[show abstract][hide abstract] ABSTRACT: In order to determine the whole-body exposure level in small animals inside a reverberation chamber (RC), an ideal electromagnetic (EM) environment has often been assumed inside an RC for numerical dosimetry analyses. In an RC where a strong EM coupling between antennas and animals exists, however, such an EM environment is difficult to be realized, so that it remains unknown whether or not the RC could provide a target dosimetry. In this study, we adopt the Poggio-Miller-Chang-Harrington-Wu-Tsai formulation of the method of moments to directly determine the specific absorption rate (SAR) of a rectangular dielectric phantom placed in an RC, and we demonstrated its validity via comparing the numerical temperature rise with those obtained from experiments. We then demonstrated a nonideal EM environment inside an RC using a prolate spheroid as an exposure target. Furthermore, we performed dosimetry evaluation for multiple rat-shaped tissue-equivalent phantoms inside the RC and determined their whole-body average SARs for many orientations and arrangements of the phantoms. Finally, based on our numerical results, we offered a design rule when using an RC as a whole-body exposure system for small animals.
IEEE Transactions on Microwave Theory and Techniques 08/2013; 61(9):3435 - 3445. · 2.23 Impact Factor
[show abstract][hide abstract] ABSTRACT: The eye is said to be one of the most sensitive organs to microwave heating. According to previous studies, the possibility of microwave-induced cataract formation has been experimentally investigated in rabbit and monkey eyes, but not for the human eye due to ethical reasons. In the present study, the temperature elevation in the lens, the skin around the eye and the core temperature of numerical human and rabbit models for far-field and near-field exposures at 2.45 GHz are investigated. The temperature elevations in the human and rabbit models were compared with the threshold temperatures for inducing cataracts, thermal pain in the skin and reversible health effects such as heat exhaustion or heat stroke. For plane-wave exposure, the core temperature elevation is shown to be essential both in the human and in the rabbit models as suggested in the international guidelines and standards. For localised exposure of the human eye, the temperature elevation of the skin was essential, and the lens temperature did not reach its threshold for thermal pain. On the other hand, the lens temperature elevation was found to be dominant for the rabbit eye.
[show abstract][hide abstract] ABSTRACT: In view of actual electromagnetic environment of multiple-frequency exposure, we have developed a whole-body exposure system for a multigeneration bio-effect test with rats at frequencies between 800 MHz and 5.2 GHz. We first experimentally evaluated the system performances such as antenna performance, electric field distribution, and specific absorption rate (SAR) in the exposure system. Then, after an experimental validation of the finite-difference time-domain modeling of the exposure system, we employed it to make a statistical SAR analysis for anatomical rat models. We first took documentary photographs of rat activity inside the exposure system every 5 min in real time. Then, by identifying the rat positions in each photograph, we derived the frequency of rats staying in various positions in the exposure system. Using the stay frequency as a weighting factor, we derived the whole-body average SAR statistical characteristics for the multiple-frequency exposure system during the entire multigeneration experimental period. The developed system and demonstrated approach provide a high-quality means for biological effect test of radio frequency exposure in wide frequency bands.
IEEE Transactions on Microwave Theory and Techniques 01/2013; 61(1):326-334. · 2.23 Impact Factor
[show abstract][hide abstract] ABSTRACT: According to the international guidelines, the whole-body averaged specific absorption rate (WBA-SAR) is used as a metric of basic restriction for radio-frequency whole-body exposure. It is well known that the WBA-SAR largely depends on the frequency of the incident wave for a given incident power density. The frequency at which the WBA-SAR becomes maximal is called the 'resonance frequency'. Our previous study proposed a scheme for estimating the WBA-SAR at this resonance frequency based on an analogy between the power absorption characteristic of human models in free space and that of a dipole antenna. However, a scheme for estimating the WBA-SAR in a grounded human has not been discussed sufficiently, even though the WBA-SAR in a grounded human is larger than that in an ungrounded human. In this study, with the use of the finite-difference time-domain method, the grounded condition is confirmed to be the worst-case exposure for human body models in a standing posture. Then, WBA-SARs in grounded human models are calculated at their respective resonant frequencies. A formula for estimating the WBA-SAR of a human standing on the ground is proposed based on an analogy with a quarter-wavelength monopole antenna. First, homogenized human body models are shown to provide the conservative WBA-SAR as compared with anatomically based models. Based on the formula proposed here, the WBA-SARs in grounded human models are approximately 10% larger than those in free space. The variability of the WBA-SAR was shown to be ±30% even for humans of the same age, which is caused by the body shape.
Physics in Medicine and Biology 11/2012; 57(24):8427-8442. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: The World Health Organization's (WHO) recommendation on the need for further research for radio-frequency dosimetry has promoted studies on the whole-body average-specific absorption rate (WBA-SAR) in various kinds of anatomical-based numerical models. For experimental validation of GHz-band WBA-SARs in a real human, however, there have not so far been any published papers, despite the fact that, in 1982, Hill measured WBA-SARs at frequencies less than 40 MHz in human volunteers using a TEM-cell exposure system. In this study, we provide a measurement technique with a reverberation chamber for validating numerical dosimetry results on GHz-band WBA-SARs in living humans. We measured WBA-SARs at 1, 1.5 and 2 GHz for a 22 year old male volunteer, with a height of 173 cm and a weight of 73 kg, in the reverberation chamber, and compared the results with the finite-difference time-domain (FDTD) simulation. The reverberation chamber was excited by using a signal generator through an amplifier with an output power of 30-40 mW, which produced inside the chamber with the volunteer an average electric field strength of 5 V m(-1) equivalent to an average power spectral density of 6.6 μW cm(-2). The WBA-SARs were obtained from the measured S(11) and S(21) together with the power density. On the other hand, the WBA-SARs have been calculated using the FDTD method for an adult male model with almost the same physique as that of the volunteer exposed to the electromagnetic field in the reverberation chamber. From the comparison between the measured and the calculated WBA-SARs, we could confirm that the measured GHz-band WBA-SARs approximately agree with the FDTD calculated results.
Physics in Medicine and Biology 11/2012; 57(23):7893-7903. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: To investigate the effects of built-in inductors on current rising
waveform for contact discharges of electrostatic discharge generators
(ESD guns), we simultaneously measured discharge currents and the
resultant magnetic near-fields, and compared their results with those
for an ESD gun with a metal cylinder in lieu of the inductor. As a
result, we found that the magnetic near-field waveform is deformed by
the distortion of the current rising waveform for an ESD gun with an
inductor, while the ESD gun with a metal cylinder produces no distortion
for the current rising waveform despite its rise time (400 ps) shorter
than the International Electrotechnical Commission (IEC) specification
so that there is no distortion for the corresponding magnetic near-field
waveform. Using the Heidler's formula for typical discharge current
waveform specified in the IEC, we calculated current rising waveform and
the resultant induced output voltage of a magnetic probe, which shows
that these results do not agree with those for the built-in inductor,
while there is good agreement for the metal cylinder. This finding
implies that immunity test results can be affected by the frequency
characteristics of a built-in inductor inside an ESD gun, and thus that
not only rise time but also current rising waveform of discharge
currents should be specified for ESD immunity testing.
IEEJ Transactions on Fundamentals and Materials 01/2012; 132(5):362-367.
[show abstract][hide abstract] ABSTRACT: In the present study, using a 50Ω tapered coaxial type adapter and
a S-parameter technique, we measured the transfer impedance of a
commercially available IEC calibration current target used for the
immunity testing against electrostatic discharges (ESDs), which is being
prescribed in the International Electrotechnical Commission (IEC)
61000-4-2, and thereby reconstructed the waveforms of discharge currents
injected onto the target from their observed output voltages for contact
and air discharges of an ESD generator. As a result, we found that the
transfer impedance is almost 1+j0Ω at frequencies below 1GHz,
while due to resonance phenomena occurring at frequencies around 2GHz
and 5GHz, resistive and reactive components significantly change at
frequencies over 1GHz, though their absolute values are slightly larger
than 1Ω. This result has revealed that the reconstructed discharge
currents agree well with the observed voltages with a rise time of
almost 1ns for the contact discharges and air discharges with slow
approach of an ESD gun with charge voltages above 2kV, while in the case
for the observed voltage with a rapid rise time shorter than a hundred
pico-seconds for the air discharges with fast approach, the
reconstructed discharge current has a bit gentle rising slope and a
slightly lower level of the first current peak.
IEEJ Transactions on Fundamentals and Materials 01/2012; 132(5):350-355.
[show abstract][hide abstract] ABSTRACT: The micro-gap discharge as the low voltage ESD shows very fast
transition-duration of about 32 ps or less. Furthermore, the breakdown
field strength in the gap was very high of about 80 MV/m in low voltage
discharging of below 400V. The relationship between the breakdown field
strength in the gap and the amplitude of radiated electromagnetic field
was examined in experimental study. The amplitude of radiated
electromagnetic field was proportion to the breakdown field strength at
the gap in the resonance experimental system using the discharge
electrode with dipole configuration. In this time, we present an
improved experimental system to measure the amplitude of radiated
electromagnetic filed in more wideband region using spherical electrodes
and a horn antenna. As a result, the amplitude of radiated
electromagnetic field is proportion to the discharge voltage from 300V
to 620V, and the amplitude of radiated electromagnetic field was
according to the diameter of spherical electrode in 1-3GHz frequency
IEEJ Transactions on Fundamentals and Materials 01/2012; 132(5):345-349.
[show abstract][hide abstract] ABSTRACT: In this study, we measured the time variation of burst electric fields
leaked from a heating device in the ion cyclotron range of
high-frequency in an experimental fusion facility, and analyzed their
statistical characteristics such as the amplitude probability
distribution (APD) and crossing rate distribution (CRD). As a result, we
found that the variation of the leaked electric field level is very
irregular, far from the normal distribution. Moreover, the leaked
electric field variation with time may reach 400 times in one second to
cross its mode value. Although so, the maximum electric field intensity
itself is much smaller than the ICNIRP safety guideline. In addition, we
also evaluated the possibility of electromagnetic interference to an
implanted cardiac pacemaker in the measured electromagnetic environment.
We found that even in the worst case the interference voltage induced in
the output of the pacemaker sensing circuit does still not exceed the
threshold for a malfunction.
IEEJ Transactions on Fundamentals and Materials 01/2012; 132(5):356-361.
[show abstract][hide abstract] ABSTRACT: A 2 GHz whole-body exposure to rats over a multigeneration has been conducted as part of bio-effect research in Japan. In this study, the rats moved freely in the cage inside the exposure system. From observation of the activity of rats in the cage, we found that the rats do not stay in each position with uniform possibility. In order to determine the specific absorption rate (SAR) during the entire exposure period with high accuracy, we present a new approach to statistically determine the SAR level in an exposure system. First, we divided the rat cage in the exposure system into several small areas, and derived the fraction of time the rats spent in each small area based on the classification of the documentary photos of rat activity. Then, using the fraction of time spent in each small area as a weighting factor, we calculated the statistical characteristics of the whole-body average SAR for pregnant rats and young rats during the entire exposure period. As a result, this approach gave the statistical distribution as well as the corresponding mean value, median value and mode value for the whole-body SAR so that we can reasonably clarify the relationship between the exposure level and possible biological effect.
Physics in Medicine and Biology 11/2011; 57(1):143-54. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: The dominant effect of human exposures to microwaves is caused by temperature elevation ('thermal effect'). In the safety guidelines/standards, the specific absorption rate averaged over a specific volume is used as a metric for human protection from localized exposure. Further investigation on the use of this metric is required, especially in terms of thermophysiology. The World Health Organization (2006 RF research agenda) has given high priority to research into the extent and consequences of microwave-induced temperature elevation in children. In this study, an electromagnetic-thermal computational code was developed to model electromagnetic power absorption and resulting temperature elevation leading to changes in active blood flow in response to localized 1.457 GHz exposure in rat heads. Both juvenile (4 week old) and young adult (8 week old) rats were considered. The computational code was validated against measurements for 4 and 8 week old rats. Our computational results suggest that the blood flow rate depends on both brain and core temperature elevations. No significant difference was observed between thermophysiological responses in 4 and 8 week old rats under these exposure conditions. The computational model developed herein is thus applicable to set exposure conditions for rats in laboratory investigations, as well as in planning treatment protocols in the thermal therapy.
Physics in Medicine and Biology 11/2011; 56(23):7639-57. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: For magnetic field exposures at extremely low frequencies, the electrostimulatory response with the lowest threshold is the magnetophosphene, a response that corresponds to an adult exposed to a 20 Hz magnetic field of nominally 8.14 mT. In the IEEE standard C95.6 (2002), the corresponding in situ field in the retinal locus of an adult-sized ellipsoidal was calculated to be 53 mV m(-1). However, the associated dose in the retina and brain at a high level of resolution in anatomically correct human models is incompletely characterized. Furthermore, the dose maxima in tissue computed with voxel human models are prone to staircasing errors, particularly for the low-frequency dosimetry. In the analyses presented in this paper, analytical and quasi-static finite-difference time-domain (FDTD) solutions were first compared for a three-layer sphere exposed to a uniform 50 Hz magnetic field. Staircasing errors in the FDTD results were observed at the tissue interface, and were greatest at the skin-air boundary. The 99th percentile value was within 3% of the analytic maximum, depending on model resolution, and thus may be considered a close approximation of the analytic maximum. For the adult anatomical model, TARO, exposed to a uniform magnetic field, the differences in the 99th percentile value of in situ electric fields for 2 mm and 1 mm voxel models were at most several per cent. For various human models exposed at the magnetophosphene threshold at three orthogonal field orientations, the in situ electric field in the brain was between 10% and 70% greater than the analytical IEEE threshold of 53 mV m(-1), and in the retina was lower by roughly 50% for two horizontal orientations (anterior-posterior and lateral), and greater by about 15% for a vertically oriented field. Considering a reduction factor or safety factors of several folds applied to electrostimulatory thresholds, the 99th percentile dose to a tissue calculated with voxel human models may be used as an estimate of the tissue's maximum dose.
Physics in Medicine and Biology 07/2011; 56(13):4091-101. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: For safety management at a magnetic confinement fusion-test facility, protection from not only ionising radiation, but also non-ionising radiation such as the leakage of static magnetic and electromagnetic fields is an important issue. Accordingly, the use of a commercially available personal RF monitor for multipoint area monitoring is proposed. In this study, the performance of both fast- and slow-type personal RF monitors was investigated by using a transverse electromagnetic cell system. The range of target frequencies was between 10 and 300 MHz, corresponding to the ion cyclotron range of frequency in a fusion device. The personal RF monitor was found to have good linearity, frequency dependence and isotropic response. However, the time constant for the electric field sensor of the slow-type monitor was much longer than that for the fast-type monitor. Considering the time-varying field at the facility, it is found that the fast-type monitor is suitable for multipoint monitoring at magnetic confinement fusion test facilities.
[show abstract][hide abstract] ABSTRACT: For a whole-body exposure setup of rats, the rats can freely move in the cage in most cases. In order to determine the specific absorption rate (SAR) variation during the entire exposure period, a statistical analysis for the rat activity behavior in the exposure period is necessary. In this study, we divide the rat cage in the exposure box into nine small areas, and derive the stay frequency of rat in each area based on the classification of the documentary photos of rat activity. Using the stay frequency as a weighting factor, we calculate the statistical characteristics of the whole-body average SAR for the rats during the entire exposure period. Such an approach gives a high quality determination of the exposure level.
[show abstract][hide abstract] ABSTRACT: In order to calculate discharge current waveforms for air discharges of ESD-Guns, an equivalent circuit representation is proposed by means of applying a spark resistance formula for our previously proposed circuit modeling for contact discharges. The circuit modeling consists of a voltage source derived from a spark resistance formula, the output impedance of an ESD-gun, and an injecting-point impedance of EUT or discharge target. For validation of the equivalent circuit modeling, discharge current waveforms for air discharges onto an IEC calibration target are calculated and are compared to measured waveforms. As a result, it is found that the calculated discharge current waveforms for air discharges approximately agree with measured ones.