Peak Spatial Specific Absorption Rate Research Articles

Deep learning-based local SAR prediction using B1 maps and structural MRI of the head for parallel transmission at 7 T.

To predict subject-specific local specific absorption rate (SAR) distributions of the human head for parallel transmission (pTx) systems at 7 T. Electromagnetic energy deposition in tissues is nonuniform at 7 T, and interference patterns due to individual channels of pTx systems may result in increased local SAR values, which can only be estimated with very high safety margins. We proposed, designed, and demonstrated a multichannel 3D convolutional neural network (CNN) architecture to predict local SAR maps as well as peak-spatial SAR (ps-SAR) levels. We hypothesized that utilizing a three-channel 3D CNN, in which each channel is fed by a map, a phase-reversed map, and an MR image, would improve prediction accuracies and decrease uncertainties in the predictions. We generated 10 new head-neck body models, along with 389 3D pTx MRI data having different RF shim settings, with their B1 and local SAR maps to support efforts in this field. The proposed three-channel 3D CNN predicted ps-SAR10g levels with an average overestimation error of 20%, which was better than the virtual observation points-based estimation error (i.e., 152% average overestimation). The proposed method decreased prediction uncertainties over 20% (i.e., 22.5%-17.7%) compared to other methods. A safety factor of 1.20 would be enough to avoid underestimations for the dataset generated in this work. Multichannel 3D CNN networks can be promising in predicting local SAR values and perform predictions within a second, making them clinically useful as an alternative to virtual observation points-based methods.

The impact of respiratory motion on electromagnetic fields and specific absorption rate in cardiac imaging at 7T.

To present electromagnetic simulation setups for detailed analyses of respiration's impact on and E-fields, local specific absorption rate (SAR) and associated safety-limits for 7T cardiac imaging. Finite-difference time-domain electromagnetic field simulations were performed at five respiratory states using a breathing body model and a 16-element 7T body transceiver RF-coil array. and SAR are analyzed for fixed and moving coil configurations. SAR variations are investigated using phase/amplitude shimming considering (i) a local SAR-controlled mode (here SAR calculations consider RF amplitudes and phases) and (ii) a channel-wise power-controlled mode (SAR boundary calculation is independent of the channels' phases, only dependent on the channels' maximum amplitude). Respiration-induced variations of both amplitude and phase are observed. The flip angle homogeneity depends on the respiratory state used for shimming; best results were achieved for shimming on inhale and exhale simultaneously ( ). The results reflect that respiration impacts position and amplitude of the local SAR maximum. With the local-SAR-control mode, a safety factor of up to 1.4 is needed to accommodate for respiratory variations while the power control mode appears respiration-robust when the coil moves with respiration (SAR peak decrease: 9% exhale→inhale). Instead, a spatially fixed coil setup yields higher SAR variations with respiration. Respiratory motion does not only affect the distribution and hence the image contrast, but also location and magnitude of the peak spatial SAR. Therefore, respiration effects may need to be included in safety analyses of RF coils applied to the human thorax.

Numerical Modeling of Smartphones with WCDMA, LTE, and WLAN Bands for Epidemiological Studies

This paper presents the representative numerical modeling of smartphones with wideband code division multiple access (WCDMA), long-term evaluation (LTE), and wireless local area network (WLAN) bands for epidemiological studies. For this purpose, based on specific absorption rate (SAR) test reports for commercial smartphones released in South Korea from 2013 to 2019, we determined the smartphone size, frequency band categorization, antenna locations, and target 1-g peak-spatial SAR (psSAR) values for a specific anthropomorphic mannequin (SAM) phantom. Numerical results showed that the designed numerical smartphone models yielded good matching and radiation performance, and more importantly, their 1-g psSAR values were within ±16% of the target 1-g psSAR values.

Numerical Study of Maximum Peak Spatial SAR Reduction in a Mobile Phone

The government regulates specific absorption rate (SAR) levels of mobile phones and beyond that limit, mobile phones are not available on the market. The SAR measurement for a specific anthropomorphic mannequin (SAM) phantom is performed for four different positions and all 1-g peak spatial SAR values should meet the requirement. In this work, we propose a novel approach to reduce the maximum 1-g peak spatial SAR value by balancing the SARs. This SAR balancing can be achieved by modifying ground current distributions. Numerical examples are employed to validate our approach. It is observed that the maximum 1-g peak spatial SAR value can be reduced by 40% compared to the case without modifying ground current distributions.

Mobile phone types and SAR characteristics of the human brain

Mobile phones differ in terms of their operating frequency, outer shape, and form and location of the antennae, all of which affect the spatial distributions of their electromagnetic field and the level of electromagnetic absorption in the human head or brain. For this paper, the specific absorption rate (SAR) was calculated for four anatomical head models at different ages using 11 numerical phone models of different shapes and antenna configurations. The 11 models represent phone types accounting for around 86% of the approximately 1400 commercial phone models released into the Korean market since 2002. Seven of the phone models selected have an internal dual-band antenna, and the remaining four possess an external antenna. Each model was intended to generate an average absorption level equivalent to that of the same type of commercial phone model operating at the maximum available output power. The 1 g peak spatial SAR and ipsilateral and contralateral brain-averaged SARs were reported for all 11 phone models. The effects of the phone type, phone position, operating frequency, and age of head models on the brain SAR were comprehensively determined.

Numerical dosimetry of CDMA/GSM, DCS/PCS and 3G signal jammers

In this study, numerical dosimetry of a code division multiple access (CDMA)/global system of mobile, digital cellular service (DCS)/personal communications service (PCS) and third generation signal jammer working with three antennas is presented. Three anatomical full human body models of different ages and gender exposed to the signal jammer electromagnetic (EM) waves at 900, 1800 and 2100 MHz, simultaneously, are numerically modelled using a high-resolution finite-difference time-domain gridding; solved by a full-wave three-dimensional EM simulation software on a high-performance workstation machine. The average specific absorption rate (SAR) in the whole human body and the peak spatial SAR averaged over 1 g induced in the head and torso biological tissues were evaluated at different distances from the signal jammer above a ground consisting of concrete and soil layers. The maximum radiated power by the signal jammer at different distances against the human body was calculated for the safe exposure in compliance with the Institute of Electrical and Electronics Engineers (IEEE)/American National Standards Institute (ANSI)/Federal Communications Commission (FCC) standard limits.

Dosimetric Simulations of Brain Absorption of Mobile Phone Radiation–The Relationship Between psSAR and Age

As children develop, they differ from adults in a number of important ways, including anatomy, metabolism, immune system, and the extent of myelination of the nervous system. As a consequence, equivalent exposures to radiation from mobile phones result in different doses to specific tissues in children compared with adults. Higher doses are likely to have more severe implications in the young. A young child's skull is not only smaller and thinner than an adult's, but also has dielectric characteristics closer to those of soft tissues, probably due to a higher water content. The young skull better matches the electromagnetic characteristics of the skin and brain. As a result, finite-difference time-domain (FDTD) simulations confirm field penetration and higher specific absorption rate (SAR) in deeper structures in the young brain. If the peak spatial SAR (psSAR) is modeled in the entire head, as current testing standards recommend, the results for adults and children are equivalent. Our anatomically based evaluations rely on FDTD simulations of different tissues within the brain and confirm that the psSAR in a child's brain is higher than in an adult's brain.

Are Children More Exposed to Radio Frequency Energy From Mobile Phones Than Adults?

There has been long-standing controversy, both among scientists and in the public, about whether children absorb more radio frequency (RF) energy in their heads than adults when using a mobile telephone. This review summarizes the current understanding of this issue, and some of the complexities in comparing the absorption of RF energy in different individuals from use of mobile phones. The discussion is limited to dosimetric issues, i.e., possible age-related differences in absorption of RF energy in the heads of mobile phone users. For most metrics of exposure, in particular those relevant to assessing the compliance of handsets with regulatory limits, there is no clear evidence for age-related differences in exposure. For two metrics of exposure, there is a clear evidence that age can play a factor: 1) the local specific absorption rate (SAR), in particular anatomically defined locations within the brain, will vary with head size and hence with age and 2) the SAR, in particular tissues, (e.g., bone marrow in the skull) can vary with age due to age-related differences in the dielectric properties of tissue. However, these differences involve SAR levels that are below the 1-g or 10-g peak spatial SAR (psSAR averaged over 1 or 10 g of tissue) and have no significance for compliance assessment. Age-related differences observed in worst case simulations such as presently considered are difficult to generalize to human populations under real-world exposure conditions due to many variables that determine SAR during realistic usages.

Why children absorb more microwave radiation than adults

History of exposure limits can be listed as follows; American National Standard Institute (ANSI) has identified that exposure limits were 10 mW/cm2 averaged over 6 minutes in 1966 and 1974. Partial body irradiation must be included since it has been shown that some parts of the human body (e.g., eyes, testicles) may be harmed if exposed to incident radiation levels significantly in excess of the recommended levels. Today’s standards allow partial body irradiation to be 20 times higher than whole body radiation. ANSI recognized the “the invariable presence of electrical hotspots in the irradiated body.” In 1982. A threshold of behavioral disruption was established: 4 W/kg” therefore a 10-fold “safety factor” SARWB=0.4 W/kg. Behavioral disruption is irreversible: “The assumption is that reversible disruption during an acute exposure is tantamount to irreversible injury during chronic exposure”. In 1991, Institute of Electrical and Electron Engineers (IEEE) mentioned two tier exposure level; partial body specific absorption rate (SAR) and whole body SAR are no longer identic. It was searched electrical workers and general public. Exposure of general public was calculated as SARWB=0.08 W/kg; SARSP=1.6 W/kg, 30 minute average, also exposure of electrical workers calculated as SARWB=0.4 W/kg; SARSP=8 W/kg, 6 minute average. It has observed 5 fold lower SAR; 5-fold longer exposure between groups. Specific absorption rate is formulated as SAR= (σ/ρ)*E2, where σ is the conductivity of the tissue, ρ is the tissue density, and E is the electric field. However, E is a function of the electrical parameter permittivity, e,“the resistance that is encountered when forming an electric field in a medium. Higher conductivity increases; higher tissue density decreases; higher permittivity decreases the SAR. However, children’s absorption of microwave radiation is greater than adults. It was found that ICNIRP’s 2 W/kg, 10 g spatial peak SAR results in 2.3-3 times higher SAR than FCC’s 1.6 W/kg, 1g spatial peak SAR. It was showed that 1-g SAR of brain tissues of children is about two times higher than adults. At the same time another group of researchers asserted that hypocampus and hypothalamus receive 1.6-3.1 higher SAR in children compared to adults; children’s bone marrow receive 10 times higher SAR than adults; children receive higher SAR to the eyes than adults; children’s cerebellum receive >2.5xhigher SAR than adults. Greatly underestimating the SAR for typical mobile phone users, especially children. A superior computer simulation certification process has been approved by the Federal Communications Commission (FCC) but is not employed to certify cell phones. In the United States, the FCC determines maximum allowed exposures. Many countries, especially European Union members, use the “guidelines” of International Commission on Non-Ionizing Radiation Protection (ICNIRP), a non governmental agency. The SAR for a 10-year old is up to 153% higher than the SAR for the SAM model. When electrical properties are considered, a child’s head’s absorption can be over two times greater, and absorption of the skull’s bone marrow can be ten times greater than adults. Therefore, a new certification process is needed that incorporates different modes of use, head sizes, and tissue properties.

SAR exposure from UHF RFID reader in adult, child, pregnant woman, and fetus anatomical models

The spread of radio frequency identification (RFID) devices in ubiquitous applications without their simultaneous exposure assessment could give rise to public concerns about their potential adverse health effects. Among the various RFID system categories, the ultra high frequency (UHF) RFID systems have recently started to be widely used in many applications. This study addresses a computational exposure assessment of the electromagnetic radiation generated by a realistic UHF RFID reader, quantifying the exposure levels in different exposure scenarios and subjects (two adults, four children, and two anatomical models of women 7 and 9 months pregnant). The results of the computations are presented in terms of the whole-body and peak spatial specific absorption rate (SAR) averaged over 10 g of tissue to allow comparison with the basic restrictions of the exposure guidelines. The SAR levels in the adults and children were below 0.02 and 0.8 W/kg in whole-body SAR and maximum peak SAR levels, respectively, for all tested positions of the antenna. On the contrary, exposure of pregnant women and fetuses resulted in maximum peak SAR(10 g) values close to the values suggested by the guidelines (2 W/kg) in some of the exposure scenarios with the antenna positioned in front of the abdomen and with a 100% duty cycle and 1 W radiated power.

Mechanisms of RF Electromagnetic Field Absorption in Human Hands and Fingers

The absorption of electromagnetic fields in the hand is investigated over the 900-3700-MHz frequency range. This enables the determination of the envelope of the peak spatial specific absorption rate in the hand. It also provides a basis for deriving measurement procedures for evaluating compliance of wireless devices with specific absorption rate limits in the hands. Both plane waves and dipole antennas are used to investigate the patterns of RF absorption in hand and finger tissue models for far- and near-field exposures. The results demonstrate that absorption enhancements are found in the hand that are not present in a standardized flat phantom. Enhancements of several decibels are observed, depending on the model parameters. A method to conservatively estimate the exposure in the hand based on flat phantom measurements is proposed.

Exposure system to study hypotheses of ELF and RF electromagnetic field interactions of mobile phones with the central nervous system

A novel exposure system for double-blind human electromagnetic provocation studies has been developed that satisfies the precision, control of fields and potential artifacts, and provides the flexibility to investigate the response of hypotheses-driven electromagnetic field exposure schemes on brain function, ranging from extremely low frequency (ELF) to radio frequency (RF) fields. The system can provide the same exposure of the lateral cerebral cortex at two different RF frequencies (900 and 2140 MHz) but with different exposure levels at subcortical structures, and also allows uniform ELF magnetic field exposure of the brain. The RF modulation and ELF signal are obtained by a freely programmable arbitrary signal generator allowing a wide range of worst-case exposure scenarios to be simulated, including those caused by wireless devices. The maximum achievable RF exposure is larger than 60 W/kg peak spatial specific absorption rate averaged over 10 g of tissue. The maximum ELF magnetic field exposure of the brain is 800 A/m at 50 Hz with a deviation from uniformity of 8% (SD).

AN INTERACTION STUDY BETWEEN PIFAS HANDSET ANTENNA AND A HUMAN HAND-HEAD IN PERSONAL COMMUNICATIONS

Abstract—In this paper, the interaction of a planar inverted-F antennas array, mounted on a mobile handset, with a human hand-head phantom is investigated in the 1.9GHz band. The hybrid approach involving the particle swarm optimization (PSO) and Nelder-Mead (NM) algorithm is considered to optimize the complex excitations of the adaptive array elements in a mutual coupling environment for different beamforming synthesis. Firstly, the effect of the human handhead on the handset radiation characteristics is studied. Then, the spatial-peak specific absorption rate (SAR) values of 2and 4-element PIFA arrays for mobile handset in the vicinity of a human handhead are evaluated numerically for different scenarios. The antenna is analyzed completely using finite difference time domain (FDTD) method while the interaction is performed using the CST Microwave Studio software.

Estimation Formulas for the Specific Absorption Rate in Humans Exposed to Base-Station Antennas

The demonstration of compliance with guidelines for human exposure to base-station antennas can be a time consuming process or often results in overly conservative estimates. To alleviate this burden and reduce the overestimation, approximation formulas for the whole-body average specific absorption rate (SAR) and the peak spatial SAR of human bodies using readily available basic antenna parameters have been developed and validated in this study. The formulas can be used for adults standing in the radiating near field of base-station antennas operating between 300 MHz and 5 GHz, at distances larger than 200 mm. It is shown that the 95th-percentile absorption for the human population can be well approximated by the absorption mechanism and statistical data of weight, height, and body-mass index of the human population. The validation was performed numerically using three anatomical human models (Duke, Ella, and Thelonious) exposed to 12 generic base-station antennas in the frequency range 300 MHz to 5 GHz at six distances between 10 mm and 3 m. From the 432 evaluated configurations, the estimation formulas for adult models are proven to be conservative in predicting the SAR exposure values of the two adults, but as expected not of the child.

Head exposure system for a human provocation study to assess the possible influence of UMTS‐like electromagnetic fields on cerebral blood circulation using near‐infrared imaging

A head exposure setup for efficient and precisely defined exposure of human subjects equipped with a near-infrared imaging (NIRI) sensor is presented. In a partially shielded anechoic chamber the subjects were exposed to Universal Mobile Telecommunications System (UMTS)-like electromagnetic fields (EMF) by using a patch antenna at a distance of 4 cm from the head. The non-contact design of the exposure setup enabled NIRI sensors to easily attach to the head. Moreover, different regions of the head were chosen for localised exposure and simultaneous NIRI investigation. The control software enabled the simple adaptation of the test parameters during exploratory testing as well as the performance of controlled, randomised, crossover and double-blind provocation studies. Four different signals with a carrier frequency of 1900 MHz were chosen for the exposure: a simple continuous wave signal and three different UMTS signals. Furthermore, three exposure doses were available: sham, low (spatial peak specific absorption rate (SAR) = 0.18 W/kg averaged over 10 g) and high (spatial peak SAR = 1.8 W/kg averaged over 10 g). The SAR assessment was performed by measurement and simulation. Direct comparison of measurement and numerical results showed good agreement in terms of spatial peak SAR and SAR distribution. The variability analysis of the spatial peak SAR over 10 g was assessed by numerical simulations. Maximal deviations of -22% and +32% from the nominal situation were observed. Compared to other exposure setups, the present setup allows for low exposure uncertainty, combined with high SAR efficiency, easy access for the NIRI sensor and minimal impairment of test subjects.

Evaluation and Optimization of the Specific Absorption Rate for Multiantenna Systems

This paper proposes an effective and efficient technique for the evaluation and optimization of the specific absorption rate for multiantenna wireless systems. This technique is based on a decomposition/superposition procedure with two different measurement scenarios. Using this proposed approach, we can estimate the lower and upper bounds of specific absorption rate within anatomically correct human head models as well as minimize the specific absorption rate within selected tissues in the head models. Furthermore, this technique can be combined with a beamforming technique to maximize the overall system reception and minimize peak spatial specific absorption rate. Numerical examples are used to demonstrate the effectiveness of this proposed approach.

Analysis of the local worst-case SAR exposure caused by an MRI multi-transmit body coil in anatomical models of the human body

Multi-transmit coils are increasingly being employed in high-field magnetic resonance imaging, along with a growing interest in multi-transmit body coils. However, they can lead to an increase in whole-body and local specific absorption rate (SAR) compared to conventional body coils excited in circular polarization for the same total incident input power. In this study, the maximum increase of SAR for three significantly different human anatomies is investigated for a large 3 T (128 MHz) multi-transmit body coil using numerical simulations and a (generalized) eigenvalue-based approach. The results demonstrate that the increase of SAR strongly depends on the anatomy. For the three models and normalization to the sum of the rung currents squared, the whole-body averaged SAR increases by up to a factor of 1.6 compared to conventional excitation and the peak spatial SAR (averaged over any 10 cm3 of tissue) by up to 13.4. For some locations the local averaged SAR goes up as much as 800 times (130 when looking only at regions where it is above 1% of the peak spatial SAR). The ratio of the peak spatial SAR to the whole-body SAR increases by a factor of up to 47 and can reach values above 800. Due to the potentially much larger power deposition, additional, preferably patient-specific, considerations are necessary to avoid injuries by such systems.

Sleep EEG alterations: effects of different pulse‐modulated radio frequency electromagnetic fields

Previous studies have observed increases in electroencephalographic power during sleep in the spindle frequency range (approximately 11-15 Hz) after exposure to mobile phone-like radio frequency electromagnetic fields (RF EMF). Results also suggest that pulse modulation of the signal is crucial to induce these effects. Nevertheless, it remains unclear which specific elements of the field are responsible for the observed changes. We investigated whether pulse-modulation frequency components in the range of sleep spindles may be involved in mediating these effects. Thirty young healthy men were exposed, at weekly intervals, to three different conditions for 30 min directly prior to an 8-h sleep period. Exposure consisted of a 900-MHz RF EMF, pulse modulated at 14 Hz or 217 Hz, and a sham control condition. Both active conditions had a peak spatial specific absorption rate of 2 W kg(-1) . During exposure subjects performed three different cognitive tasks (measuring attention, reaction speed and working memory), which were presented in a fixed order. Electroencephalographic power in the spindle frequency range was increased during non-rapid eye movement sleep (2nd episode) following the 14-Hz pulse-modulated condition. A similar but non-significant increase was also observed following the 217-Hz pulse-modulated condition. Importantly, this exposure-induced effect showed considerable individual variability. Regarding cognitive performance, no clear exposure-related effects were seen. Consistent with previous findings, our results provide further evidence that pulse-modulated RF EMF alter brain physiology, although the time-course of the effect remains variable across studies. Additionally, we demonstrated that modulation frequency components within a physiological range may be sufficient to induce these effects.

Exposure assessment in front of a multi‐band base station antenna

This study investigates occupational exposure to electromagnetic fields in front of a multi-band base station antenna for mobile communications at 900, 1800, and 2100 MHz. Finite-difference time-domain method was used to first validate the antenna model against measurement results published in the literature and then investigate the specific absorption rate (SAR) in two heterogeneous, anatomically correct human models (Virtual Family male and female) at distances from 10 to 1000 mm. Special attention was given to simultaneous exposure to fields of three different frequencies, their interaction and the additivity of SAR resulting from each frequency. The results show that the highest frequency--2100 MHz--results in the highest spatial-peak SAR averaged over 10 g of tissue, while the whole-body SAR is similar at all three frequencies. At distances > 200 mm from the antenna, the whole-body SAR is a more limiting factor for compliance to exposure guidelines, while at shorter distances the spatial-peak SAR may be more limiting. For the evaluation of combined exposure, a simple summation of spatial-peak SAR maxima at each frequency gives a good estimation for combined exposure, which was also found to depend on the distribution of transmitting power between the different frequency bands.

Impact of pinna compression on the RF absorption in the heads of adult and juvenile cell phone users

The electromagnetic exposure of cell phone users depends on several parameters. One of the most dominant of these is the distance between the cell phone and the head tissue. The pinna can be regarded as a spacer between the top of the phone and the head tissue. The size of this spacer has not yet been systematically studied. The objective of this article is to investigate the variations of distance as a function of age of the exposed person, and the mechanical force on the pinna and how it affects the peak spatial specific absorption rate (psSAR). The distances were measured for adults and children (6-8 years of age) while applying a well-defined force on the pinna using a custom-developed measurement device. The average distances of the pinnae to the heads and their standard deviations showed no major differences between the two age groups: 10.5 +/- 2.0 mm for children (6-8 years) and 9.5 +/- 2.0 mm for adults. The pinnae of our anatomical high-resolution head models of one adult and two children were transformed according to the measurement results. The numerical exposure analysis showed that the reduced distance due to the pinna compression can increase the maximum 10 g psSAR by approximately 2 dB for adults and children, if the exposure maximum is associated with the upper part of the phone.