Maximum Specific Absorption Rate Value Research Articles

Enhancing antenna performance and SAR reduction by a conductive composite loaded with carbon-silica hybrid filler

In this paper, we describe a technique for reducing the specific absorption rate (SAR) and human effects on antenna performance, based on combined use of rubber conductive composite loaded with carbon-silica hybrid filler used like a single director and a resonant half wave dipole. The finite-difference time-domain method was used to evaluate SAR and antenna performance for different antenna-composite configurations and distances. Bandwidth, gain, and radiation efficiency for different antenna-composite configurations were compared. It was found that the presence of a conductive composite loaded with carbon-silica hybrid filler can effectively reduce maximum SAR value (up to 70%) in a human head model, increasing the radiation efficiency (up to 67%) and the bandwidth (up to 110%) of the antenna.

Surface Effects and Critical Dimensions of Iron Nanoparticles

The surface effects on the critical dimensions of ferromagnetic nanoparticles have been studied. Iron nanoparticles with different mean diameter from 5.9 nm to 21.4 nm were prepared by thermal decomposition of iron pentacarbonyl in the presence of oleic acid/octyl ether. The heating response of these ferromagnetic nanoparticles suspended in water were measured experimentally during which the same amount of iron nanoparticles and di-ionized water were irradiated by an alternating magnetic field and the increase in temperature of the system was measured. The heating performance of the nanoparticles was described in terms of Specific Absorption Rate (SAR) which depends on the heating rate. The heating rate was calculated from the initial slope of the heating curve at an inflection point whereby there is minimum heat loss to the surrounding. Results were analyzed to find the critical diameters for the transition from single-domain to superparamagnetic regime and from single-domain to multi-domain regime. Also, the frequency and current dependence of SAR were studied. The maximum value of SAR was obtained when the applied frequency and current were at 175 kHz and 15 A, respectively. An equation for the critical radius for the transition from single-domain to multi-domain regime with low anisotropy was derived and numerically solved by using a program written in C++ and results were analyzed to find the effect of surface parameters on the critical diameter of nanoparticles. The SAR as a function of nanoparticle’s diameter shows two maxima which can be connected with the two critical dimensions. One is DC1 at 18 nm for the transition from single-domain to multi-domain configuration and the second is DC2 at 10 nm for the transition from single-domain to superparamagnetic regime. Comparison of these experimental results with the bond order-length-strength correlation theory was discussed.

FeCo nanowires with enhanced heating powers and controllable dimensions for magnetic hyperthermia

A detailed study of the magnetic properties and heating capacities of electrodeposited FeCo nanowires with varying lengths (2–40 μm) and diameters (100 and 300 nm) is reported. We find that specific absorption rate (SAR) increases rapidly with increasing wire length up to 10 μm, followed by a gradual increase for larger lengths. Magnetic and hyperthermia measurements have revealed the important effect of dipolar interactions between the nanowires on their magnetic and inductive heating responses. Both calorimetric and AC magnetometry methods consistently show that the physical movement contribution of the nanowires to the SAR is small, and that for applied fields exceeding the coercive field, the nanowires tend to align parallel to the field, thus enhancing the SAR. Maximum SAR values of ∼1500 W/g have been achieved for the largest wires at H = 300 Oe and f = 310 kHz.

NOVEL WEARABLE EAGLE SHAPE MICROSTRIP ANTENNA ARRAY WITH MUTUAL COUPLING REDUCTION

Novel eagle shape microstrip wearable antennas (element and array) are presented. The single- and two-element antenna arrays are designed and fabricated on a Roger RT/Duroid 5880 substrate with dielectric constant of 2.2, thickness of 1.5748 mm, and tan δ =0 .001. The measured results show that a reduction in mutual coupling of 36 dB is achieved at the first band (1.68-2.65) GHz and 22.1 dB over the second band (6.5-8.86) GHz due to introducing electromagnetic bandgap (EBG) structures. EBG structure has an eagle-like shape with more gaps. By increasing the number of EBG cells and varying the gap distance between cells to certain limit, the mutual coupling reduction is improved. Also, a size reduction of 80% is achieved. The microstrip array was simulated by CST simulator version 2014 and fabricated by proto laser machine with precision 25µm. The specific absorption rate (SAR) investigation is carried out on CST2014 Simulator. Maximum SAR value is 1.953 W/kg which indicates that the eagle-shaped microstrip wearable antennas are safe for human. The antennas can be used in the official or RFID applications.

Same magnetic nanoparticles, different heating behavior: Influence of the arrangement and dispersive medium

The heating ability of the same magnetic nanoparticles (MNPs) dispersed in different media has been studied in the 170–310 K temperature range. For this purpose, the biggest non-twinned nanoparticles have been selected among a series of magnetite nanoparticles of increasing sizes synthesized via a seeded growth method. The sample with nanoparticles dispersed in n-tetracosane, thermally quenched from 100 °C and solid in the whole measuring range, follows the linear response theoretical behavior for non-interacting nanoparticles, and displays a remarkably large maximum specific absorption rate (SAR) value comparable to that of magnetosomes at the alternating magnetic fields used in the measurements. The other samples, with nanoparticles dispersed either in alkane solvents of sub-ambient melting temperatures or in epoxy resin, display different thermal behaviors and maximum SAR values ranging between 11 and 65% of that achieved for the sample with n-tetracosane as dispersive medium. These results highlight the importance of the MNPs environment and arrangement to maintain optimal SAR values, and may help to understand the disparity sometimes found between MNPs heating performance measured in a ferrofluid and after injection in an animal model, where MNP arrangement and environment are not the same.

Size dependence study on magnetic heating properties of superparamagnetic iron oxide nanoparticles suspension

We studied the magnetic heating properties of nearly monodispersed iron oxide nanoparticles (NPs) suspended in liquid media. Iron oxide NPs with a narrow size distribution (σ ∼ 0.1) with various median diameters were synthesized by the seed-mediated growth method. The effect of size on specific absorption rate (SAR) was investigated for median diameters ranging from 9 to 15 nm at intervals of 1 nm. The maximum value of SAR was found at about 13 nm for the samples dispersed in a viscous liquid media. Calculated SAR using linear response theory explains the results qualitatively but showed quantitative disagreement. This discrepancy was explained by assuming the existence of a magnetically frustrated layer on the seed NPs.

Mobile Phone Antenna with Reduced Radiation into Inner Ear

Health hazards are of great concern to cellular phone users. One important measure of the effect of electromagnetic radiation into human body is the specific absorption rate (SAR). If the human body is exposed to electromagnetic radiation, the amount of power absorbed by its tissues per mass volume should be limited and not to exceed a maximum SAR value. The cellular phone radiated through its antenna in all directions a certain amount of electromagnetic energy. This energy is concentrated in the near field region, where the user’s head is located during the call. The closest organ that is very sensitive to temperature change is the inner ear (it is just under the cellular phone antenna) where it contains a controlled viscosity liquid. In this paper we devise a two-antenna design to generate a low radiation in the direction of user’s head while using the cellular phone. By creating a null in the radiation pattern in the direction of user’s head we minimize the risk of hazards on the user. We optimize the null steering such that the device maintains a good connection to its base station and keeps the SAR level under the allowed maximum value using Lagrange method. To implement the analytical solution in real time simulated annealing (SA) algorithm is used. Results showed that we could steer the radiation pattern to optimize the radiated power in the direction of base station under the limited SAR level constraint. Simulated annealing algorithm is adopted to find the near optimal delay value to steer the antenna radiation pattern since it finds the global optimal point. It shows that a real time processing on the mobile unit can be performed to solve for the best null direction while the device is active.

Four Element Wideband Rectangular Dielectric Resonator Antenna Terminated in a Bio-medium

In this paper, a new four element rectangular dielectric resonator antenna (RDRA) for 5.0 GHz WLAN/WiMAX band is proposed. The simulation results for its radiation characteristics and the specific absorption rate (SAR) distribution in a homogenous bio-medium (muscle layer) for different antenna-to-muscle layer spacings are presented at different frequencies in 4.9---5.9 GHz band. The input characteristics of the proposed antenna and a single element RDRA of same size as the element are compared. The effect of changing antenna input power on maximum SAR value is also analyzed. The simulation study has been carried out using CST Microwave Studio software.

Analysis of heating effects (magnetic hyperthermia) in FeCrSiBCuNb amorphous and nanocrystalline wires

The induction heating effects in amorphous and nanocrystalline wires, Fe73.5-xCrxSi13.5Cu1B9Nb3 (x = 3, 7, and 10), are analyzed in this work. In these alloys, the Curie temperature of the amorphous phase, TC, can be tailored through the Cr content of the alloy or the volume crystalline fraction after nanocrystallization. Four samples were selected; amorphous with x = 0 and 10 and nanocrystalline x = 7 with different crystalline fractions. The Curie temperature of the residual amorphous phase, TCa, was experimentally determined by the temperature dependence of the self-inductance of the samples. The analysis of the frequency dependence of the complex magnetic susceptibility enabled the estimation of the magnetic power losses in the samples. The heating effects on the wires were analyzed under the application of an ac magnetic field employing a home-made hyperthermia set-up. A single piece of a wire was immersed in a water bath (initial temperature from 291 K to 325 K) and subjected to the ac magnetic field. The specific absorption rate (SAR) was estimated through the initial slope of the temperature increase as a function of time. Maximum SAR values were obtained in the amorphous sample (x = 3) with the highest TC and enhanced magnetic power losses. In the nanocrystalline samples (x = 7), the detected heating effects above TCa are interpreted as a consequence of the magnetization process of the ferromagnetic grains. However, in spite of the low SAR displayed by the amorphous wire with TC ≈ 300 K (x = 10), interesting self-regulated characteristics are observed in this sample.

Magnetic induction heating of FeCr nanocrystalline alloys

In this work the thermal effects of magnetic induction heating in (FeCr)73.5Si13.5Cu1B9Nb3 amorphous and nanocrystalline wires were analyzed. A single piece of wire was immersed in a glass capillary filled with water and subjected to an ac magnetic field (frequency, 320kHz). The initial temperature rise enabled the determination of the effective Specific Absorption Rate (SAR). Maximum SAR values are achieved for those samples displaying high magnetic susceptibility, where the eddy current losses dominate the induction heating behavior. Moreover, the amorphous sample with Curie temperature around room temperature displays characteristic features of self-regulated hyperthermia.

Apparatus for ``in vivo'' exposure at 1.8 GHz microwaves

In a previous study, an apparatus generating 1.8 GHz electromagnetic radiation for ``in vivo'' biomedical study was designed and implemented. The apparatus consisted of a reverberation chamber and it reproduced a habitat similar to the usual one for the laboratory animals. Plexiglas boxes with 300 cc physiological liquid were utilized as simple phantoms. For the measurements a small Electric Field Probe was used. The maximum SAR (Specific Absorption Rate) and average Power Efficiency Pe (SAR/input power) values obtained were quite low (``in vivo experiments'') and this was the drawback of the apparatus. In the present work, different modifications introduced in order to increase SAR and Power Efficiency are presented. In the new configuration, the dosimetry for the previous phantoms and for oil-in-gelatine phantoms was investigated and quite satisfactory SAR and Power Efficiency values were obtained, overcoming the previous drawback. In the first case the sensor was waterproofed, as in the previous study; in the other case a Plexiglas box with inside a tight shaped allocation for the measurement probe was realized. These measurement technologies could be applied to other media used for the phantoms.

Evaluation of the Specific Absorption Rate and the Temperature Rise in the Human Eyes With Account for Resonance

We propose two methods for the evaluation of the maximum specific absorption rate (SAR) in the human eyes due to radio-frequency exposure from handheld devices. They account for the existence of resonance in the eye and are applied to the case of near-field exposure. The first method is semi-analytical. As an input, it requires the measured or simulated open-space near field of the device under test in the absence of the eye. As an output, depending on the mutual position and orientation of the eye and the device, it produces the maximum SAR value in the eye averaged over 1 and 10 g of tissue. The second method is experimental. It requires the fabrication of a simple eye phantom and relies on a measurement with an SAR robot. The proposed methods allow for the fast and reliable SAR evaluation of newly developed handheld devices in an industrial environment. We also present results concerning the temperature rise in the eyes which utilizes a detailed simulation eye model.

Effects of Dielectric Parameters of Human Body on Radiation Characteristics of Ingestible Wireless Device at Operating Frequency of 430 MHz

In order to assess the sensitivities of the radiation characteristics and the compliance of ingestible wireless device (IWD) in human body due to the uncertainty and intersubject variability of dielectric properties of human body tissues, the specific absorption rate (SAR) and radiation characteristics of the IWD in two realistic human body models with changed and unchanged dielectric values are quantitatively compared using the finite-difference time-domain method. Simulations are carried out in 13 scenarios where the IWD is placed in the center positions of abdomens in the two body models at the operating frequency of 430 MHz with three orientations. Results show that the variation of radiation intensity near the surface of abdomen is around 2.5 dB within 20% variation of dielectric values. The maximum SAR values increase with the increase in conductivities of human body tissues and decrease with the increase in relative permittivities of human body tissues. A variation of up to 20% in conductivities and relative permittivities, alone or simultaneously, always causes a variation of SAR to be less than 10%. As far as the compliance of safety is concerned, the maxima of 1-g-averaged and 10-g-averaged SARs can reach 3.16 and 0.89 W/kg at the input power of 25 mW.

Size dependent heat generation of magnetite nanoparticles under AC magnetic field for cancer therapy

BackgroundWe have developed magnetic cationic liposomes (MCLs) that contained magnetic nanoparticles as heating mediator for applying them to local hyperthermia. The heating performance of the MCLs is significantly affected by the property of the incorporated magnetite nanoparticles. We estimated heating capacity of magnetite nanoparticles by measuring its specific absorption rate (SAR) against irradiation of the alternating magnetic field (AMF).MethodMagnetite nanoparticles which have various specific-surface-area (SSA) are dispersed in the sample tubes, subjected to various AMF and studied SAR.ResultHeat generation of magnetite particles under variable AMF conditions was summarized by the SSA. There were two maximum SAR values locally between 12 m2/g to 190 m2/g of the SSA in all ranges of applied AMF frequency and those values increased followed by the intensity of AMF power. One of the maximum values was observed at approximately 90 m2/g of the SSA particles and the other was observed at approximately 120 m2/g of the SSA particles. A boundary value of the SAR for heat generation was observed around 110 m2/g of SSA particles and the effects of the AMF power were different on both hand. Smaller SSA particles showed strong correlation of the SAR value to the intensity of the AMF power though larger SSA particles showed weaker correlation.ConclusionThose results suggest that two maximum SAR value stand for the heating mechanism of magnetite nanoparticles represented by hysteresis loss and relaxation loss.

SAR and radiation performance of balanced and unbalanced mobile antennas using a hybrid computational electromagnetics formulation

A procedure to reduce the effect of the mobile antenna on the handset by using balanced antennas has been investigated. Use of this type of antenna may degrade the antenna performance, such as bandwidth and gain, although it can cause less effect on the body to which they are adjacent. If the antennas are well designed, the maximum specific absorption rate (SAR) values are likely to be reduced when placed next to the head, since the coupling of such antennas to the body of the handset is very weak. A study on balanced and unbalanced antennas for mobile handsets next to the human head is presented, using a hybrid electromagnetics method for the analysis. The method uses the hybridisation technique between the frequency-domain method of moments (MoM) and the finite-difference time-domain method (FDTD). The antenna was modelled using MoM whereas the head tissues were modelled using FDTD. Two antennas were designed and investigated with respect to the SAR and radiation performance for two different antenna positions on the top edge of a mobile handset. Radiation patterns are presented and compared, with and without the head, and the maximum SAR values and field distributions inside the head are discussed. The balanced antenna shows good improvements with respect to the unbalanced antenna in terms of the SAR values and variations of the input impedances.

Power absorption and temperature elevations induced in the human head by a dual-band monopole-helix antenna phone

A numerically efficient way to evaluate specific absorption rate (SAR) deposition and temperature elevation inside the head of a user of a cellular phone equipped with a dual-band monopole-helix antenna is proposed. The considered antenna operates at both frequencies (900 and 1800 MHz) employed in global system for mobile communication. The results obtained show that, for a given radiated power, although the maximum SAR value as averaged over 1 g in the brain is higher at 900 MHz than at 1800 MHz, the maximum temperature increase in the brain is higher at 1800 MHz. However, taking into account that the average power levels radiated at the two operating frequencies are different (250 mW at 900 MHz and 125 mW at 1800 MHz), higher temperature elevations are obtained at 900 MHz. In this last case, the temperature increases are of the order of 0.2/spl deg/C in the ear, and less than 0.1/spl deg/C in the external brain region close to the phone. When the heating effect due to the contact of the ear and cheek with the phone is also taken into account, it is found that the predominant heating effect in the ear, able to cause temperature increases as high as 1.5/spl deg/C, is the one due to the phone contact, while SAR deposition plays a significant role only in the heating of the external brain region.

The specific absorption rate in a spherical head model from a dipole with metallic walls nearby

The head of the user of a mobile telephone is modeled by a lossy dielectric sphere excited by the near field of a dipole antenna and the effects of a metallic wall placed close to this system are analyzed. Three wall orientations are used and the changes in specific absorption rate (SAR) with wall or antenna distance from the sphere are investigated. When the wall is closer than 100 mm to the dipole-sphere system, it can have a strong influence on the input impedance of the antenna and can cause significant increases in the SAR in the head. When a constant radiated power of the dipole antenna is assumed, the maximum value of SAR averaged over 10 g of tissue within the sphere is increased by 60% when the wall is as close as 30 mm from the surface of the sphere. For a dipole excited by a constant current source, the increase is less than 10% and occurs when the wall is 120 mm from the surface of the sphere. Although the distances and configurations analyzed may not represent the way the majority of people use mobile telephones, it is important to examine unusual situations to determine the extent of the SAR enhancement problem.

Heating pattern of helical microwave intracavitary oesophageal applicator

Helical microwave intracavitary oesophageal (HMIO) applicators were designed to operate at frequencies of 433 MHz and 915 MHz. Heating patterns were studied within muscle-equivalent phantom by thermographic camera and fibreoptic thermometers. The results showed that frequency significantly influenced the microwave heating pattern. The 433 MHz applicator had a single power deposition region, the longitudinal specific absorption rate (SAR) distribution appeared to be nearly even, and the maximum SAR value occurred close to the centre of the active length of the applicator. The 915 MHz applicator had two power deposition regions, the peak SAR values occurred at about 1/4 and 3/4 of the active length respectively. The radial SAR distribution suggested that there is no obvious difference between the 433 MHz and 915 MHz applicators in that the average radial penetration of 50% surface SAR (RP50) was about 0.65 cm. It was also shown that power deposition was axially symmetric for both 433 MHz and 915 MHz HMIO applicators. It is shown that better impedance matching is more important for intracavitary hyperthermia than for external hyperthermia. Choosing HMIO applicators in clinical practice is also discussed.

Broadband proton decoupled natural abundance 13C NMR spectroscopy of humans at 1.5 T.

The feasibility of broadband proton decoupled in vivo 13C NMR spectroscopy of humans at 1.5 T was explored. A dual surface coil set-up was used, comprising a circular 13C coil and a butterfly 1H decoupling coil placed at one third of its width away from the body. A calibration procedure was introduced to evaluate the specific absorption rate (SAR) in any gram of tissue for the inhomogeneous decoupling field generated by a surface coil. For the WALTZ-4 sequence it was demonstrated that broadband decoupled spectra of both subcutaneous adipose and underlying muscle or liver tissue could be obtained at 1.5 T without exceeding recommended maximum SAR values. Broadband decoupling caused an additional resolution enhancement ascribed to the removal of (1H-13C) long range couplings. Broadband proton decoupled spectra of subcutaneous adipose tissue were obtained in less than 10 min showing highly resolved and intense signals of fully relaxed carbon spin systems of triacylglycerols. Broadband proton decoupled 13C NMR spectra of calf muscle showed several resonances for metabolites resolved from triacylglycerol signals (e.g. C1-C5 of glycogen, C4 of histidine, aromatic and carbonyl carbons of aminoacids and N linked carbons of ethanolamine, choline and creatine). With an acquisition time of 20-30 min, the C1 glycogen signal was observed with a root mean square signal-to-noise ratio of about 15. Not only the glycogen C1 signal but also its C2-C6 signals could be monitored in dynamic studies. Finally broadband proton decoupled 13C spectra were obtained with signals from liver tissue (notably the carbons of glycogen).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of insertion depth on the theoretical SAR patterns of 915 MHz dipole antenna arrays for hyperthermia.

Interstitial microwave antenna array hyperthermia (IMAAH) is presently used clinically in the treatment of cancer. This paper presents the theoretical specific absorption rate (SAR) patterns of 915 MHz microwave antenna arrays for varying insertion depths. The antennas were oriented in a 2 x 2 cm square array. Insertion depth, defined as distance from skin to antenna tip, ranged from 5.9 to 17.6 cm. Two different antenna configurations were considered. In the first the antenna had a distal section a quarter-wavelength long (resonant case), while the second had a distal section approximately 13% longer than a quarter-wavelength (non-resonant case). SAR patterns were calculated from theoretical expressions, and displayed as lines of constant SAR normalized to the maximum SAR value in the array. The results show that regions of concentrated power deposition or 'hotspots' occurred in the centre of the array and moved in a complex but predictable fashion as insertion depth was varied. For insertion depths shorter than a resonant half-wavelength, there occurred one hotspot distal to the antenna junctions. As insertion depth was increased beyond a resonant half-wavelength, the hotspot moved proximal to the antenna junctions and eventually split in two. For depths very much longer than a resonant half-wavelength a hotspot centred about the antenna junction dominated the SAR pattern. For the resonant case the maximum SAR was often along the central axis of the array, while for the non-resonant case the maximum SAR was at the antennas with a local maximum on the central axis.