribution but does not increase maximum local SAR

Development of Exposure Setups

Assessment of Experimental Setups

• Quantify exposure condition• Important for the improvement of reproducibility• Both numerical and experimental exposure assessment

Air VentilationDuctSDRat inaTube

Stopper

4 rats

8 rats

2 rats0.01

0.1

1

SAR

[W/k

g]

Results of numerical calculations

Thermography camera

Divided phantom

Exposure setup and rat phantoms.

Experimental SAR Measurement Thermograph Method

Exposure Setup for Long-term Cancer Promotion Study

• ENU-initiated brain tumor of rats• Two-year NTP study at a GLP facility• Localized exposure in the brain with least whole-body

SAR • Many animals

Numerical Dosimetry

X-ray CT images

Numerical models

Calculated SARs

126 g 263 g 359 gGrowth

Experimental DosimetryPhantom for Termograph Method

Water

Ager

NaCl

NaN3

TX-151

Polyethylene

　phantom recipe[g]

1687.5

52.3

7.52

1.0

42.2

168.75

Comparison between Numerical and Experimental Dosimetry

SAR by Thermography

0

1

2

3

4

5

6

7

8

9

0 10 20 30

Depth[mm]

SAR[W

/kg

]

Calculated SAR

Experimental

Numerical

Exposure Setup for Brain Circulation Study

• SD rat with Cranial Window• Allows in situ observation of

circulation• Very localized exposure in the

brain by a loop antennaSmall loop

Electromagnetic Field Distribution

H-FieldComputedMeasured

E-FieldComputedMeasured

SAR Distributions

Measured by Thermography

FDTD Calculation

Exposure Setup for MW Effect on Eye

Antenna

Stub tuner

Generator

• 2.45 GHz• Dielectric-loaded waveguide antenna• Pulsed field/CW

In Vitro Exposure Setup

What else engineering could contribute ?

Exercises for engineers and physicists

(1) Microwave Hearing (MWH)

• Buzzing or clicking auditory sensation caused by the exposure to high peak power microwave pulses on the head

• Thermo-elastic waves generated by the small but rapid local heating should cause the sensation.

• An established phenomenon specific to pulse modulated microwaves.

Questions

• What is the mechanism ?– Thermoelastic waves (J.C. Lin, 1970’)

• Does the heterogeneity of head affect the phenomenon ?

• What waveform do we actually perceive ?• How strong is the acoustic waves in the brain ?• -------

SAR Distribution and Elastic Waves

0.5

0.2

0Loca

l SA

R [W

/kg]

• Amplitude, power, displacement of MWH is very small

• Acoustic resonance in the head governs the waveform

(2) Intrinsic Current

• Cells lying near an axon on which action potentials propagate are exposed to strong field

• Are these cells affected by the field ?

(3) Circularly Polarized Magnetic Field

• Kato et al. (1994) reported inhibition of melatonin secretion only by exposure to circularly polarized B-field.

• He did not find any effect for linearly polarized field.

Can difference in induced current characteristics explain the difference ?

Induced Current by Circularly Polarized B

How “circular” the current in brain is ?

The result is still puzzling us.

Needs an experiment for X-y plane rotating field.

Local SAR of 10W/kgBasic restriction on local SAR for Occupational exposure

Internal E-field ~100V/m（σRF ~1 S/m）

Threshold of Magnetophosphene

~10mA/m2

（σELF ~1 S/m）　　　　～0.01V/m？

Input OutputNonlinear response

Demodulation of the Baseband Component due to Nonlinearity

(4) Demodulation of AM Signals

SAR

E-field

SAR and E-Field Distribution by RF

Std. Dipole

900 MHz

5cm

Antenna input

1 W

Peak Local SAR (10 g av.)

1.35 W/kg

Peak SAR near retina

1.3 W/kg

Result of Search for “MW Phosphene”

• So far no phosphene-like phenomenon has been found.• Induced current by modulated RF may differ from that

by ELF magnetic field. • However, we know that the threshold current density for

electrophosphenes is similar to magnetophosphenes.• ??????

Induced by BInduced by E or RF

(5) Exposure Assessment of Epidemiological Studies

• What “dose-effect relationship” could explain the elevation of RR ?

• How strong should the effect be to explain the RR ?

Epidemiological Studies

RR～2.0

?

>4 mG <4mG

Laboratory Studies

Animal Studies

Volunteer Study ?

～1　???

Concluding Remarks• We would like to emphasize the importance of

engineering and physics in EMF health research. • Good engineering provides:

– Quantitative understanding of the events in the body– Hints of mechanisms of interaction– Reproducible experiments– Means to hypothesis driven experiments

• We can ask a number of “good questions” to ourselves which could be answered by investigations based on engineering, physics, and mathematics.