SAR CHARACTERIZATION OF FOCUSED PLANAR ARRAY OF WATER-LOADED MODIFIED BOX-HORNS FOR HYPERTHERMIA

A 4×4 planar array of modified box-horns as a microwave hyperthermia applicator is theoretically studied to characterize power deposition (SAR) in heating tissue (muscle) at 2450 MHz. A modified box-horn is a novel improved version of conventional box-horn in which horn exciting the box waveguide is flared in both E-and H-planes. Modified box-horn supports TE₁₀ and TE₃₀ modes. The amplitude distribution over the H-plane of the box-horn aperture is a closer approximation to the uniform distribution. It is proposed that the interior of the box-horn be filled with water to provide a better impedance match to biological tissue. By applying Fresnel-Kirchhoff scalar diffraction field theory, the expression for electric field in heating region is derived and distribution of specific absorption rate (SAR) in that region due to planar array of modified box-horns as direct contact applicator is evaluated at 2450 MHz. The results of modified box-horn array are compared with those of a single modified box-horn operating at the same frequency. Results demonstrate that planar array of modified box-horns offers improvement in SAR distribution and penetration depth. It is shown that by changing the phase and amplitude of excitation of the modified box-horns of the array, the relative amplitude and position of the hot spot can be changed. The present analysis is validated through the results obtained by plane wave spectral technique.     

Mutual Coupling Between Box-Horn Elements of a Phased Array Terminated in Three-layered Bio-Media

The ldquoplane wave spectral techniquerdquo is used to investigate theoretically the extent of mutual coupling between two box-horn elements of a phased array terminated in three-layered bio-media (skin, fat, and muscle layers). The results obtained have been validated against published results in the literature. Each box-horn of the array is assumed to be filled with water to provide good impedance match to the bio-media. Also, it reduces the dimensions of each box-horn which makes it suitable for array configuration. The effects of relative position of elements, spacing between elements, frequency, aperture size of each element, relative phase and amplitude of excitation of the elements on mutual coupling coefficients are numerically investigated and presented for an array of two box-horns terminated in bio-media. The specific absorption rate (SAR) distributions along the direction of the array axis in muscle layer of the bio-media in direct contact with a collinear array of two box-horns both with and without mutual coupling considerations are also computed and presented. The results of the present research work can provide useful design guidelines for the development of prototypes of box-horn array for hyperthermia treatment of tumors. Bio-media, box-horn, hyperthermia, mutual coupling, phased array, plane wave spectra.     

男性生殖器在915和2450MHz微波作用下的比吸收率分布

应用模型实验和数字化红外热象技术,定量地测定了男性生殖器在915和2450MHz微波远场和近场作用下的SAR分布。实验表明:男性生殖器对915和2450MHz微波的吸收有极化效应。当微波电场矢量E与阴茎平行时,阴茎吸收最强,最大SAR出现在阴茎基部;当微波电场矢量E与阴茎垂直时,对915MHz微波,最大SAR出现在阴囊中心,对2450MHz微波,最失SAR出现在睾丸内,显示同一器官对不同频率,不同极化微波的吸收特性间的差别。本文对实验结果进行了讨论并与过去的理论分析和实验结果作了比较,显示相当接近。     

On the determination of an optimum microwave diathermy frequency for a direct contact applicator

An approximate theoretical analysis is presented for determining the relative heating in a two layer fat-muscle medium due to a dielectric loaded dipole-corner reflector applicator in direct contact with the finite fat layer. The results indicate that convenient applicator sizes, equivalent to those now used at 2450 Mc/s, operating at a frequency in the neighborhood of 750 Mc/s provide the maximum ratio of relative heat per unit volume in the muscle as compared to that in the fat. Experimental data taken with the aid of an electrical model of the fat-muscle layers shows reasonable agreement with the results of the approximate analysis.     

Absorption of microwave energy by muscle models and by birds of differing mass and geometry

Spheres composed of phantom muscle of radius 1.5, 2, 3, 4, 6 and 8 cm, as well as birds (parakeets, quail, pigeons, chickens, turkeys) were exposed to far-field plane waves at power densities of incident radiation between 182 and 560 mW/cm2 and at frequencies of 775, 915 and 2450 MHz. Specific absorption rate (SAR) patterns were determined by thermographic techniques for both spheres and birds. The measured SAR patterns in spheres were comparable to those from theoretical predictions. The SAR patterns in birds, however, varied markedly from those obtained from spheres of comparable mass. The results indicate that the geometrically complex animal is not represented by simple geometric models for making absorption studies. Thermograms of birds exposed in the flying position indicated that the SAR is high in the wings. The behavioral response of the birds to the exposure was variable. Threshold power densities for biological or behavioral reactions were determined for each bird at all three frequencies. The lowest power density associated with reactivity by the chicken was 5.8 mW/cm2 (corresponding to SARs of 3.1 W/kg in the head and 3.9 W/kg in the neck) at 775 MHz.     

Comparison of power deposition by in-phase 433 MHz andphase-modulated 915 MHz interstitial antenna array hyperthermia systems

The interstitial microwave antenna array hyperthermia (IMAAH) system produces a pattern of specific absorption rate (SAR) that is nonuniform within a 2-cm square array when driven in phase at 915 MHz. It was found that phase modulation makes the time-averaged SAR pattern significantly more uniform in planes perpendicular to the antennas. To drive antennas in phase at 433 MHz similarly improves SAR uniformity when the antennas are of resonance length     

Development and Testing of a 2450-MHz Lens Applicator for Localized Microwave Hyperthermia (Short Paper)

A new type of applicator with a convergent lens for localized microwave hyperthermia is developed. A lens applicator of direct contact type was designed to conduct actual and progressive experiments with phantoms of simulated fat and muscle tissues heated at 2450 MHz. The experimental results showed that the heating power penetration depth increased 40 percent with this applicator as compared to a simple rectangular waveguide applicator with the same size aperture that had generally been used for microwave hyperthermia. Our applicator had a concave-shaped aperture and was designed to contact well with the heating medium whose shape was cylindrical like a human body.     

Hearing microwaves: the microwave auditory phenomenon

The microwave auditory phenomenon, or the microwave hearing effect, pertains to the hearing of short pulses of modulated microwave radiation at high peak power by humans and laboratory animals. Anecdotal and journalistic reports of the hearing of microwave pulses persisted throughout the 1940s; and 1950s. The first scientific report of the phenomenon appeared in 1961. The effect has been observed for RF exposures across a wide range of frequencies (450-3000 MHz). It can arise, for example, at an incident energy-density threshold of 400 mJ/m2 for a single 10-microsecond-wide pulse of 2450 MHz microwave energy, incident on the head of a human subject. And it has been shown to occur at an SAR threshold of 1.6 kW/kg for a single 10-microsecond-wide pulse of 2450 MHz microwave energy, impinging on the head. A single microwave pulse can be perceived as an acoustic click or knocking sound, and a train of microwave pulses to the head can be sensed as an audible tune, with a pitch corresponding to the pulse-repetition rate (a buzz or chirp). Note that the SAR threshold of 1.6 kW/kg is about 1000 times higher than that allowable by FCC rules for cellular mobile telephones     

Effectiveness analysis of lossy dielectric shields for athree-layered human model

In this paper, we discuss the shielding effects of lossy dielectric materials located in front of a human model. Using the method of moments, we investigated the shielding effects by calculating the “whole average specific absorption rate (SAR)” and the “local SAR” for a three-layered elliptical model of the human body, which simulates the skin, fat, and muscle tissues. According to the results, in the low-frequency range of 200-800 MHz, the multiple reflection between the shield and human model gives rise to an increase in the whole average SAR when a low-loss material shield is placed in front of the human model. On the other hand, the local SAR increased not only at the skin layer, but also at the muscle layer. At higher frequencies, the SAR became a continuously decreasing function of frequency     

A 50-kW CW Ferrite Circulator in S Band

A 50-kW CW differential phase-shift circulator at 2450 MHz has been developed. Its insertion loss and isolation are 0.18 dB and 20 dB, respectively, over a bandwidth of 25 MHz at 2450 MHz. It is useful for protecting the microwave source of a high-power system from reflected power. The design and experimental results of the circulator are presented.     

SAR distributions for 915 MHz interstitial microwave antennas usedin hyperthermia for cancer therapy

Theoretical and experimental specific absorption rate (SAR) distributions are presented for single insulated antennas operating at 915 MHz in muscle phantom; the SAR is deduced from measured temperature rise. Results show that dipoles with lengths much shorter than their resonant length have a characteristically large reactive input impedance component and a substantially smaller absolute SAR magnitude than resonant dipoles. All cases investigated demonstrate that the maximum SAR occurs near the junction, regardless of insertion depth. Experimental results show that an antenna with the tip section equal to a quarter-wavelength and the insertion depth equal to a half-wavelength achieves a substantial increase in the longitudinal power distribution compared to other antenna designs that were evaluated     

An insulated dipole applicator for intracavitary hyperthermia

Specific absorption rate (SAR) patterns for an insulated sleeve dipole applicator, operating at 433 MHz, were measured by infrared thermography. The applicator was modeled using classical transmission-line theory, and experimental and theoretical results were compared. In general, agreement between measured and calculated SAR values was good. However, at the antenna feedpoint, the measured values were appreciably higher than the calculated values. This indicates that inclusion of junction effects would improve the model     

The Propagation Characteristics of Radio Frequency Signals for Wireless Sensor Networks in Large-Scale Farmland

For configuring wireless sensor network and deploying nodes, the propagation characteristics of wireless channel at frequency of 433 MHz and 2.4 GHz are investigated. Through the analysis of the received signal strength indicator (RSSI) and packet loss rate (PLR), we find that the RSSI (PLR) decreases (increases) as the transceiver nodes distance increases. It is also found that the path loss decreases with the antenna height increasing, and the path loss at 2.4 GHz is more serious than that at 433 MHz. Through the regression analysis in Matlab, we find that the optimal fitting model is the parametric exponential decay (OFPED) model, and the second-best is the linear logarithmic model. For OFPED model, the values of R² vary from 0.9347 to 0.9893, and the values of root mean square error (RMSE) range from 0.7469 to 2.243 at frequency of 433 MHz; while at frequency of 2.4 GHz, the values of R² change from 0.9612 to 0.9857, and the values of RMSE range from 1.375 to 3.181. Moreover, we make a comparison analysis with several modified exponential decay (MED) models, and the validation results show that the MED models can be used as conservative upper and lower bounds of path loss, at least for wheat field.     

Numerical computation of fat layer effects on microwave near-fieldradiation to the abdomen of a full-scale human body model

Numerical computation results of fat layer effects on the microwave near field radiation to the abdomen of a three-dimensional (3-D) full-scale human body model are presented. The human body is modeled as a 3-D homogeneous muscle phantom with a fat layer covering the abdomen part. The dipole wire-antenna located proximate to the abdomen is used as the microwave radiation source at 915 MHz. This is to study the effects on hyperthermia heating by using the microwave applicator (at 915 MHz) or the near-field exposure from the proximate handset antenna to the human body at ISM band wireless communication band (902-928 MHz). Coupled integral equations (CIE) and the method of moments (MoM) are employed to numerically compute electromagnetic (EM) energy deposition specific absorption rate (SAR) from the radio frequency (RF) antenna applicator into the proximate fat layer covered abdomen. The antenna input impedance (proximate to the body), return loss (RL), and the resonant antenna length (proximate to the body) will also be numerically determined to increase the microwave power delivered into the body. The study of fat layer effects is important for microwave hyperthermia applications. It is also important for the investigation of the potential health hazard from the near-field radiation of a wireless communication antenna     

Parametric dependence of SAR on permittivity values in a man model

The development and widespread use of advanced three-dimensional digital anatomical models to calculate specific absorption rate (SAR) values in biological material has resulted in the need to understand how model parameters (e.g., permittivity value) affect the predicted whole-body and localized SAR values. The application of the man dosimetry model requires that permittivity values (dielectric value and conductivity) be allocated to the various tissues at all the frequencies to which the model will be exposed. In the 3-mm-resolution man model, the permittivity values for all 39 tissue-types were altered simultaneously for each orientation and applied frequency. In addition, permittivity values for muscle, fat, skin, and bone marrow were manipulated independently. The finite-difference time-domain code was used to predict localized and whole-body normalized SAR values. The model was processed in the far-field conditions at the resonant frequency (70 MHz) and above (200, 400, 918, and 2060 MHz) for E orientation. In addition, other orientations (K, H) of the model to the incident fields were used where no substantial resonant frequency exists. Variability in permittivity values did not substantially influence whole-body SAR values, while localized SAR values for individual tissues were substantially affected by these changes. Changes in permittivity had greatest effect on localized SAR values when they were low compare to the whole-body SAR value or when errors involved tissues that represent a substantial proportion of the body mass (i.e., muscle). Furthermore, we establish the partial derivative of whole-body and localized SAR values with respect to the dielectric value and conductivity for muscle independently. It was shown that uncertainties in dielectric value or conductivity do not substantially influence normalized whole-body SAR. Detailed investigation on localized SAR ratios showed that conductivity presents a more substantial factor in absorption of energy in tissues than dielectric value for almost all applied exposure conditions.     

Average SAR and SAR Distributions in Man Exposed to 450-MHz Radiofrequency Radiation

Fifth-scale phantom models were exposed to 2450-MHz electromagnetic fields to obtain the average specific absorption rate (SAR) and SAR distribution in man exposed to 1 mW/cm/sup 2/ 450-MHz radiofrequency radiation for various polarizations and body positions. The average SAR was measured calorimetrically and SAR distribution was determined thermographically using an interactive computer system, The mean SAR, as averaged over the body, remained relatively constant at 0.050 W/kg, with a standard deviation of +-0.007 W/kg for all polarizations and body postures considered in the study. Peak SAR values were as high as 0.650 W/kg, occuring typicaly in the wrist.     

Microstrip Loop Radiators for Medical Applications

Three microstrip loop radiators designed to operate at frequencies of 433, 915, and 1300 MHz are described. Empirical design methods and experimental results obtained with phantoms and human tissues are presented. The radiators are relatively well matched when applied to water boluses followed by muscle phantoms or human tissues. When used with the boluses, the radiators have circular surface-temperature distribution while the in-depth heating patterns are similar to those of the aperture-type radiators.     

Focused Array Hyperthermia Applicator: Theory and Experiment

The design and analysis of a focused linear array at 2450 MHz for microwave hyperthermia research is described. The array, which was submerged in deionized water to reduce its size and to provide a better impedance match to a high dielectric medium representative of human tissues, consisted of four titanium dioxide loaded horn antennas with apertures of 2.0 x 1.4 cm and a feed network with weighted phase shifts, Power radiation pattern measurements were made in planes ranging from 7.6 to 10.2 cm from the array to determine the focusing characteristics and beam spot size. Due to high attenuation in the medium, planes beyond 10.2 cm were not considered. The half-power beamwidth (HPBW) measured at the focal point was approximately 1.3 cm. The measured patterns were found to be in close agreement with theoretical predictions.     

International intercomparison of specific absorption rates in a flat absorbing phantom in the near-field of dipole antennas

This paper reports the results of an international intercomparison of the specific absorption rates (SARs) measured in a flat-bottomed container (flat phantom), filled with human head tissue simulant fluid, placed in the near-field of custom-built dipole antennas operating at 900 and 1800 MHz, respectively. These tests of the reliability of experimental SAR measurements have been conducted as part of a verification of the ways in which wireless phones are tested and certified for compliance with safety standards. The measurements are made using small electric-field probes scanned in the simulant fluid in the phantom to record the spatial SAR distribution. The intercomparison involved a standard flat phantom, antennas, power meters, and RF components being circulated among 15 different governmental and industrial laboratories. At the conclusion of each laboratory's measurements, the following results were communicated to the coordinators: Spatial SAR scans at 900 and 1800 MHz and 1 and 10 g maximum spatial SAR averages for cubic volumes at 900 and 1800 MHz. The overall results, given as mean standard deviation, are the following: at 900 MHz, 1 g average 7.850.76; 10 g average 5.160.45; at 1800 MHz, 1 g average 18.44/spl plusmn/1.65; 10 g average 10.14/spl plusmn/0.85, all measured in units of watt per kilogram, per watt of radiated power.     

Analysis of current density and specific absorption rate in biological tissue surrounding transcutaneous transformer for an artificial heart

This paper reports on the current density and specific absorption rate (SAR) analysis of biological tissue surrounding an air-core transcutaneous transformer for an artificial heart. The electromagnetic field in the biological tissue is analyzed by the transmission line modeling method, and the current density and SAR as a function of frequency, output voltage, output power, and coil dimension are calculated. The biological tissue of the model has three layers including the skin, fat, and muscle. The results of simulation analysis show SARs to be very small at any given transmission conditions, about 2-14 mW/kg, compared to the basic restrictions of the International Commission on nonionizing radiation protection (ICNIRP; 2 W/kg), while the current density divided by the ICNIRP's basic restrictions gets smaller as the frequency rises and the output voltage falls. It is possible to transfer energy below the ICNIRP's basic restrictions when the frequency is over 250 kHz and the output voltage is under 24 V. Also, the parts of the biological tissue that maximized the current density differ by frequencies; in the low frequency is muscle and in the high frequency is skin. The boundary is in the vicinity of the frequency 600-1000 kHz.