Whole-body Average Specific Absorption Rate Research Articles

Impact of RF Shimming on RF-Induced Heating Near Implantable Medical Electrodes in a 3T MRI Coil

Radio frequency (RF) shimming is a widely used method to improve the MRI quality by adjusting the magnitude and phase of the transmit elements in the RF coil and eliminating the field inhomogeneity in 3T MRI system. This process alters the field distribution inside the MRI borehole and changes both the level and location of the absorbed RF energy in patients. It is necessary to assess the impact of RF shimming on the risk of RF-induced heating for patients especially those who have implantable longitude medical electrodes in place during the MRI procedure. This paper investigates the impacts of RF shimming on the RF-induced heating of a generic pacemaker electrode using a transfer function method. Two different shimming schemes were studied. The first excitation scheme was designed to obtain a homogeneous ${B_{1 + }}$ field in the image region-chest region. The second condition maximized the temperature rise around the generic pacemaker electrode with different implant trajectories. All the results were normalized to 2 W/kg average whole-body specific absorption rate (SAR) and a Q-matrices method was used to improve the SAR calculation efficiency. The average temperature rise at the electrode tip under quadrature excitation was 3.7 °C while under optimized excitation was 2.3 °C. The worst-case temperature rise for a selected trajectory was found to be 33.17 °C, which was almost 6 times of the temperature rise under quadrature excitation. Therefore, the RF-shimming condition can significantly change the RF-induced heating of electrodes. To ensure patient safety under MRI, RF-shimming condition should be considered and thoroughly integrated into the RF heating assessment.

Improvement of SAR Accuracy by Combining Two SAR Quantification Methods for Small Animals in Reverberation Chamber Above 10 GHz

Two measurement-based methods are presented to quantify the whole-body-averaged specific absorption rate (WBA-SAR) and local SAR for small animals in a medium-size reverberation chamber (RC) for a bio-effect experiment at frequencies above 10 GHz. S-parameter-based method is a totally experimental approach to quantify the WBA-SAR in small animals based on measuring the mean power density and loss power expressed by S-parameters. Two-step method is a measurement-simulation hybrid approach, which links the mean power density to the finite difference time domain (FDTD) calculated WBA-SAR and local SAR in specified organs. However, at a frequency as high as 10 GHz, the RC is usually not ideal because loss on the RC walls is not negligible. In addition, a numerical-only approach is also unrealistic to quantify the SAR because of the heavy calculation burden and difficulty in obtaining converged results. To address this issue, we proposed an improved two-step hybrid method. The method uses the S-parameter-based method to derive the mean power density by matching its WBA-SAR to that of the two-step hybrid method, and then calculate the local SAR in anatomical numerical animal models using the two-step hybrid method under this mean power density. In such a way, in addition to the advantage of obtaining local SAR, the two-step hybrid method has significantly improved its accuracy of SAR quantification.

Investigation of RF-Induced Heating Near Interventional Catheters at 1.5 T MRI: A Combined Modeling and Experimental Study

This study investigated the radiofrequency-induced heating due to interventional catheters at 1,5 T magnetic resonance image (MRI) based on a combined modeling and experimental method. Two types of interventional catheters, a “single wire” and a “dual wire” catheter, were studied. They were modeled inside a high-resolution anatomical human model along four trajectories. In total, 4 insertion depths, from 17.5 to 70 cm, and 13 scanning landmarks were studied to cover various clinically relevant scenarios. The computational model for the catheters was based on a transfer function approach, measured using the reciprocity theorem. Results of the study showed that the upper limit of the temperature rises near the catheter tip may reach up to 100 °C when scaled to the 2 W/kg average whole-body specific absorption rate (SAR). The computational model was validated experimentally by measuring the induced heating near the catheter in a gel-filled phantom. The data showed a good agreement between the result obtained by the combined method and the direct measurement method with a difference of less than 1 °C. The results suggest that the proposed combined approach, based on the transfer function method, may accurately and efficiently predict the temperature rises near ablation catheters at 1,5 T MRI exposure.

The specific absorption rate in different brain regions of rats exposed to electromagnetic plane waves

Accurate dosimetry of a specific brain region in rats exposed to an electromagnetic field (EMF) is essential for studies focusing on dose-effect relationship of the region. However, only dosimetry of whole brain or whole body were evaluated in most of previous studies. In this study, a numerical voxel rat model with 10 segmented brain regions was constructed. Then, the effects of frequency, incidence direction, and E-polarization direction of plane wave EMF on brain region averaged specific absorption rate (BRSAR) of rats were investigated. At last, the reliability of using whole-body averaged SAR (WBDSAR) and whole-brain averaged SAR (WBRSAR) as estimations of BRSAR were also evaluated. Our results demonstrated that the BRSAR depended on the frequency, incidence direction, and E-polarization direction of the EMF. Besides, the largest deviation could be up to 13.1 dB between BRSAR and WBDSAR and 9.59 dB between BRSAR and WBRSAR. The results suggested that to establish an accurate dose-effect relationship, the variance of the BRSAR induced by alteration of frequency, incidence direction, and E-polarization direction of EMF should be avoided or carefully evaluated. Furthermore, the use of WBDSAR and WBRSAR as estimations of BRSAR should be restricted to certain conditions such that the deviations are not too large.

Whole-body average SAR measurement using flat phantoms for radio base station antennas and its applicability to adult and child human models

The specific absorption rate (SAR) measurement procedure for radio base station antennas using flat phantoms has been investigated and standardized by the International Electrotechnical Commission (IEC). This paper discusses the effectiveness of this procedure for considering Japanese human models based on numerical simulations with validation of whole-body average SAR (WBSAR) measurement. The WBSAR in two Japanese anatomical human models (adult male and 3-year-old child) and two box-shaped phantoms (large and small) using a standardized SAR measurement procedure are compared at 788 MHz and 3.5 GHz. Computational results show that the SAR measurement procedure in the IEC leads to overestimated WBSAR compared to those in Japanese anatomical human models. These results imply that the SAR measurement procedure above is applicable to not only some commonly used but Japanese human models. The WBSAR obtained using SAR estimation formulae in the IEC is also overestimated compared to those in Japanese anatomical human models. In addition, to reduce the measurement time and simplify the post-processing, this paper introduces a SAR measurement procedure based on the two-dimensional SAR distribution around the surface of the bottom of the phantom and the one-dimensional exponential decay of the SAR distribution in the direction of the phantom depth. SAR measurement examples of antennas for small radio base stations are also presented.

2.45 GHz microwave radiation impairs learning, memory, and hippocampal synaptic plasticity in the rat.

Microwave (MW) radiation has a close relationship with neurobehavioral disorders. Due to the widespread usage of MW radiation, especially in our homes, it is essential to investigate the direct effect of MW radiation on the central nervous system. Therefore, this study was carried out to determine the effect of MW radiation on memory and hippocampal synaptic plasticity. The rats were exposed to 2.45 GHz MW radiation (continuous wave with overall average power density of 0.016 mW/cm2 and overall average whole-body specific absorption rate value of 0.017 W/kg) for 2 h/day over a period of 40 days. Spatial learning and memory were tested by radial maze and passive avoidance tests. We evaluated the synaptic plasticity and hippocampal neuronal cells number by field potential recording and Giemsa staining, respectively. Our results showed that MW radiation exposure decreased the learning and memory performance that was associated with decrement of long-term potentiation induction and excitability of CA1 neurons. However, MW radiation did not have any effects on short-term plasticity and paired-pulse ratio as a good indirect index for measurement of glutamate release probability. The evaluation of hippocampal morphology indicated that the neuronal density in the hippocampal CA1 area was significantly decreased by MW.

Evaluation of MRI RF electromagnetic field induced heating near leads of cochlear implants

A typical cochlear implant system under magnetic resonance imaging (MRI) procedures may couple with the radio frequency (RF) electromagnetic field (EMF) and results in an intensified electric field at the lead tip. As a result, the RF energy deposited in human tissues around the lead tip may induce heating and cause tissue damage concerns. The purpose of this work is to evaluate the RF-EMF-induced heating for cochlear implant system in 1.5 T MRI coil and highlight the factors that have significant effects on the heating. The potential factors involved in the RF-EMF-induced heating including the lead type, lead trajectory, human model and MRI landmark. In this paper, the RF-EMF-induced heating for three types of leads is evaluated in two virtual human models. A total of 24 lead trajectories and 23 anatomical landmark positions are studied using the transfer function method. The average temperature rise for all the studied cases in the human models is 0.79 °C, and the maximum value is 2.80 °C for a maximum whole-body average specific absorption rate of 2 W kg−1 in an RF body coil. It is found that the lead trajectory and MRI landmark are two primary influencing factors. The maximum temperature rises for different lead trajectories can vary from 0.82 to 2.80 °C. A difference in heating of 2.80 °C is observed when the landmark changes from −100 to 700 mm. This work demonstrates that it is necessary to take these factors into account when evaluating the RF-EMF-induced heating for implanted medical devices.

Assessment of MRI safety issues for stainless steel sutures used for microtia reconstruction

Potential magnetic resonance imaging issues for stainless steel sutures used for microtia reconstruction could be clinically significant for safety and diagnostic yield considerations. Therefore, the purpose of this investigation was to assess magnetic resonance issues (magnetic field interactions, heating, and artifacts) for different types of stainless steel sutures used for microtia reconstruction. Small gauge, commonly used stainless steel sutures from four different manufacturers (5/0 Steelex, Aesculap/B, Braun Medical, Inc.; Nagata 38 Gauge Microtia Wire, Bear Medical Corporation; Auricular Reco Wire, Medicon Surgical Inc.; and 5-0 B&S 35 Surgical Steel Suture, Ethicon, Inc.) were tested using standardized ex vivo techniques to assess magnetic field interactions, heating, and artifacts at 3Tesla. Before testing, the stainless steel sutures were configured in a manner same as that for cartilage reconstruction used to treat microtia. Each stainless steel suture exhibited minor magnetic field interactions at 3Tesla (translational attraction, deflection angle <10°, and no torque). Heating associated with a whole-body averaged specific absorption rate of 2.9 W/kg was not excessive (highest temperature changes, ≤1.8 °C). Artifacts were relatively minor in relation to the size and shape of each stainless steel suture (artifact size in relation to the size and shape of each stainless steel suture extending ≤5 mm). The stainless steel sutures that underwent testing do not present additional risks to patients in a 3-Tesla or less magnetic field setting (i.e., magnetic resonance conditional). Artifacts for these sutures may only be an issue within close proximity to the reconstructed ear.

Modeling Human Body Using Four-Pole Debye Model in Piecewise Linear Recursive Convolution FDTD Method for the SAR Calculation in the Case of Vehicular Antenna

We propose an efficient finite difference time domain (FDTD) method based on the piecewise linear recursive convolution (PLRC) technique to evaluate the human body exposure to electromagnetic (EM) radiation. The source of radiation considered in this study is a high-power antenna, mounted on a military vehicle, covering a broad band of frequency (100 MHz–3 GHz). The simulation is carried out using a nonhomogeneous human body model which takes into consideration most of the internal body tissues. The human tissues are modeled by a four-pole Debye model which is derived from experimental data by using particle swarm optimization (PSO). The human exposure to EM radiation is evaluated by computing the local and whole-body average specific absorption rate (SAR) for each occupant. The higher in-tissue electric field intensity points are localized, and the SAR values are compared with the crew safety standard recommendations. The accuracy of the proposed PLRC-FDTD approach and the matching of the Debye model with the experimental data are verified in this study.

Numerical analysis for infant's unintentional exposure to 3.5 GHz plane wave radiofrequency electromagnetic fields by field test of fifth generation wireless technologies

AbstractIn this study, the plane wave exposure of an infant to radiofrequency electromagnetic fields of 3.5 GHz was numerically analyzed to investigate the unintentional electromagnetic field (EMF) exposure of fifth generation (5G) signals during field test. The dosimetric influence of age‐dependent dielectric properties and the influence of an adult body were evaluated using an infant model of 12 month old and an adult female model. The results demonstrated that the whole body‐averaged specific absorption rate (WBASAR) was not significantly affected by age‐dependent dielectric properties and the influence of the adult body did not enhance WBASAR. Taking the magnitude of the in situ E field strength into consideration, realistic WBASAR was far below the basic restriction. Age‐dependent dielectric properties could significantly change the tissue specified specific absorption rate (TSSAR) of internal organs. However, the variation was not significant because the absolute values were marginal. Among the factors that influenced TSSAR variation, change in dielectric properties demonstrated a close correlation. In general, at 3.5 GHz, the infant did not absorb more power than the case of EMF exposure to third generation (3G) and fourth generation (4G) signals. The work was helpful for network operators and device manufactures to estimate the potential exposure risk during the field test, especially for the infant.

Whole-Body Specific Absorption Rate Assessment of Lossy Objects Exposed to a Diffuse Field Inside a Reverberant Environment

In this study, we provide a novel approach for exposure assessment in terms of whole-body average specific absorption rate ( WBSAR ) due to the electromagnetic fields inside reverberant environments. The approach utilizes power balance theory and the lossy object's absorption cross section ( ACS ). We use this to present a noninvasive technique for determining the WBSAR in reflective environments. Measurements were performed inside a reverberation chamber on spherically-shaped phantoms filled with lossy liquids. The approach was verified by performing invasive temperature measurements and showed excellent agreement (within 5%). This approach can be used for human exposure assessment inside reverberant environments since human body ACS has been well studied in the literature. The main advantage of the proposed approach is that it overcomes the need for complex invasive measurements in order to determine WBSAR .

A Novel Design of Implantable Esophageal Stent to Reduce the MRI RF-Induced Heating

In this work, the magnetic resonance imaging (MRI) radio frequency (RF)-induced heating mechanism for an esophageal stent is discussed from the prospective of forward scattering and the reciprocity theorem. We conclude that the RF-induced heating largely depends on the induced currents on the surface of the stent. Based on this understanding, a novel implantable segmented stent structure is developed to reduce the RF-induced heating of an esophageal stent under MRI procedures. By using a segmented structure, the specific absorption rate (SAR) for a 10-mm-radius and 98-mm-length stent decreased from 53.0 to 14.2 W/kg at a whole-body average SAR of 2 W/kg, and the maximum temperature rise over 15-min MRI exam is reduced from 4.52 to 1.72 $^\circ$ C. The simulated results and experimental measurements agree with each other within the combined uncertainty and demonstrate the effectiveness of the structured stent design. Related to RF-induced heating, this novel design can be used to develop inherently safe esophageal stents.

Caculation about influence of gap on shield effectiveness of protective suits

The protective suits model on human were established on the basis of the perfect electric conductor and the dimensions of a Chinese male. By means of finite-difference time-domain method, the protective characteristics of the protective suits were studied and the influence of the leakage microwave energy caused by the gap on suits were discussed in detail. The calculation results show that the energy leakage is more likely to happen when the electric field vectors are vertical to the gap. It is found that reduction of whole body average specific absorption rate(WBSAR) in cases of vertical incidence and horizontal incidence is significant when wearing protective suits. The shielding effectiveness(SE) of the protective suits increases with the increase of electromagnetic field frequency (from 300 MHz to 3 GHz), and finally becomes stable. Since the gap is inevitable, special treatment need to be performed to avoid the leakage of energy because SE of the protective suits with gap cannot reach 30 dB.

Exposure time-dependent thermal effects of radiofrequency electromagnetic field exposure on the whole body of rats.

We investigated the thermal effects of radiofrequency electromagnetic fields (RF-EMFs) on the variation in core temperature and gene expression of some stress markers in rats. Sprague-Dawley rats were exposed to 2.14 GHz wideband code division multiple access (W-CDMA) RF signals at a whole-body averaged specific absorption rate (WBA-SAR) of 4 W/kg, which causes behavioral disruption in laboratory animals, and 0.4 W/kg, which is the limit for the occupational exposure set by the International Commission on Non-Ionizing Radiation Protection guideline. It is important to understand the possible in vivo effects derived from RF-EMF exposures at these intensities. Because of inadequate data on real-time core temperature analyses using free-moving animal and the association between stress and thermal effects of RF-EMF exposure, we analyzed the core body temperature under nonanesthetic condition during RF-EMF exposure. The results revealed that the core temperature increased by approximately 1.5°C compared with the baseline and reached a plateau till the end of RF-EMF exposure. Furthermore, we analyzed the gene expression of heat-shock proteins (Hsp) and heat-shock transcription factors (Hsf) family after RF-EMF exposure. At WBA-SAR of 4 W/kg, some Hsp and Hsf gene expression levels were significantly upregulated in the cerebral cortex and cerebellum following exposure for 6 hr/day but were not upregulated after exposure for 3 hr/day. On the other hand, there was no significant change in the core temperature and gene expression at WBA-SAR of 0.4 W/kg. Thus, 2.14-GHz RF-EMF exposure at WBA-SAR of 4 W/kg induced increases in the core temperature and upregulation of some stress markers, particularly in the cerebellum.

Convergence of a Single-Frequency FDTD Solution in Numerical Dosimetry

In this paper, we propose a new approach for determining convergence in finite-difference time-domain (FDTD) method for numerical dosimetry at a single frequency. Both amplitude and phase of the FDTD calculation are simultaneously determined and traced in the time domain in order to evaluate the convergence of the solution and terminate the FDTD computation. Validity of our approach has been demonstrated via dosimetry of a homogeneous dielectric sphere in comparison with Mie’s analytical results at frequencies from 100 kHz to 100 MHz. Induced SARs calculated by the proposed approach are in good agreement with Mie’s results, having differences in the whole-body average SAR for less than 9%. It is also found that convergence of the FDTD solutions is related only to permittivity, conductivity, and time-step interval. Finally, the proposed approach has been applied to analysis of an inhomogeneous realistic human body of a Japanese adult exposed to an electromagnetic (EM) plane wave and to localized EM fields radiated from a wireless device at intermediate frequencies around 10 MHz.

Desktop exposure system and dosimetry for small scale in vivo radiofrequency exposure experiments.

This paper describes a new approach to the risk assessment of exposure from wireless network devices, including an exposure setup and dosimetric assessment for in vivo studies. A novel desktop reverberation chamber has been developed for well-controlled exposure of mice for up to 24 h per day to address the biological impact of human exposure scenarios by wireless networks. The carrier frequency of 2.45 GHz corresponds to one of the major bands used in data communication networks and is modulated by various modulation schemes, including Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Radio Frequency Identification (RFID), and wireless local area network, etc. The system has been designed to enable exposures of whole-body averaged specific absorption rate (SAR) of up to 15 W/kg for six mice of an average weight of 25 g or of up to 320 V/m incident time-averaged fields under loaded conditions without distortion of the signal. The dosimetry for whole-body SAR and organ-averaged SAR of the exposed mice, with analysis of uncertainty and variation analysis, is assessed. The experimental dosimetry based on temperature measurement agrees well with the numerical dosimetry, with a very good SAR uniformity of 0.4 dB in the chamber. Furthermore, a thermal analysis and measurements were performed to provide better understanding of the temperature load and distribution in the mice during exposure.

Cylindrical Antenna Theory for the Analysis of Whole-Body Averaged Specific Absorption Rate

International guidelines and standards on whole-body radio-frequency (RF) dosimetry use the whole-body averaged specific absorption rate (WBA-SAR) as a surrogate metric to quantify the temperature rise in the body. This study proposes the analysis of whole-body RF dosimetry for far-field exposure of a grounded human body in the frequency range of 1–150 MHz based on a semianalytic approach of cylindrical antenna theory. The human body is represented by a lossy homogeneous cylindrical monopole antenna. For the first time, an explicit model for the resonance frequency of a grounded human body is proposed. The model captures the effects of the human body weight, height, and the dielectric properties. This study also addresses the effect of shoes on WBA-SAR. It is found that the resonance frequency for the WBA-SAR with shoe effect is higher than reported from using the bare-footed models, as confirmed by theory and measurement.

Calculation of Whole‐Body Average SARs in Pregnant Woman and Child Simultaneously Exposed to Multiple Radio‐Frequency Waves in Real Environment

SUMMARYThere are few published papers on the whole‐body average specific absorption rates (WBA‐SARs) in humans for simultaneous exposure to multiple radio frequencies (RFs). In order to evaluate human safety of radiowaves in real environments, in this paper we calculate the WBA‐SARs in models of a pregnant woman and 3‐year‐old child for multiple RF exposure, using measured results on electric field intensities from multiple base station antennas installed for cellular phones with four different radiowave frequencies in an underground shopping mall and with five radiowave frequencies in the neighborhood of an elementary school. The Japan Ministry of Internal Affairs and Communications officially reported these data in 2008. Statistical analyses show that the squared sum ratio of the measured electric field to the corresponding Japanese guideline for the general public is smaller than one‐hundredth and one‐thousand at most in the underground shopping mall and elementary school areas, respectively. As a result, we found that the WBA‐SARs in the pregnant woman and 3‐year‐old child models calculated at each of the frequencies for base station antennas in both areas are significantly lower than the The International Commission on Non‐Ionizing Radiation Protection (ICNIRP) basic restriction (0.08 W/kg) for the general public, and that the sum of the WBA‐SARs never exceeds one‐hundredth of 0.08 W/kg for simultaneous exposure to multiple RF.

Development of Chinese reference man deformable surface phantom and its application to the influence of physique on electromagnetic dosimetry

A reference man is a theoretical individual that represents the average anatomical structure and physiological and metabolic features of a specific group of people and has been widely used in radiation safety research. With the help of an advantage in deformation, the present work proposed a Chinese reference man adult-male polygon-mesh surface phantom based on the Visible Chinese Human segment image dataset by surface rendering and deforming. To investigate the influence of physique on electromagnetic dosimetry in humans, a series of human phantoms with 10th, 50th and 90th body mass index and body circumference percentile physiques for Chinese adult males were further constructed by deforming the Chinese reference man surface phantom. All the surface phantoms were then voxelized to perform electromagnetic field simulation in a frequency range of 20 MHz to 3 GHz using the finite-difference time-domain method and evaluate the whole-body average and organ average specific absorption rate and the ratios of absorbed energy in skin, fat and muscle to the whole body. The results indicate thinner physique leads to higher WBSAR and the volume of subcutaneous fat, the penetration depth of the electromagnetic field in tissues and standing-wave occurrence may be the influence factors of physique on electromagnetic dosimetry.

Whole-Body Averaged Specific Absorption Rate Estimation Using a Personal, Distributed Exposimeter

For the first time, a body area network (BAN) is used to construct a personal, distributed exposimeter (PDE), which can measure the whole-body averaged specific absorption rate ( ${{\rm SAR}_{\rm wb}}$ ) in real life, together with the incident power density ( ${{\rm S}_{\rm inc}}$ ). The BAN consists of four textile antennas with integrated radio frequency receiver nodes tuned to the Global System for Mobile Communications (GSM) 900 downlink band. Calibration measurements at 942.5 MHz, using a human subject, are performed in an anechoic chamber. These are combined with numerical simulations to estimate both ${{\rm SAR}_{\rm wb}}$ and ${{\rm S}_{\rm inc}}$ from the averaged received power on the PDE. The PDE has 50% prediction intervals of 3 dB on ${{\rm S}_{\rm inc}}$ and 3.3 dB on the ${{\rm SAR}_{\rm wb}}$ , caused by the presence of the human body, whereas the best single textile antenna in our measurements exhibits ${{\rm PI}_{50}}$ ’s of 7.1 dB on ${\rm {S}}_{{\rm {inc}}}$ and 5 dB on ${{\rm SAR}_{\rm wb}}$ . Measurements using the PDE are carried out in Ghent, Belgium, during which a median ${{\rm S}_{\rm inc}} = 47~\mu\hbox{W}/\hbox{m}^{2}$ and ${{\rm SAR}_{\rm wb}} = 0.25~\mu\hbox{W}/{kg}$ are measured.