Om P. Gandhi

Electromagnetic absorption in the human head and neck for mobile telephones at 835 and 1900 MHz

The authors have used the finite-difference time-domain method and a new millimeter-resolution anatomically based model of the human to study electromagnetic energy coupled to the head due to mobile telephones at 835 and 1900 MHz. Assuming reduced dimensions characteristic of todays mobile telephones, the authors have obtained SAR distributions for two different lengths of monopole antennas of lengths /spl lambda//4 and 3/spl lambda//8 for a model of the adult male and reduced-scale models of 10- and 5-year-old children and find that peak one-voxel and 1-g SARs are larger for the smaller models of children, particularly at 835 MHz. Also, a larger in-depth penetration of absorbed energy for these smaller models is obtained. The authors have also studied the effect of using the widely disparate tissue properties reported in the literature and of using homogeneous instead of the anatomically realistic heterogeneous models on the SAR distributions. Homogeneous models are shown to grossly overestimate both the peak 1-voxel and 1-g SARs. Last, the authors show that it is possible to use truncated one-half or one-third models of the human head with negligible errors in the calculated SAR distributions. This simplification will allow considerable savings in computer memory and computation times.

A frequency-dependent finite-difference time-domain formulation for general dispersive media

A weakness of the finite-difference-time-domain (FDTD) method is that dispersion of the dielectric properties of the scattering/absorbing body is often ignored and frequency-independent properties are generally taken. While this is not a disadvantage for CW or narrowband irradiation, the results thus obtained may be highly erroneous for short pulses where ultrawide bandwidths are involved. In some recent publications, procedures based on a convolution integral describing D(t) in terms of E(t) are given for media for which the complex permittivity in *( omega ) may be described by a single-order Debye relaxation equation or a modified version thereof. Procedures are, however, needed for general dispersive media for which in *( omega ) and mu *( omega ) may be expressible in terms of rational functions, or for human tissues for which multiterm Debye relaxation equations must generally be used. The authors describe a new differential equation approach, which can be used for general dispersive media. In this method D(t) in terms of E(t) by means of a differential equation involving E, and their time derivatives. The method is illustrated for several examples. >

Use of the Finite-Difference Time-Domain Method in Calculating EM Absorption in Human Tissues

Although there are acceptable methods for calculating whole body electromagnetic absorption, no completely acceptable method for calculating the local specific absorption rate (SAR) at points within the body has been developed. Frequency domain methods, such as the method of moments (MoM) have achieved some success; however, MoM requires computer storage on the order of (3N) 2 and computation time on the order of (3N) 3 where N is the number of cells. The finite-difference time-domain (FDTD) method has been employed extensively in calculating the scattering of metallic objects, and recently is seeing some use in calculating the interaction of EM fields with complex, lossy dielectric bodies. Since the FDTD method has storage and time requirements proportional to N, it presents an attractive alternative to calculating SAR distribution in large bodies. This paper describes the FDTD method and evaluates it by comparing its results to analytic solutions in two and three dimensions. The utility of the FDTD method is demonstrated by a 3D scan of the human torso. The results obtained demonstrate that the FDTD method is capable of calculating internal SAR distribution with acceptable accuracy. With the availability of supercomputers, such as the CRAY II, the calculation of SAR distribution in a man model of 50 000 cells (1.27 cm per cell) appears to be feasible.

A 3-D impedance method to calculate power deposition in biological bodies subjected to time varying magnetic fields

A previously described two-dimensional impedance method (ibid., vol.31, p.644-51, 1984) for modeling the response of biological bodies exposed to time-varying electromagnetic fields has been extended to three dimensions. This method is useful at those frequencies where the quasistatic approximation is valid and calculates the fields, current densities, and power depositions in the bodies. Solutions using this method for homogeneous spheres in plane waves are presented and compared to the analytic solutions for the same configuration. Solutions for a man exposed to a uniform radio-frequency magnetic field at 30 MHz, are presented, as well as for a man exposed to either circularly or linearly polarized magnetic fields at 63 MHz, uniform within a portion of his body and linearly decreasing outside of that portion, which approximates the exposure in some nuclear-magnetic-resonance imaging devices.<<ETX>>

Computation of electric and magnetic stimulation in human head using the 3-D impedance method

A comparative, computational study of the modeling of transcranial magnetic stimulation (TMS) and electroconvulsive therapy (ECT) is presented using a human head model. The magnetic fields from a typical TMS coil of figure-eight type is modeled using the Biot-Savart law. The TMS coil is placed in a position used clinically for treatment of depression. Induced current densities and electric field distributions are calculated in the model using the impedance method. The calculations are made using driving currents and wave forms typical in the clinical setting. The obtained results are compared and contrasted with the corresponding ECT results. In the ECT case, a uniform current density is injected on one side of the head and extracted from the equal area on the opposite side of the head. The area of the injected currents corresponds to the electrode placement used in the clinic. The currents and electric fields, thus, produced within the model are computed using the same three-dimensional impedance method as used for the TMS case. The ECT calculations are made using currents and wave forms typical in the clinic. The electrical tissue properties are obtained from a 4-Cole-Cole model. The numerical results obtained are shown on a two-dimenaional cross section of the model. In this study, we find that the current densities and electric fields in the ECT case are stronger and deeper penetrating than the corresponding TMS quantities but both methods show biologically interesting current levels deep inside the brain.

Use of the finite-difference time-domain method for calculating EM absorption in man models

The finite-difference time-domain method is used to calculate the specific absorption rate (SAR) within the human body. SAR distributions are calculated using incident frequencies of 100 MHz and 350 MHz for three different cases: (1) a homogeneous man model in free space; (2) an inhomogeneous man model in free space; and (3) an inhomogeneous man model standing on a ground plane. These various cases are used to evaluate the advantage of inhomogeneous models over homogeneous models, and grounded models versus free space models. Comparison is made between the results obtained here and those obtained using the method of moments.<<ETX>>

Numerical Calculation of Electromagnetic Energy Deposition for a Realistic Model of Man

Numerical calculations of absorbed energy deposition have been made for a block model of man that is defined with careful attention given to the biometric and anatomical features of a human being. CalcuIated post-resonant absorption and distribution of energy deposition through the body have better agreement with experimental results than previous calculations made using less realistic models.

Impedence Method for Calculation of Power Deposition Patterns in Magnetically Induced Hyperthermia

To obtain a detailed view of the power deposition pattern resulting from time-varying magnetic fields used in hyperthermia, we have developed a method of modeling portions of the human body using an impedance network. The region of interest is subdivided into a number of cells, each of which is then replaced by an equivalent impedance, and the currents induced in the resulting network due to the prescribed magnetic field are found by the application of circuit theory.

Some numerical methods for dosimetry: Extremely low frequencies to microwave frequencies

This paper describes some of the numerical methods that have been developed for calculations of induced electric fields, current densities, and specific absorption rates for anatomically based heterogeneous models of the human body with increasingly finer resolutions. These methods, namely, the impedance method and the finite difference time domain (FDTD) method, have been used for dosimetric calculations for a number of bioelectromagnetic problems for whole-body or partial-body exposures, for far-field or near-field sources, and for CW or transient fields. The paper gives detailed calculations for some recent applications such as currents induced in the users body by the electromagnetic fields (emfs) of electric blankets using the impedance method, coupling of an ultrawideband pulse using the frequency-dependent FDTD method incorporating dispersive properties of the various tissues, and specific absorption rate distributions in the head for emfs of cellular telephones. Because of accurate modeling of tissue heterogeneities and shapes, these methods are likely to play an increasing role in emerging technologies with bioelectromagnetic concerns.

State of the knowledge for electromagnetic absorbed dose in man and animals

The paper gives the EM absorbed dose for man and animals at various frequencies for the plane wave irradiation condition for different orientations of the body relative to incident fields. Also included are the results for the whole-body absorption for conditions of electrical contact with ground and in the presence of reflecting surfaces of high conductivity and multiple animals. The data are given for the ditribution of power deposition in man models for the resonance conditions of highest whole-body electromagnetic absorption. The highlights of the results obtained with proportionately scaled saline- and biological-phantom-filled models of man have been confirmed by experiments with small laboratory animals, from 25-g mice to 2250-g rabbits.