Three-dimensional SAR distributions computed in a multilayeredcylindrical model for electromagnetic hyperthermia

A model consisting of multilayered, concentric, circular cylinders is used to investigate numerically specific absorption rate (SAR) distributions for electromagnetic hyperthermia. The fields in the cylinders are expanded in eigenfunctions, and axial confinement is achieved via Fourier transformation. Only axisymmetric SAR distributions are considered. TM₀ modes have SAR distributions that appear to be most useful for hyperthermia of deep-seated tumors. As the SAR is more confined axially: (1) the radial components of the TM₀ mode fields increase, and (2) the attenuation in the radial direction increases. Differences in SAR distributions are more apparent near the surface of a model than they are near the core. The effect of axial confinement on the optimal frequency of operation is discussed     

Temperature distributions in the human leg for VLF-VHF exposures atthe ANSI-recommended safety levels

Using a block model of 1532 cubical cells, temperature distributions are calculated for the lowest 21 cm of the human leg for electric fields recommended in the ANSI RF safety guideline. The thermal model uses inhomogeneous volume-averaged tissue properties: blood-flow rate, metabolism, thermal conductivity, specific heat, etc. The SARs are obtained using the impedance method. A modified finite-difference technique is used to solve the 3-D heat-conduction equation for the thermal model. Numerical results are obtained for RF currents at 3 and 40 MHz projected for the E fields recommended by the ANSI Standard (614 and 61.4 V/m, respectively) and also for power densities one-tenth of that level. Temperatures as high as 41.6°C are obtained for some internal cells for the higher E fields while relatively moderate temperatures on the order of 37°C are obtained for the lower E fields. Some of the calculated results for the surface temperature have been compared and found to be in good agreement with the experimental data for initial rates of heating     

Evaluation of the SAR distribution in the human head for cellularphones used in a partially closed environment

The purpose of this paper is to calculate the specific absorption rate (SAR) distribution in a human head exposed to the electromagnetic field emitted from a handheld cellular phone operating in the 900 MHz range in a partially closed environment. The environment could be, for example, the interior of a car, a condition of exposure which is largely diffused nowadays. The presence of reflecting surfaces near the phone modifies the current distribution on, and the emitting properties of, the phone antenna. Therefore, the distribution of the absorbed power inside the head is different from that absorbed in the free space exposure condition. The finite-difference time-domain (FDTD) method has been used to evaluate the SAR in a realistic anatomically based model of the human head for different antenna-handset configurations and for different antenna-head distances. The environmental effects have been simulated through partially or totally reflecting walls located in various positions with reference to the phone. It is found that the presence of a horizontal reflecting wall over the head decreases the SAR values in the part of the head directly exposed to the phone antenna, while it increases the SAR values in the part not directly exposed. On the contrary, the presence of a vertical wall, located in proximity of the phone and parallel to it, raises the SAR values everywhere into the head     

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     

Spatio-temporal EEG source localization using a three-dimensional subspace FINE approach in a realistic geometry inhomogeneous head model

The subspace source localization approach, i.e., first principle vectors (FINE), is able to enhance the spatial resolvability and localization accuracy for closely-spaced neural sources from EEG and MEG measurements. Computer simulations were conducted to evaluate the performance of the FINE algorithm in an inhomogeneous realistic geometry head model under a variety of conditions. The source localization abilities of FINE were examined at different cortical regions and at different depths. The present computer simulation results indicate that FINE has enhanced source localization capability, as compared with MUSIC and RAP-MUSIC, when sources are closely spaced, highly noise-contaminated, or inter-correlated. The source localization accuracy of FINE is better, for closely-spaced sources, than MUSIC at various noise levels, i.e., signal-to-noise ratio (SNR) from 6 dB to 16 dB, and RAP-MUSIC at relatively low noise levels, i.e., 6 dB to 12 dB. The FINE approach has been further applied to localize brain sources of motor potentials, obtained during the finger tapping tasks in a human subject. The experimental results suggest that the detailed neural activity distribution could be revealed by FINE. The present study suggests that FINE provides enhanced performance in localizing multiple closely spaced, and inter-correlated sources under low SNR, and may become an important alternative to brain source localization from EEG or MEG.     

Temperature increase in the human head due to a dipole antenna at microwave frequencies

The temperature increases in a human head due to electromagnetic (EM) wave exposure from a dipole antenna are investigated in the frequency range of 900 MHz to 2.45 GHz. The maximum temperature increases in the head and brain are compared with the values of 10/spl deg/C and 3.5/spl deg/C (found in literature pertaining to microwave-induced physiological damage). In particular, the estimation scheme for maximum temperature increases of the head and brain tissues is discussed in terms of a peak average specific absorption rate (SAR) as prescribed in safety standards. The rationale for this attempt is that maximum temperature increases and peak average SARs have not been well correlated yet. For this purpose, the SAR in the head model is initially calculated by the finite-difference time-domain method. The temperature increase in the model is then calculated by substituting the SAR into the bioheat equation. Numerical results demonstrate that the temperature increase distribution in the head is largely dependent on the frequency of EM waves. This is mainly because of the frequency dependency of the SAR distribution. Similarly, maximum temperature increases in the head and brain are significantly affected by the frequency and polarization of the EM wave. The maximum temperature increases in the head (excluding auricles) and brain are determined through linear extrapolation of the peak average SAR in these regions. According to this scheme, it is found that the peak SAR averaged over 1 g of tissue in the head should be approximately 65 W/kg to achieve the maximum temperature increase of 10/spl deg/C induced in the head excluding auricles. This corresponds to a factor of about 40 compared to the FCC standard. On the other hand, the peak SAR for 10 g of tissue should be around 40 W/kg, which implies a factor of about 20 compared to the ICNIRP standard.     

Improved method for computation of potentials in a realistic headshape model

The lead field analysis (LFA) algorithm, a new computational technique for the calculation of potentials on the surface of a realistic head shaped volume conductor model based on the boundary element method and the reciprocity theorem, is presented. The new algorithm, in comparison to the standard boundary element method, offers improved computational efficiency and lower storage requirements. It also yields more accurate surface potential results in the face of varying dipole source locations for a head shape boundary element model with a given number of nodes. Additionally, the algorithm results in quasi-analytic expressions of the derivatives of the surface potential with respect to the location of the sources, allowing the use of optimization techniques with better convergence properties. A set of simulations demonstrating the increased robustness of the LFA algorithm in the face of varying dipole source parameters is also described     

Field distributions in a three-layer prolate spheroidal human body model for a loop antenna irradiation

Analysis of the field distributions in a three-layer prolate spheroidal human body model for a loop antenna irradiation is described. The loop radiator is assumed to be placed symmetrically with respect to the spheroid axis. Spheroidal wave functions are employed to expand the field components inside and outside of the prolate spheroid. The boundary conditions on the interface layers are satisfied by using a mode matching procedure. The loop radiator can be very close to the human body. Numerical results are given for the 13 MHz frequency.     

Numerical optimization of 3-D SAR distributions in cylindricalmodels for electromagnetic hyperthermia

Numerically optimized SAR (specific absorption rate) distributions in a source free 3-D multilayered concentric cylindrical (MCC) model are presented. The fields were expanded in the modes of the MCC. Cost functions which specify mathematically the relative weight assigned to differences between an SAR distribution and a desired SAR distribution were defined. The coefficients of the modes, which minimize the cost function, were obtained using gradient search optimization methods. The optimized SAR distributions shown were computed using three different cost functions and two different radial locations for the center of the region where the desired SAR is largest. A five-layered model, including the outer water layer for cooling and improved matching with the source, was used. The frequency was 70 MHz. The current and charge distributions computed on a perfectly conducting cylindrical surface just outside the model are also shown. The surface current and charge distributions depend strongly on the relative importance of the cost for acute heat and systemic heat. A technique is developed for generating a new set of basis functions for reducing the number of unknowns to be optimized. We suggest that the approach shown could be useful in designing hyperthermia applicators.     

Preventive ablation strategies in a biophysical model of atrial fibrillation based on realistic anatomical data.

Ablation strategies to prevent episodes of paroxysmal atrial fibrillation (AF) have been subject to many clinical studies. The issues mainly concern pattern and transmurality of the lesions. This paper investigates ten different ablation strategies on a multilayered 3-D anatomical model of the atria with respect to 23 different setups of AF initiation in a biophysical computer model. There were 495 simulations carried out showing that circumferential lesions around the pulmonary veins (PVs) yield the highest success rate if at least two additional linear lesions are carried out. The findings compare with clinical studies as well as with other computer simulations. The anatomy and the setup of ectopic beats play an important role in the initiation and maintenance of AF as well as the resulting therapy. The computer model presented in this paper is a suitable tool to investigate different ablation strategies. By including individual patient anatomy and electrophysiological measurement, the model could be parameterized to yield an effective tool for future investigation of tailored ablation strategies and their effects on atrial fibrillation.     

Correlation of maximum temperature increase and peak SAR in the human head due to handset antennas

This paper attempts to correlate the maximum temperature increase in the head and brain with the peak specific absorption rate (SAR) value due to handset antennas. The rationale for this study is that physiological effects and damage to humans through electromagnetic-wave exposure are induced by temperature increases, while the safety standards are regulated in terms of the local peak SAR. For investigating these correlations thoroughly, the total of 660 situations is considered. The numerical results are analyzed on the basis of statistics. We find that the maximum temperature increases in the head and brain can be estimated in terms of peak SARs averaged over 1 and 10 g of tissue in these regions. These correlations are less affected by the positions, polarizations, and frequencies of a dipole antenna. Also, they are reasonably valid for different antennas and head models. Further, we discuss possible maximum temperature increases in the head and brain for the SAR values prescribed in the safety standards. They are found to be 0.31/spl deg/C and 0.13/spl deg/C for the Federal Communications Commission Standard (1.6 W/kg for 1 g of tissue), while 0.60/spl deg/C and 0.25/spl deg/C for the International Commission on Non-Ionizing Radiation Protection Standard (2.0 W/kg for 10 g of tissue).     

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.     

SAR amplitude probability density function estimation based on a generalized Gaussian model.

In the context of remotely sensed data analysis, an important problem is the development of accurate models for the statistics of the pixel intensities. Focusing on synthetic aperture radar (SAR) data, this modeling process turns out to be a crucial task, for instance, for classification or for denoising purposes. In this paper, an innovative parametric estimation methodology for SAR amplitude data is proposed that adopts a generalized Gaussian (GG) model for the complex SAR backscattered signal. A closed-form expression for the corresponding amplitude probability density function (PDF) is derived and a specific parameter estimation algorithm is developed in order to deal with the proposed model. Specifically, the recently proposed "method-of-log-cumulants" (MoLC) is applied, which stems from the adoption of the Mellin transform (instead of the usual Fourier transform) in the computation of characteristic functions and from the corresponding generalization of the concepts of moment and cumulant. For the developed GG-based amplitude model, the resulting MoLC estimates turn out to be numerically feasible and are also analytically proved to be consistent. The proposed parametric approach was validated by using several real ERS-1, XSAR, E-SAR, and NASA/JPL airborne SAR images, and the experimental results prove that the method models the amplitude PDF better than several previously proposed parametric models for backscattering phenomena.     

Determination of a small-signal model for ion-implanted microwave transistors

Most microwave transistor models are useful within specific frequency ranges; outside those ranges, they become inaccurate essentially because of the distributed nature of the active base resistance which is modeled as a lumped element. In this paper, a quantitative study of the two-dimensional current flow in the active region of an ion-implanted device leads to the synthesis of a small-signal model, emphasizing its relations to the distributed parameters defined. Methods for extracting those parameters are proposed via expressions that relate them to electrical measurements at the terminals. A final section deals with the analysis of experimental data to verify the validity of a distributed equivalent circuit at frequencies up to 10 GHz.     

Analysis of the SAR distributions in three-layered bio-media in direct contact with a water-loaded modified box-horn applicator

This paper presents theoretical analysis of the specific absorption rate (SAR) distribution in three-layered bio-media (skin, fat, and muscle layers) in direct contact with a modified box-horn applicator in which the horn is flared in both the E- and H-planes for hyperthermia treatment of cancer. The modified box-horn is assumed to be filled with water to provide a better impedance match to the bio-media. The present analysis is based on a plane-wave spectral technique. The spatial distribution of SAR in skin, fat, and muscle layers are computed and presented at 433 and 2450 MHz. Higher SAR value in each of the bio-layers, higher penetration depth, and lower resolution in the muscle layer are observed at 433 MHz in comparison to those at 2450 MHz for the water-loaded modified box-horn. The effect of substituting a pyramidal horn in place of an H-plane sectoral horn in the design of the box-horn on the SAR distribution is also examined. The results have been validated against published results in the literature.     

Computed SAR and thermal elevation in a 0.25-mm 2-D model of the human eye and head in response to an implanted retinal stimulator - part II: results

This is the second of a series of two papers on the thermal increase in the human eye and head in response to an implanted retinal stimulator. This paper provides specific absorption rates induced in the human head by the extraocular unit and the temperature increases associated with induced electromagnetic fields and power dissipation of the implanted microchip. Results are provided for different assumptions about choroid blood flow. It is shown that computed results associated with the power dissipation of the implanted microchip, corresponding to temperature increases of approximately 0.6/spl deg/C in the midvitreous of the eye and 0.2/spl deg/C in the retina, closely parallel in-vivo experimental results in animals.     

Review on non-thermal effects of microwave irradiation in organic synthesis.

The aim of this review is to show the occurrence of non-thermal effects in organic synthesis. The effect of microwave irradiation is a consequence of the interaction of radiation with matter and a combination of thermal and non-thermal effects. Thermal effects are well-described and arise from the heating rate, sometimes non accessible by conventional heating, superheating, "hot spots" and the selective absorption of radiation by polar substances. The existence of non-thermal effects of the highly polarized radiation is still a controversial topic, and one usually masked by thermal effects. Separation and identification of thermal and non-thermal effects is a complex matter, but essential to the study of non-thermal effects. Some predictive models have also been described.     

人体头部过载模型阶次的辨识与降阶研究

针对在系统辨识过程中面临的人体头部过载模型阶次辨识问题,介绍了常用的模型阶次辨识方法及利用残差平方和即损失函数J估计模型阶次的原理,并基于20组试验数据开展人体头部过载模型阶次辨识研究。通过系统辨识获得了人体头部过载模型后,采用Gram阵对Hankel奇异值分解方法实施模型简化和降阶处理。通过比较降阶前后模型,表明模型降阶方法是有效和正确的。