Numerical Dosimetry Research Articles

SAR and efficiency evaluation of a 900 MHz waveguide chamber for cell exposure

In this work we present the results of numerical and experimental dosimetry carried out for an in vitro exposure device to irradiate sample groups at 900 MHz. The cells are kept in 8 and 15 ml cell cultures, contained, respectively in T25 and T75 rectangular flasks. The dosimetric assessment of the distribution of the specific absorption rate (SAR) is performed for both the bottom of the flask and the whole volume of the sample to provide results for experiments on either the cell layer or the cell suspension. The irradiating chamber is a rectangular waveguide (WG). Different configurations are considered to assess the optimum orientation and positioning of the cell cultures inside the WG. The system performance is optimal when the electric field is parallel to the sample and the WG is terminated by a matched load. In this condition two 15 or four 8 ml cells cultures can be exposed. The efficiency (ratio between the power absorbed by the sample and the incident power) and the non-uniformity degree (ratio between the standard deviation of SAR values and the average SAR over the sample) are calculated and successfully verified through measurements of the scattering parameters and local temperature increases. In the chosen exposure configuration, the efficiency is 0.40 and the non-uniformity degree is 39% for the 15 ml samples. For the 8 ml samples, the efficiency is 0.19 and a low non-uniformity degree (15%) is found.

Variability analysis of SAR from 20 MHz to 2.4 GHz for different adult and child models using finite-difference time-domain

This paper deals with the variability of body models used in numerical dosimetry studies. Six adult anthropomorphic voxel models have been collected and used to build 5-, 8- and 12-year-old children using a morphing method respecting anatomical parameters. Finite-difference time-domain calculations of a specific absorption rate (SAR) have been performed for a range of frequencies from 20 MHz to 2.4 GHz for isolated models illuminated by plane waves. A whole-body-averaged SAR is presented as well as the average on specific tissues such as skin, muscles, fat or bones and the average on specific parts of the body such as head, legs, arms or torso. Results point out the variability of adult models. The standard deviation of whole-body-averaged SAR of adult models can reach 40%. All phantoms are exposed to the ICNIRP reference levels. Results show that for adults, compliance with reference levels ensures compliance with basic restrictions, but concerning children models involved in this study, the whole-body-averaged SAR goes over the fundamental safety limits up to 40%.For more information on this article, see medicalphysicsweb.org

Development of a numerical model connecting electromagnetism, thermal and hydrodynamics to analyse in vitro exposure system

For bioelectromagnetic studies, a complete dosimetry is essential for optimised experiments and for analysis of the biological results. In this paper, we focused on the numerical dosimetry based on electromagnetic, thermal and convection simulations. The finite difference time domain (FDTD) method is used to obtain electromagnetic fields and specific absorption rate (SAR) distributions. Often metallic losses exist and cannot be neglected, and they are considered with SAR spatial distribution to evaluate temperature elevation. Time-scaled algorithms of heat transfer equation and incompressible Navier–Stokes equations applied to in vitro bioelectromagnetic studies are presented. For hydrodynamic thermal convection, the biological medium density has to be considered variable and the heat masses can move. Two different in vitro exposure systems are presented. A test tube with high temperature gradient induced in the biological medium is studied and illustrate convection phenomena. A wire patch cell with metallic elements highlights the role of metallic losses in the increase of temperature.

Specific absorption rate assessment near a base-station antenna (2,140 MHz): some key points

In this paper, a numerical dosimetry study for a typical base-station antenna used in third generation system is presented. This study is based on the limits defined by the International Commission on Non-Ionizing Radiation Protection. The primary goal is to analyze the relationship between the antenna input powers required to reach the specific absorption rate (SAR) limit and those ones which are required to reach the power density limit. The secondary goal is to underline the key points which impact the local SAR in the case of such a directive antenna. Moreover, for these studies, two numerical approaches are used: the first one is only based on the finite difference time domain method and, in the second one, this algorithm is coupled with a method of moments simulator via a Huygens box. Investigations are made to ensure that a more suitable numerical approach is used.

A Real-Time Exposure System for Electrophysiological Recording in Brain Slices

In order to study possible effects of microwave electromagnetic (EM) fields on neuronal activity, a real-time exposure system was designed and fabricated. The structure is suitable for electrophysiological recording in brain slices, a widely used in vitro methodology for investigations on neuronal function. The EM features of the structure were characterized both numerically and experimentally. Results indicate that the system is slightly radiating and that its field configuration produces minimum coupling with laboratory equipment. To determine the device efficiency, numerical and experimental dosimetry was conducted. The system exhibits high efficiency, with uniform distribution of the E- and H-fields and the specific absorption rate (SAR) in the biological sample.

An anatomically realistic whole-body pregnant-woman model and specific absorption rates for pregnant-woman exposure to electromagnetic plane waves from 10 MHz to 2 GHz

The numerical dosimetry of pregnant women is an important issue in electromagnetic-field safety. However, an anatomically realistic whole-body pregnant-woman model for electromagnetic dosimetry has not been developed. Therefore, we have developed a high-resolution whole-body model of pregnant women. A new fetus model including inherent tissues of pregnant women was constructed on the basis of abdominal magnetic resonance imaging data of a 26-week-pregnant woman. The whole-body pregnant-woman model was developed by combining the fetus model and a nonpregnant-woman model that was developed previously. The developed model consists of about 7 million cubical voxels of 2 mm size and is segmented into 56 tissues and organs. This pregnant-woman model is the first completely anatomically realistic voxel model that includes a realistic fetus model and enables a numerical simulation of electromagnetic dosimetry up to the gigahertz band. In this paper, we also present the basic specific absorption rate characteristics of the pregnant-woman model exposed to vertically and horizontally polarized electromagnetic waves from 10 MHz to 2 GHz.

Electric emissions from electrical appliances

Electric emissions from electric appliances are frequently considered negligible, and standards consider electric appliances to comply without testing. By investigating 122 household devices of 63 different categories, it could be shown that emitted electric field levels do not justify general disregard. Electric reference values can be exceeded up to 11-fold. By numerical dosimetry with homogeneous human models, induced intracorporal electric current densities were determined and factors calculated to elevate reference levels to accounting for reduced induction efficiency of inhomogeneous fields. These factors were found not high enough to allow generally concluding on compliance with basic restrictions without testing. Electric appliances usually simultaneously emit both electric and magnetic fields exposing almost the same body region. Since the sum of induced current densities is limited, one field component reduces the available margin for the other. Therefore, superposition of electric current densities induced by either field would merit consideration.

Influence of a 902.4 MHz GSM signal on the human visual system: Investigation of the discrimination threshold

The proximity of a mobile phone to the human eye raises the question as to whether radiofrequency (RF) electromagnetic fields (EMF) affect the visual system. A basic characteristic of the human eye is its light sensitivity, making the visual discrimination threshold (VDThr) a suitable parameter for the investigation of potential effects of RF exposure on the eye. The VDThr was measured for 33 subjects under standardized conditions. Each subject took part in two experiments (RF-exposure and sham-exposure experiment) on different days. In each experiment, the VDThr was measured continuously in time intervals of about 10 s for two periods of 30 min, having a break of 5 min in between. The sequence of the two experiments was randomized, and the study was single blinded. During the RF exposure, a GSM signal of 902.4 MHz (pulsed with 217 Hz) was applied to the subjects. The power flux density of the electromagnetic field at the subject location (in the absence of the subject) was 1 W/m(2), and numerical dosimetry calculations determined corresponding maximum local averaged specific absorption rate (SAR) values in the retina of SAR(1 g) = 0.007 W/kg and SAR(10 g) = 0.003 W/kg. No statistically significant differences in the VDThr were found in comparing the data obtained for RF exposure with those for sham exposure.

Behavioral and Cognitive Effects of MW Electromagnetic Field Exposures

Published studies of MW exposure thresholds for human adverse perception and cognitive effects with an emphasis on replicated results and the weight of scientific evidence, form the scientific basis for IEEE and ICNIRP exposure limits. Established adverse MW effects are associated with whole body heating at levels that usually increase temperature by approximately 1◦C or more. In human cognitive studies with low-level MW exposures, high quality replication studies appear to have eliminated many of the “false positives”. Although human cognitive research continues there are no established human cognitive effects at or below standard or guideline limits. In animal spatial memory studies the weight of evidence from six independent research groups is that there is no effect on spatial learning at whole body or local (head) MW exposures within guideline limits and also well above the limits, when whole body temperature did not measurably rise. These replications are of particular note because of the existing large body of scientific evidence of the neurophysiological and neuroanatomical nature of and homology of spatial memory in animals and man. But confirmatory MW exposure, spatial memory experiments in humans are required for health risk assessment. Controversy has followed reports that low level MW exposure caused toxic leakage through the blood brain barrier [BBB] that could lead to cognitive impairment. The BBB area of research remains open while we await the publication of further BBB attempted replication studies. Since changes in temperature are the biologically relevant information, experimental and numerical dosimetry is underway to predict more precisely MW thermophysiological dose-effects on human or animal nervous functions. DOI: 10.2529/PIERS060907150019

Effects of geometry discretization aspects on the numerical solution of the bioheat transfer equation with the FDTD technique

In this work, we highlight two issues that have to be taken into consideration for accurate thermal modelling with the finite-difference time-domain (FDTD) method, namely the tissue interfaces and the staircasing effect. The former appears less critical in the overall accuracy of the results, whereas the latter may have an influence on the worst-case approach used in numerical dosimetry of non-ionizing radiation.

A high-efficiency waveguide applicator for in vitro exposure of mammalian cells at 1.95 GHz

A standardized exposure device for a set of in vitro bioelectromagnetic experiments, to be carried out in different laboratories under strictly standardized exposure conditions, was designed and tested on the basis of efficiency and field uniformity criteria. In particular, a WR-430 waveguide was chosen in order to expose mammalian cells cultures at 1.95 GHz. The cells could be exposed in either one, two, or four 35-mm Petri dishes filled with 3-mL samples. Numerical and experimental dosimetry was carried out in order to assess the optimum orientation and positioning of the circular Petri dishes inside the simple waveguide chamber, obtaining very satisfactory performances of the system when the electric field is parallel to the sample. The efficiency and the nonuniformity degree simulated were successfully verified through measurements of the scattering parameters and local temperature increases.

Quasi-Static Electromagnetic Dosimetry: From Basic Principles to Examples of Applications

An overview of quasi-static electromagnetic dosimetry is presented. After an introductive description of quantities and standards and a quick look at experimental and analytical approaches, attention is focused on numerical dosimetry. The process that leads to the calculation of results is analyzed in its basic steps, including the representation of the human body by means of a realistic voxel phantom. The most popular numerical methods are then described. An analysis of different methods in the same framework emphasizes common features and differences. This can help in choosing a more suitable method to solve a particular problem. An example of an application is finally reported.

Dosimetric comparison of the specific anthropomorphic mannequin (SAM) to 14 anatomical head models using a novel definition for the mobile phone positioning

This paper presents new definitions for obtaining reproducible results in numerical phone dosimetry. Numerous numerical dosimetric studies have been published about the exposure of mobile phone users which concluded with conflicting results. However, many of these studies lack reproducibility due to shortcomings in the description of the phone positioning. The new approach was tested by two groups applying two different numerical program packages to compare the specific anthropomorphic mannequin (SAM) to 14 anatomically correct head models. A novel definition for the positioning of mobile phones next to anatomically correct head models is given along with other essential parameters to be reported. The definition is solely based on anatomical characteristics of the head. A simple up-to-date phone model was used to determine the peak spatial specific absorption rate (SAR) of mobile phones in SAM and in the anatomically correct head models. The results were validated by measurements. The study clearly shows that SAM gives a conservative estimate of the exposure in anatomically correct head models for head only tissue. Depending on frequency, phone position and head size the numerically calculated 10 g averaged SAR in the pinna can be up to 2.1 times greater than the peak spatial SAR in SAM. Measurements in small structures, such as the pinna, will significantly increase the uncertainty; therefore SAM was designed for SAR assessment in the head only. Whether SAM will provide a conservative value for the pinna depends on the pinna SAR limit of the safety standard considered.

Numerical dosimetry for cells under millimetre-wave irradiation using Petri dish exposure set-ups

We carried out a numerical dosimetry study for in vitro experiments on millimetre-wave (MMW) biological effects on cells. The cell layers are cultured in 35 mm Petri dishes placed in the far-field region of a rectangular horn irradiator generating a 50.0 GHz continuous sinusoidal MMW. The finite-difference time-domain (FDTD) method and the second-order approximation of the absorbing boundary conditions (ABCs) are applied in calculating the specific absorption rate (SAR) in the cells. The 0.125 mm and 0.25 mm voxel models of the Petri dish are used. The permittivity and the conductivity of the cells and those of the culture medium are obtained with Debye's dispersion equation. We measured the power pattern of the irradiator using the plane wave expansion (PWE) method, and developed a program module to calculate the FDTD-compatible incident field from the irradiator, whose position and orientation are adjustable. The MMW multiple reflection between the Petri dish and the irradiator is evaluated before being neglected. For the single-dish, double-dish and quadruple-dish exposure set-ups, the SAR intensity and the SAR uniformity are analysed and compared. The meniscus effect on the SAR distribution over the cell layer is evaluated for the single-dish set-up. The influence of the Petri dish interaction on the SAR distribution is examined for the double-dish and quadruple-dish set-ups.

Numerical modeling and dosimetry of the 35 mm Petri dish under 46 GHz millimeter wave exposure

Experimental studies on effects of millimeter wave (MMW) exposure on cells cultured in Petri dishes have attracted interest in recent decades. To improve the quantification of the biological responses toward the MMW energy, an accurate and precise MMW dosimetry is to be provided. By using the finite difference time domain (FDTD) method, the numerical dosimetry is performed for a typical 35 mm Petri dish under 46 GHz continuous MMW exposure from an irradiator of a specified power pattern. With the aim of building a precise model, the meniscus at the interface between the culture solution and the Petri dish sidewall is taken into account, followed by the modeling of smooth edges of the Petri dish. The trilinear interpolation is introduced to assist the FDTD method to obtain a more precise dosimetric assessment. The specific absorption rate (SAR) distributions in the cornea cells covered by culture solution in the Petri dish are calculated and compared to display the effects of using Petri dish models of various precision and the trilinear interpolation on dosimetry results. In addition, the SAR distribution in the cells is analyzed to study its homogeneity. The results indicate that the precise Petri dish model and the application of the trilinear interpolation are helpful in improving the precision of numerical dosimetry. It is also revealed that the inhomogeneity of the SAR distribution is well beyond neglect, which deserves cautious consideration in experiments investigating MMW effects on cells in vitro.

Numerical Dosimetry for Cells Cultured in Petri Dishes Exposed to a Six-degree-freedom Millimeter-Wave Radiator

A numerical dosimetry is performed for experiments studying effects on cornea cells cultured in Petri dishes exposed to 50 GHz millimeter wave (MMW). An algorithm is developed to calculate the incident field from a six-degree-freedom MMW radiator, with respect to its specified position and orientation. With the algorithm, the finite-difference time-domain (FDTD) method simulated the interaction between the incident MMW and the Petri dishes. The specific absorption rate (SAR) in cells and the homogeneity of the SAR distribution are calculated and analyzed for dosimetric assessment in related MMW experiments on cells in vitro.

Dielectric properties of biological tissue: Variation with age

The dielectric properties of whole brain, skin, and skull were determined experimentally in the frequency range 300 KHz-300 MHz. Tissue samples were excised from 10, 30, and 70 day old Wistar strain rats. The data are presented in graphical format and compared to previously published data in the frequency range 0.1-20 GHz. Good agreement is observed between the two data sets. At frequencies in excess of about 100 MHz, where the gamma dispersion is dominant, the permittivity and conductivity increase monotonically with decreasing age. At lower frequencies, the site of the beta dispersion, a change in the frequency dependence of the dielectric parameters is observed and is most evident in the spectra for brain and skin. This is attributed to changes in the tissue structure. Age-related dielectric data, available for 9 of the 34 tissues, were incorporated in a numerical plane wave exposure dosimetry study on anatomically heterogeneous rat models with body sizes corresponding to the ages of 10, 30, and 70 days at a number of spot frequencies from 27 to 2000 MHz. The results reveal that the variation in the dielectric properties affect the whole body SAR by less than 5% with the most conservative value (highest SAR) obtained when 70 day properties are used.

1800 MHz in vitro exposure device for experimental studies on the effects of mobile communication systems

A wire patch cell (WPC) operating at the uplink frequency band of GSM 1800 MHz has been designed for in vitro experiments with the aim of investigating the possible biological effects of electromagnetic radiation associated with cellular phones. The 1800 MHz WPC design is a direct descendant of the original 900 MHz WPC introduced by Laval et al. This system provides a homogeneous specific absorption rate distribution, using four 3.5 cm petri dishes simultaneously. Numerical dosimetry has been performed using a commercial code (CST Microwave Studio), in order to evaluate accurately the efficiency of the structure (in terms of W kg(-1) per 1 W input power) and the distribution in the chosen biological target. The numerical results have been confirmed by experimental measurements performed by measuring thermal increase due to a high power impulse. The efficiency of the structure is 1.25 +/- 25% W kg(-1) per 1 W input power higher than the efficiency of the 900 MHz WPC. A few adjustments have been made in order to use the WPC in a standard incubator and to avoid thermal increases related to the radio frequency exposure. This exposure system has been adopted for the experiments scheduled in the RAMP and GUARD projects (VFPE).

High Peak SAR Exposure Unit With Tight Exposure and Environmental Control for In Vitro Experiments at 1800 MHz

The objective of this study was to develop, optimize, and characterize a flexible and reliable unit for standardized and well-controlled electromagnetic-field exposure of cells in vitro. The technical requirements were high peak and time-averaged exposure of the cells with a minimal temperature rise for the cell cultures, flexible modulation schemes, high uniformity, and low variability of exposure, as well as support of blinded protocols. The developed setup is based on two R18 waveguides resonant at 1800 MHz and operated with a computer-controlled signal and monitoring unit. The cells can be exposed in 35-mm Petri dishes either cultivated as monolayers or in suspension. For cell monolayers, the system provides an efficiency for the specific absorption rate (SAR) of >50 (W/kg)/W input power, a nonuniformity of the SAR distribution of 10 (W/kg)/W, nonuniformity of <40%, SAR variability of <17%, and a temperature rise of <0.13/spl deg/C/(W/kg) is achieved. The numerical dosimetry for the field and temperature distributions within the Petri dishes was verified using E-field and temperature probes. The temperature analysis has shown that the possibility of localized hot spots can be excluded.

The GSF family of voxel phantoms

Voxel phantoms are human models based on computed tomographic or magnetic resonance images obtained from high-resolution scans of a single individual. They consist of a huge number of volume elements (voxels) and are at the moment the most precise representation of the human anatomy. The purpose of this paper is to introduce the GSF voxel phantoms, with emphasis on the new ones and highlight their characteristics and limitations. The GSF voxel family includes at the moment two paediatric and five adult phantoms of both sexes, different ages and stature and several others are under construction. Two phantoms made of physical calibration phantoms are also available to be used for validation purposes. The GSF voxel phantoms tend to cover persons of individual anatomy and were developed to be used for numerical dosimetry of radiation transport but other applications are also possible. Examples of applications in patient dosimetry in diagnostic radiology and in nuclear medicine as well as for whole-body irradiations from idealized external exposures are given and discussed.