Peak Specific Absorption Rate Values Research Articles

Complementary analysis of specific absorption rate safety for an 8Tx/16Rx array with central symmetry of elements for magnetic resonance imaging of the human heart and abdominopelvic organs at 7 T.

A complementary safety assessment of the specific absorption rate (SAR) of the electromagnetic energy was performed in a prototype 8Tx/16Rx RF array for cardiac magnetic resonance imaging (MRI) at 7 T. The study aimed to address two critical aspects of 7-T SAR safety not always explicitly examined by coil vendors: (i) the influence of an RF-array position on a peak SAR value, and (ii) the risk of exceeding the permitted maximal SAR in the tissue surrounding conductive passive implants. The full-wave 3D electromagnetic simulations for the thorax with shifted array position and the whole-body volume in the presence of a dental retainer, an intrauterine contraceptive device (IUD), and a hip joint implant, were performed for two human voxel models. The effect of the array displacement on the SAR was simulated for seven array locations on the thorax shifted from the central position in different directions on 50 mm. The peak SAR values for both models were analyzed for the three phase-only transmit vectors optimized for B1 + homogeneity and transmit efficiency. Peak SAR values due to the shifts of the array position increase up to ≈50%. The worst-case peak SAR value for a dental retainer was found to be in the range of 10% of the maximal SAR in the tissue within the array's borders. For the IUD and artificial hip joint implants the effect was found to be negligible (peak SAR < 1% of the SAR within array borders). In addition to simulations for cardiac MRI, we performed a preliminary B1 + shimming and SAR-safety analysis for the same RF-array at various positions lower on the body trunk to assess a potential application in imaging abdominopelvic organs (prostate, kidney, and liver). The most promising target for an ad hoc alternative application of the array was found to be the prostate.

Safety Analysis of Medical Implants in the Human Head Exposed to a Wireless Power Transfer System

The interactions of medical implants in the human body with electromagnetic fields from newly introduced high-field technologies such as near-field wireless power transfer (WPT) are of great concern. In this study, the effects of implants on the specific absorption rate (SAR) in a head model were computationally evaluated in the immediate vicinity of a WPT system operating at 6.78 MHz with a transferred power of 50 W. The SAR in the head model located 30 mm above the WPT system was evaluated with a skull plate, bone plates (different shapes), miniplate, fixtures, and a deep brain stimulator. The results indicated that even small implants had notable effects on the SAR distribution and corresponding local peak and mass-averaged SAR values. Apart from the size and shape of the implant, the crucial factor influencing the SAR was the implant’s position with respect to the WPT system. Therefore, various position configurations were simulated for the WPT system to determine the worst case scenario for each specific implant. Although the local peak SAR value was increased by a factor of 600 in the worst-case scenario (for the skull plate), the SAR <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{\text{1}g}$</tex-math></inline-formula> and SAR <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{\text{10}g}$</tex-math></inline-formula> were only increased by factors of seven and four, respectively. Compliance with international safety limits was studied, followed by computing the maximum allowable transmit power (MATP). It was found that, without the implant, the MATP satisfying SAR <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{\text{1}g}$</tex-math></inline-formula> was 43 W for the designed WPT system, and was reduced to 6.9 W in the presence of the skull plate.

All-Textile On-Body Antenna for Military Applications

An all-textile cavity-backed substrate integrated waveguide (SIW) antenna is proposed for military applications. The antenna matches standard military badge size. The aircraft-shape badge antenna that operates in X-band is designed for air force. SIW cavity and meandered slots that form the body and wing of the figure provide resonance at 8 GHz with 26&#x0025; bandwidth. Symmetrical boresight patterns with very low sidelobes are observed. The measured gain and efficiency of the structure are 5.2 dBi and 79.2&#x0025; at 8 GHz, respectively. This efficiency value is the highest among other SIW cavity textile-antennas. The antenna is investigated on conformal surfaces, as well. Bending of the antenna to follow body movements does not deteriorate performance of the textile antenna sensor. The peak specific absorption rate value over 10 g of tissue was 0.53 and 0.69 W&#x002F;kg for 30 dBm input power for female and male breasts, which makes it suitable as an on-body antenna. The badge antenna is fabricated with standard textile fabrication techniques and its simulation performance is confirmed successfully by measurements. Usage of standard off-the-shelf parts for the assembly and fabrication with standard textile fabrication techniques enables simple and cost-effective mass production of the proposed badge antenna.

A pattern‐reconfigurable antenna design for body area networks communications

This article presents a novel pattern-reconfigurable antenna based on theory of characteristic mode for body area networks (BAN) communication. The radiation pattern of the antenna is able to switch between the omnidirectional pattern and broadside pattern, which can satisfy on-body and off-body communication links simultaneously. An array of 4 × 2 rectangular plates with the off-body communication mode at BAN frequency band is used as the main radiator, and four copper posts between the radiator and the ground are introduced to generate the on-body mode. A simple feeding network is designed to selectively excite the two modes. The footprint of the proposed antenna is 0.43 λ0 × 0.43 λ0 × 0.05 λ0. For demonstration, the proposed antenna has been fabricated and measured both in the free space and on the phantom. The measured results agree well with the simulated ones. Furthermore, the issue of the specific absorption rate (SAR) is studied. The peak SAR values of the proposed antenna in two communication modes are far below the Federal Communication Commission standard. In addition, for practical application, the link budget of the on-body link is investigated. The stable impedance and radiation properties, the low SAR values and the reliable communication ability make the proposed antenna be a suitable candidate for BAN communications.

Metamaterial-Inspired Radiofrequency (RF) Shield With Reduced Specific Absorption Rate (SAR) and Improved Transmit Efficiency for UHF MRI.

To prevent the interferences between radiofrequency (RF) coils and other components in the magnetic resonance imaging (MRI) system such as gradient coils, it is essential to place an RF shield between the RF coils and gradient coils. However, the induced currents on conventional RF shields have negative influences on the RF coil performance. To reduce these influences, metamaterial absorbers (MA), a class of metamaterials exhibiting nearly unity absorption rate for the incident electromagnetic fields, can be employed for the design of a novel RF shield. However, the adoption of metamaterials in MRI systems is usually problematic because of the bulkiness of the metamaterial structure. In this work, capacitors and metallic interconnectors are used to miniaturize the MA so that the unit MA cell can operate at the Larmor frequencies of 7T and 9.4T MRI and stay compact. This MA-RF shield is used to improve the transmit efficiency of RF surface coils and reduce the specific absorption rate (SAR) in the region of interest (ROI). It is successfully demonstrated by simulations and experiments that, compared with conventional RF shield structure, the transmit efficiency can be enhanced by more than 32% and the peak SAR value can be reduced by 22% using the MA-RF shield. Moreover, it is observed that the transmit field penetration is improved when the surface coil is used with the MA-RF shield. This proof-of-concept study suggests a new practical way for the utilization of metamaterials in ultra-high field MRI applications.

Design of Low-SAR Mobile Phone Antenna: Theory and Applications

This article focuses on the design of a low specific absorption rate (SAR) mobile phone antennas. First, the interaction mechanism of the radiation fields of mobile phone antennas with nearby human tissues is analyzed by using boundary conditions of electromagnetics. The relations between SAR and the electric field are constructed. Then, a concept of reverse current is proposed to decrease the electric field radiated by the antenna at the air-tissue interface and, consequently, reduce the SAR of human tissues. Finally, a metal-rimmed mobile phone antenna is designed according to the proposed theory. The ground clearance of the designed antenna is 2 mm. Their measured normalized peak head and body SAR values are lower than 1.0 W/kg, much less than the SAR safety limit of 1.6 W/kg according to the guidelines of IEEE and FCC or 2.0 W/kg according to the guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The measured -6 dB impedance bandwidths are 830-1040 and 1670-2730 MHz, respectively. The measured efficiencies are higher than -5 dB at the lower band and -4 dB at the higher band. The measured results prove that the proposed theory can provide guidance for the design of low-SAR mobile phone antennas.

Miniaturised and radiation efficient implantable antenna using reactive impedance surface for biotelemetry

This study presents the realisation of reactive impedance surface (RIS) in an implantable environment to design a compact wideband antenna for biotelemetry communication. The antenna is designed to operate at 2.45 GHz industrial, scientific and medical (ISM) band using a one-layer skin model. The proposed mushroom-based circular RIS successively improves the impedance matching, gain, and bandwidth of the antenna. The improvement in −10 dB impedance bandwidth and gain is observed by 270 MHz and 7 dB, respectively. The overall volume of the antenna is compact and measured only 99.69 mm3. The peak specific absorption rate value of the loaded antenna is noted at 595 W/kg for input power of 1 W, while the radiation efficiency of the antenna is noted by 7.5% at the resonance. The fabricated prototype is inserted into the minced pork and dipped into a skin mimicking gel for the measurement purpose. Furthermore, the link margin has been analysed, which predicts a possible transmission range up to 40 m for a signal bit rate ≤5 Mbps. The impact of associated circuit components on the antenna performance has also been investigated.

Design and performance analysis of a conformal CPW fed wideband antenna with Mu-Negative metamaterial for wearable applications

AbstractIn this paper, a flexible CPW fed ultrawide band (UWB) antenna with mu-negative (MNG) metamaterial is designed, fabricated, and tested for wearable applications. Initially, a UWB antenna of size 50 mm × 43 mm is fabricated on two different substrates, viz. flexible FR4 and semi-flexible Rogers RT/duroid 5880. A metamaterial structure fabricated on flexible FR4 shows a magnetic resonance from 7.2 GHz to 9.2 GHz with maximum stop band attenuation (−49 dB) and high MNG value (−2121.6) at 7.87 GHz. Then a (3 × 3) array of designed MNG metasurface is used as ground plane with flexible UWB antenna, which improves its overall gain and radiation pattern. The performance of the flexible antenna with/without metamaterial at various distances from flat and cylindrical three-layered human phantom of skin, fat, and muscle is studied. Further, the bending characteristics at different angles and performance over thin metallic sheet is also evaluated. Additionally, the peak specific absorption rate value averaged over 1 g of tissue at three chosen frequencies from UWB range (3, 5, 10 GHz) with/without metamaterial using 0.3 and 0.1 W of input power is also analyzed. The simulated and measured results are in good agreement which confirms that the designed antenna is a good candidate for wearable applications.

Radio-Frequency Safety Assessment of Stents in Blood Vessels During Magnetic Resonance Imaging.

Purpose: The purpose of this study was to investigate the need for high-resolution detailed anatomical modeling to correctly estimate radio-frequency (RF) safety during magnetic resonance imaging (MRI). RF-induced heating near metallic implanted devices depends on the electric field tangential to the device (Etan). Etan and specific absorption rate (SAR) were analyzed in blood vessels of an anatomical model to understand if a standard gel phantom accurately represents the potential heating in tissues due to passive vascular implants such as stents.Methods: A numerical model of an RF birdcage body coil and an anatomically realistic virtual patient with a native spatial resolution of 1 mm3 were used to simulate the in vivo electric field at 64 MHz (1.5 T MRI system). Maximum values of SAR inside the blood vessels were calculated and compared with peaks in a numerical model of the ASTM gel phantom to see if the results from the simplified and homogeneous gel phantom were comparable to the results from the anatomical model. Etan values were also calculated in selected stent trajectories inside blood vessels and compared with the ASTM result.Results: Peak SAR values in blood vessels were up to ten times higher than those found in the ASTM standard gel phantom. Peaks were found in clinically significant anatomical locations, where stents are implanted as per intended use. Furthermore, Etan results showed that volume-averaged SAR values might not be sufficient to assess RF safety.Conclusion: Computational modeling with a high-resolution anatomical model indicated higher values of the incident electric field compared to the standard testing approach. Further investigation will help develop a robust safety testing method which reflects clinically realistic conditions.

A SAR Reduced mm-Wave Beam-Steerable Array Antenna With Dual-Mode Operation for Fully Metal-Covered 5G Cellular Handsets

In this letter, a specific absorption rate (SAR) reduced 28 GHz beam-steering array antenna with dual-mode operation for a fully metal-covered fifth-generation (5G) cellular handset is proposed. The proposed antenna consists of two subarrays, each of which has eight rotated slot antenna elements, which are arranged on the upper frame and on the part of the back cover of the handset. The subarrays are selected according to the modes by using a switch. The proposed array demonstrates good reflection coefficients in the frequency band ranging from 27.2 to 28.2 GHz and the mutual couplings between antenna elements are less than –11.8 dB. The proposed antenna has good beam-steering properties and a hemispherical beam coverage for a millimeter-wave (mm-wave) 5G cellular handset. The calculated peak SAR values on the head phantom by the proposed structure, for the beam scan angles of 0° and 40°, are 0.53 and 0.88 W/kg, respectively, when the input power of each subarray is 24 dBm.

Experimental Dosimetry in a Mode-Stirred Reverberation Chamber in the 60-GHz Band

This paper aims at introducing a calibration procedure for experimental dosimetry in a millimeter wave reverberation chamber. The proposed procedure is based on surface temperature measurements performed on a parallelepiped phantom using an infrared camera. The temperature rise measurement is compared to the value predicted by an analytical model. The latter is based on heat transfer formulation using the statistically homogeneous and isotropic electromagnetic field generated in the reverberation chamber. Temperature measurements are performed, at 59.3 GHz, on a dielectric phantom constituted of distilled water with a concentration of 4% of agar. Good agreement is observed between experimental and theoretical models. The difference between the peak specific absorption rate values estimated by two different approaches is about 16%.

SAR in human head model due to resonant wireless power transfer system.

Efficient mid-range wireless power transfer between transmitter and the receiver has been achieved based on the magnetic resonant coupling method. The influence of electromagnetic field on the human body due to resonant wireless power transfer system (RWPT) should be taken into account during the design process of the system. To analyze the transfer performance of the RWPT system and the change rules of the specific absorption rate (SAR) in the human head model due to the RWPT system. The circuit-field coupling method for a RWPT system with consideration of the displacement current was presented. The relationship between the spiral coil parameters and transfer performance was studied. The SAR in the human head model was calculated under two different exposure conditions. A system with output power higher than 10 W at 0.2 m distance operating at a frequency of approximately 1 MHz was designed. The FEM simulation results show the peak SAR value is below the safety limit which appeared when the human head model is in front of the transmitter. The simulation results agreed well with the experimental results, which verified the validity of the analysis and design.

The numerical modelling studies performed at the Gazi Biophysics Laboratory

In recent years humans are more exposed to human-made electromagnetic (EM) fields than natural fields with developing technologies. Especially, widespread use of wireless communication technologies in all areas of daily life and getting closer to sensitive organs like brain caused an increase in possible risks and worries about human health. For this reason, the potential health effects from the use of mobile phones has been studied intensively in scientific field. Specific Absorption Rate (SAR) with the dose limits are used to prevent possible health effects of Radio Frequency (RF) radiation. Due to lack of SAR value measurement on human or animal tissues directly or in a lab environment, the value of the dose from exposure to the EM Field was determined from measurements of phantoms filled with tissue equivalent liquid or can be made by using a computer simulation environment. The Numerical Modelling studies performed at the Gazi Biophysics Laboratory are reviewed in this paper: 1. Simulations of human head for mobile phone (CP) usage: In this study, the effects of 835 MHz and 900 MHz RFR exposures on human head while using CP were investigated. The effects of CP usage on SAR were calculated by SEMCAD X software which uses FDTD method in details. Some parameters as the different head dimensions and dielectric properties of the head (adult and child), positions of the mobile phone (cheek and tilt), and rectangular metal frame spectacles as a widely used metallic accessory were considered. With this aim, dose values in the tissue for 10 g peak spatial-average SAR value were calculated. At both of the frequencies of 835 MHz and 900 MHz, higher SAR values were obtained in the cheek positions than the tilt positions for conditions of with or without metal frame spectacles. 2. Simulations of “in vivo” experimental set-up: 2.1.Numerical simulations were performed on a simplified model of mouse contained in a square shaped glass cage and Generic Mobile Phone located 1 cm below the cage. 10 g peak spatial average SARs at 900 MHz in the head and trunk was determined respectively. 2.2.Numerical simulations were performed on a simplified model of rat located from the horn antenna as an RF source. 10 g peak spatial average SARs at 900 and 1800 MHz in the head was determined. 3. Simulations of “in vitro” experimental set-up: In vitro exposure system was consist of a horn antenna and 24-well micro titer plates. Quad Ridged Guide Horn antenna was used for a radio frequency field source. 24 well microtiter plates centered on 1 mm above the horn antenna. Numerical models for the DMEM medium inside a 24 well microtiter plates were used to assess peak SAR values averaged over 10 g of tissue at 2.1 GHz.

자기공명영상장치에서 전자파흡수율 분석

본 연구는 자기공명영상장치의 특성변수인 숙임각(flip angle), 반복시간(repetition time, TR), 에코시간(echo time, TE)을 사용하여 전자파흡수율(specific absorption rate, SAR)을 알아보고자 하였다. 연구대상은 체중 10 kg부터 90 kg까지를 대상으로 하여 동일한 검사기법을 적용하였고, 매개변수의 변화에 따른 평균 SAR 및 peak SAR 값을 측정하였다. 체중에 따른 SAR는 TE에서 변화는 없지만 숙임각이 커지고 TR이 짧을수록 증가하였다. SAR 값은 두부 허용 기준치 범위에 포함이 되었고 분절별 체중에 따른 영상의 신호 대 잡음비(signal to noise ratio, SNR)는 체중이 증가함에 따라 SNR은 증가하지는 않았다. 본 결과를 바탕으로 적절한 특성변수를 사용하여 다양한 대조도와 SNR을 얻어 진단의 가치를 높일 수 있을 것이다. In this paper, we measured specific absorption rate (SAR) using characteristic variables such as flip angle, repetition time (TR) and echo time (TE) at magnetic resonance imaging. The subject was applied to same scan technique from body weight 10 kg to 90 kg, were measured for the average SAR and the peak SAR values according to the change of parameter. SAR with different body weight levels was not seen a significant change at TE but it increased in the larger flip angle and the shorter TR. SAR value was within the limits of human head acceptable standard and SNR in segmental body weights was not proportional to the increase of body weights. In conclusion, this study can be helpful for diagnosis by using appropriate parameters which obtained the various contrast and SNR.

New head models extracted from thermal infrared (IR) images for dosimetry computations

In electromagnetic dosimetry, anatomical human models are commonly obtained by segmentation of magnetic resonance imaging or computed tomography scans. In this paper, a human head model extracted from thermal infrared images is examined in terms of its applicability to specific absorption rate (SAR) calculations. Since thermal scans are two-dimensional (2D) representation of surface temperature, this allows researchers to overcome the extensive computational demand associated with 3D simulation. The numerical calculations are performed using the finite-difference time-domain method with mesh sizes of 2 mm at 900 MHz plane wave irradiation. The power density of the incident plane wave is assumed to be 10 W/m2. Computations were compared with a realistic anatomical head model. The results show that although there were marked differences in the local SAR distribution in the various tissues in the two models, the 1 g peak SAR values are approximately similar in the two models.

Numerical Calculation of Peak-to-Average Specific Absorption Rate on Different Human Thorax Models for Magnetic Resonance Safety Considerations

Magnetic resonance imaging (MRI) is considered a safe technology since it relies only on spatial encoding of the position of atomic nuclei (mainly protons) in a static magnetic field irradiating them with radio-frequency (RF) pulses instead of ionizing radiation. Specific absorption rate (SAR) is the most frequently used parameter for monitoring and quantifying the power deposition on a subject during an MRI examination. The peak-to-average SAR ratio is important information for the MRI operator during the acquisition sequence setup. In this work, a birdcage body coil model is used for RF excitation of several inhomogeneous human thorax models with different sizes and weights. To study the peak-to-average SAR ratio correlation with sample metrics, numerical simulations using the finite difference time domain method were performed to estimate the peak and average SAR values on the entire sample volume. Results for 11 different thorax models indicate a strong correlation between the peak-to-average SAR value and the sample metrics.

COMPUTATION OF ELECTROMAGNETIC DOSIMETRY FOR HUMAN BODY USING PARALLEL FDTD ALGORITHM COMBINED WITH INTERPOLATION TECHNIQUE

To enhance the flexibility of the parallel FDTD for the analysis of the bio-electromagnetic problems, a universal and efficient interpolation technique based on the super-absorbing boundary principle is presented, which can improve the interpolation accuracy and ensure the stability of the parallel FDTD iterative procedure. Using this technique, we calculate the SAR (Specific Absorption Rate) values in the head for two different human-body postures. In the iteration procedure of parallel FDTD, the data are exchanged between adjacent subdomains with the interpolation technique. Thus, the meshes can be created in local coordinates, which makes it convenient to build the human model in the different posture and use position for FDTD computing. The results show that the change of human-body posture only brings about a slight decrease (within 6.8%) in the peak SAR values, whereas the SAR values in the brain, as a critical organ, are sensitive to the change of the body posture, and it increases by 28% at maximum for the 1-g averaged peak SAR.

A four spiral slots microstrip patch antenna for radiotelemetry capsules based on FDTD

Antenna is very crucial to radiotelemetry capsules which can measure the physiological parameters of the gastrointestinal (GI) tract. The objective of this paper is to design a novel spiral slots microstrip patch antenna for the radiotelemetry capsules communicating with external recorder at 915 MHz located in ISM (Industry, Science, and Medical) bands. The microstrip patch antenna is designed and evaluated using the finite-difference time-domain (FDTD) method. Return loss characteristics and the effect of the human body on resonant frequency are analyzed, and the performances of radiation patterns at different positions of the human alimentary tract are also estimated. Finally, specific absorption rate (SAR) computations are performed, and the peak 1-g and 10-g SAR values are calculated. According to the peak SAR values, the maximum delivered power for the designed antenna was found so that the SAR values of the antenna satisfy the ANSI (American National Standards Institute) limitations.

7 T body MRI: B1 shimming with simultaneous SAR reduction

The high frequency of the radiofrequency (RF) fields used in high field magnetic resonance imaging (MRI) results in electromagnetic field variations that can cause local regions to have a large specific absorption rate (SAR) and/or a low excitation. In this study, we evaluated the use of a B1 shimming technique which can simultaneously improve the B+1 homogeneity and reduce the SAR for whole body imaging at 7 T. Optimizations for four individual anatomies showed a reduction up to 74% of the peak SAR values with respect to a quadrature excitation and a simultaneous improvement of the B+1 homogeneity varying between 39 and 75% for different optimization parameters. The average SAR was reduced with approximately 50% for all optimizations. The optimized phase and amplitude settings from an elliptical phantom model were applied to four realistic human anatomy models to evaluate whether a generic application without prior knowledge of the detailed human anatomy is possible. This resulted in an average improvement of the B+1 homogeneity of 37% and an average reduction of the maximum and average SAR of 50 and 55%, respectively. It can be concluded that this generic method can be used as a simple method to improve the prospects of 7 T body imaging.

Performance of shorted microstrip patch antennas for mobile communications handsets at 1800 MHz

We have investigated the application of two different types of novel shorted-patch antennas for mobile communications handsets at 1800 MHz. A single shorted-patch and a stacked shorted-patch antenna offering improved bandwidth are compared with data for a /spl lambda//4 monopole. The finite-difference time-domain (FDTD) technique was used to calculate antenna characteristics such as impedance and radiation patterns for two cases: on a handset and on a handset near a (2.5-mm voxel) heterogeneous head model in an actual position of phone use. We also obtained specific absorption rate (SAR) distributions and calculated the spatial peak 1-g SAR values. In addition, the effect on SAR and antenna characteristics of including a block model of the hand was assessed. Similar performance is achieved from the single or stacked shorted-patch antenna with the latter providing greater bandwidth, 8.2% versus 9.4% with the head and hand included. Both antennas reduce the l-g spatial peak SAR value in the head by 70% relative to the monopole. The presence of the hand reduces the efficiency of all three antenna types by approximately 10%.