SAR Distribution Research Articles

FEM modeling of SAR distribution and temperature increase in human brain from RF exposure

The assessment of radiofrequency exposure level and the exposure setup are critical, because if the exposure levels (related to frequency, power, position, and other variables) are not known, the biological results are not meaningful. In this regard, this study aims to design, implement, and analyze numerical setups for the simulations of radiofrequency exposure related specific absorption rate and temperature increase in the human brain and head. For this purpose, specific models for human head and telephone antenna are chosen, and the FEM is selected for solving PDEs related to electromagnetic wave equations and bioheat equation. After the verification of the methodology chosen by achieving comparable results with the literature, the scope of the study is then turned to the effects of different parameters on the exposure levels. In the end, comprehensive information can be obtained from the simulation results so that risk management policies for electromagnetic radiation can be reevaluated to minimize the possible health hazards. Copyright © 2012 John Wiley & Sons, Ltd.

SAR distribution in the tissues of human knee subjected to microwave radiation and its dependence on the radiated power

A high resolution three-dimensional geometric and electric model is constructed for the human knee. This model is subjected to microwave radiation at a frequency of 2450 MHz. The aim of the study is to determine the SAR distributions in the knee under different exposure conditions for the purpose of determining the consequent temperature distributions. The SAR distributions in the knee tissues are obtained including the SAR at a point, SAR averaged over 1 g (SAR 1g ) and that averaged over 10 g (SAR10g). The relation between the incident power density and the SAR is presented.

Fractional Derivatives Based Scheme for FDTD Modeling of $n$th-Order Cole–Cole Dispersive Media

A finite-difference time-domain (FDTD) modeling of the wave propagation in a Cole-Cole dispersive media is presented. Since the empirical Debye and Lorentz models are not accurate for the representation of the frequency dependence of some dispersive media terms, the Cole-Cole dispersion relation was used to model the electromagnetic properties of biological tissues. The main problem in time-domain modeling of the Cole-Cole model is the approximation of the fractional derivatives that appear in the model equation. Researchers face this problem by approximating the Cole-Cole terms (poles) by a sum of Debye terms or by a sum of decaying exponentials or by polynomials. The accuracy of these models depends on the number of terms needed to model each Cole-Cole term, which may consume large amounts of time and memory. In this letter, all the FDTD fields are approximated by a linear function of time that has a closed form for its fractional derivative. The proposed scheme is considered the more general scheme that has the capability to model nth-order Debye and Cole-Cole models. The scheme is a straightforward extension that can deal with other models such as Lorenz, Drude, and the chiral media. Promising results are observed when calculating the reflection coefficient at an air/muscle material interface. The SAR distribution within a Cole-Cole equivalent brain spherical material excited by an infinitesimal dipole is calculated and compared to the normal FDTD at 900 MHz.

MICROWAVE THERMOTHERAPY IN CANCER TREATMENT: EVALUATION OF HOMOGENEITY OF SAR DISTRIBUTION

Medical applications of microwaves (i.e., a possibility to use microwave energy and/or microwave technique and technology for therapeutical purposes) are a quite new and very rapidly developing fleld. Microwave thermotherapy is being used in medicine for cancer treatment and treatment of some other diseases since early eighties. This paper is a contribution to a theory of phase array applicators to be used for a microwave thermotherapy (microwave hyperthermia) in a cancer treatment. It deals with a study and theoretical evaluation of homogeneity of SAR distribution in cylindrical agar phantom for several difierent values of its radius. Discussed SAR distribution is in our case created by simulations of EM fleld exposure done by aid of four microwave stripline type TEM mode applicators of the same type.

509 有限要素法解析による脳腫瘍ハイパーサーミア用空胴共振器加温方式の加温特性解析(G03-2医療関連技術,G03機素潤滑部門)

In this paper, we propose a new heating control method of resonant cavity applicator using a dielectric bolus. The dielectric bolus is filled with dielectric such as water and is attached to a human head inside the cavity. In this paper, first, the proposed heating method using a dielectric bolus was described. Second, the 3-D anatomical human head model, which was reconstructed from 2-D MRIs and X-ray CT images, was presented. Finally, it was shown that in the human brain could be controlled by changing the electrical properties of the dielectric bolus. From these results of the SAR distributions, it was found that the proposed method was useful for effective hyperthermia treatments.

Local SAR in parallel transmission pulse design.

The management of local and global power deposition in human subjects (specific absorption rate, SAR) is a fundamental constraint to the application of parallel transmission (pTx) systems. Even though the pTx and single channel have to meet the same SAR requirements, the complex behavior of the spatial distribution of local SAR for transmission arrays poses problems that are not encountered in conventional single-channel systems and places additional requirements on pTx radio frequency pulse design. We propose a pTx pulse design method which builds on recent work to capture the spatial distribution of local SAR in numerical tissue models in a compressed parameterization in order to incorporate local SAR constraints within computation times that accommodate pTx pulse design during an in vivo magnetic resonance imaging scan. Additionally, the algorithm yields a protocol-specific ultimate peak in local SAR, which is shown to bound the achievable peak local SAR for a given excitation profile fidelity. The performance of the approach was demonstrated using a numerical human head model and a 7 Tesla eight-channel transmit array. The method reduced peak local 10 g SAR by 14-66% for slice-selective pTx excitations and 2D selective pTx excitations compared to a pTx pulse design constrained only by global SAR. The primary tradeoff incurred for reducing peak local SAR was an increase in global SAR, up to 34% for the evaluated examples, which is favorable in cases where local SAR constraints dominate the pulse applications.

Investigation of Hand Worn Jewellery Effects on the Specific Absorption Rate in the Human Head and Hand

The presence of hand could alter the energy absorbed in the human head. Hence, a study of the interaction between mobile device antennas and human head in the presence of metallic hand worn jewellery on human hand with different holding ways is investigated. A finite-difference time domain (FDTD) method has been performed by considering an internal PIFA antenna as the radiating source mounted at the top of a commercial clamshell phone positioned nearer the ear. With the introduction of hand-worn jewellery in variation parameters, there were perceptible effects on SAR variation in the human head. SAR distribution in the SAM head was found decreases due to different hand positions. Therefore, the head and in particular the hand may further impairment radiation performance of a mobile device.

An investigation impact of user’s positions in closed space over SAR in the head induced from mobile phone

The purpose of this work was to investigate the impact of user’s positions in close space over the variations of SAR distribution in human head induced from mobile phone under the worst case conditions. The “worst case” considered in this paper refers to conditions that include (1) a half-wavelength dipole antenna, placed near the model of human head—SAM phantom; (2) a vertically oriented dipole antenna, parallel to the side walls of closed space for maximum reflection; (3) metallic closed space acts as a resonant cavity; and (4) 360 randomly selected observation points. We also used a free space model for comparative purposes. The maximum 1- and 10-g average SAR and average SAR in whole-exposed object probability density function curves have been derived to illustrate the percentage of SAR values that have been induced inside SAM for different locations of user in the closed space. The obtained results show that in more than 86.7% of all investigated user’s positions in the metallic closed space, the induced maximum 1-g average SAR in user’s head is in the boundaries of ±10% of maximum 1-g average SAR, when the user use mobile phone in free space. This relationship keep still at maximum 10-g average SAR, where in 85.9% from all investigated user’s positions, SAR values are in boundaries of ±10% of maximum 10-g average SAR in free space. The results showed that closed space causes more significant changes on average SAR in whole-exposed object than maximal SAR values. When we estimated the average SAR in whole-exposed SAM in 85% from all investigated user’s position in closed space, the induced SAR is higher than SAR values in free space model. It can be noted that the highest obtained SAR values in closed space were with 8.5% (for 1 g), 6.7% (for 10-g average SAR), and 15.1% (for average SAR in whole SAM) higher than these of the free space.

Human keratinocytes in culture exhibit no response when exposed to short duration, low amplitude, high frequency (900 MHz) electromagnetic fields in a reverberation chamber

We exposed normal human epidermal keratinocytes to short duration, high frequency, and low amplitude electromagnetic fields, similar to that used by mobile phone technologies. We paid particular attention to the control of the characteristics of the electromagnetic environment generated within a mode stirred reverberation chamber (statistical homogeneity and isotropy of the field and SAR distribution). Two non-thermal exposure conditions were tested on the epidermal cells: 10-min exposure with a field amplitude of 8 V/m, and 30 min with 41 V/m. Corresponding specific absorption rates ranged from 2.6 to 73 mW/kg (continuous wave, 900 MHz carrier frequency). We collected RNA from cells subjected to these conditions and used it for a large-scale microarray screening of over 47000 human genes. Under these conditions, exposure of keratinocytes to the electromagnetic field had little effect; only 20 genes displayed significant modulation. The expression ratios were very small (close to 1.5-fold change), and none of them were shared by the two tested conditions. Furthermore, those assayed using polymerase chain reaction did not display significant expression modulation (overall mean of the exposed samples: 1.20 ± 0.18). In conclusion, the data presented here show that cultured keratinocytes are not significantly affected by EMF exposure.

Changes in Electromagnetic Field Absorption in the Presence of Subcutaneous Implanted Devices: Minimizing Increases in Absorption

Changes in electromagnetic energy absorption were studied in the presence of subcutaneous antenna devices implanted for biotelemetry applications. We examined the influence on energy absorption of these devices as passive metallic implants when incoming RF energy is present. We aim to contribute to the optimization of the implant location in order to reduce a possible energy increase on tissues. The research was carried out using electromagnetic modeling based on the finite-difference time-domain method. The calculations were performed in terms of electric field and SAR distributions calculated for five different frequency bands (from 0.9 to 17 GHz) covering most current telecommunications standards. The focus was on both far-field and near-field exposure. The results lead us to propose the subcutaneous zone nearest to the surface of the skin as the most appropriate place to locate these devices.

Simulation of Hyperthermic Treatment Using the Matrix of Stripline Applicators

This paper describes the design of a microwave stripline applicator for hyperthermic treatment, and the design of an anatomically based biological model, which is a necessary part of hyperthermia treatment planning for measuring the distribution of SAR. In this paper we compare the SAR distribution in a cylindrical homogeneous agar phantom (which has similar characteristics to biological tissue) and in an anatomically based biological model of the femur (which has been developed from a computer tomography scan) using a matrix of two applicators of the same type.

COMPLEX IMAGE SOLUTION OF SAR INSIDE A HUMAN HEAD ILLUMINATED BY A FINITE-LENGTH DIPOLE

This paper uses the accurate and computationally efficient complex image technique (CIT) to find the electric field and SAR distributions inside a human head illuminated by a finite-length dipole. The human head model used consists of 3 planar layers of lossy dielectrics. The accuracy of the results is verified by comparison with HFSS software. It was found that the CIT requires about 1 minute and 16 MB of RAM, while HFSS requires about 2 hours and 1.5 GB of RAM to find the SAR distribution using a 2.19 GHz Core 2 Due PC. The CIT method can also efficiently solve other types of antennas on other planar head models.

Microwave applicator for hyperthermia treatment on in vivo melanoma model

In this article, we evaluated a planar microwave applicator for in vivo superficial hyperthermia treatments on small tumors in the mouse mimicking treatments for human neoplasms. The design of the applicator, was challenged by the small dimensions of the tumors and unwanted diffusion of heating in the tumor-bearing animals. The required solution was to limit the penetration of microwaves in the depth of the tissue maintaining the full efficacy of hyperthermia. The study was firstly performed by computer simulations of SAR distribution inside a flat homogeneous phantom, considering various thicknesses of the integrated water bolus. Simulations, validated by the measurements, were also used to evaluate the impedance matching. Further tests were performed on homogeneous agar phantom to simulate the temperature distribution in the biological tissue and to preliminary assess the possible modality and schedule of microwave hyperthermia delivery. The in vivo experiments showed the evidence of direct microwave-induced heating and damage of the melanoma tissue in a range of penetration coherent both with computer simulations and phantom studies. The described approach appears perspective for designing limited-microwave-delivery applicators tailored for treatments of human superficial tumors and pre-tumoral lesions.

Experimental and numerical assessment of MRI‐induced temperature change and SAR distributions in phantoms and in vivo

It is important to accurately characterize the heating of tissues due to the radiofrequency energy applied during MRI. This has led to an increase in the use of numerical methods to predict specific energy absorption rate distributions for safety assurance in MRI. To ensure these methods are accurate for actual MRI coils, however, it is necessary to compare to experimental results. Here, we report results of some recent efforts to experimentally map temperature change and specific energy absorption rate in a phantom and in vivo where the only source of heat is the radiofrequency fields produced by the imaging coil. Results in a phantom match numerical simulation well, and preliminary results in vivo show measurable temperature increase. With further development, similar methods may be useful for verifying numerical methods for predicting specific energy absorption rate distributions and in some cases for directly measuring temperature changes and specific energy absorption rate induced by the radiofrequency fields in MRI experiments.

Low‐intensity microwave irradiation does not substantially alter gene expression in late larval and adult Caenorhabditis elegans

Reports that low-intensity microwave radiation induces heat-shock reporter gene expression in the nematode, Caenorhabditis elegans, have recently been reinterpreted as a subtle thermal effect caused by slight heating. This study used a microwave exposure system (1.0 GHz, 0.5 W power input; SAR 0.9-3 mW kg(-1) for 6-well plates) that minimises temperature differentials between sham and exposed conditions (< or =0.1 degrees C). Parallel measurement and simulation studies of SAR distribution within this exposure system are presented. We compared five Affymetrix gene arrays of pooled triplicate RNA populations from sham-exposed L4/adult worms against five gene arrays of pooled RNA from microwave-exposed worms (taken from the same source population in each run). No genes showed consistent expression changes across all five comparisons, and all expression changes appeared modest after normalisation (< or =40% up- or down-regulated). The number of statistically significant differences in gene expression (846) was less than the false-positive rate expected by chance (1131). We conclude that the pattern of gene expression in L4/adult C. elegans is substantially unaffected by low-intensity microwave radiation; the minor changes observed in this study could well be false positives. As a positive control, we compared RNA samples from N2 worms subjected to a mild heat-shock treatment (30 degrees C) against controls at 26 degrees C (two gene arrays per condition). As expected, heat-shock genes are strongly up-regulated at 30 degrees C, particularly an hsp-70 family member (C12C8.1) and hsp-16.2. Under these heat-shock conditions, we confirmed that an hsp-16.2::GFP transgene was strongly up-regulated, whereas two non-heat-inducible transgenes (daf-16::GFP; cyp-34A9::GFP) showed little change in expression.

Exploring the enhancement of the imaging properties of a microwave radiometry system for possible functional imaging using a realistic human head model

During the past four years, a novel Microwave Radiometry Imaging System (MiRaIS) has been developed and experimentally tested for brain activation imaging via contactless measurements. Through analytical theoretical and experimental analysis, the system seems to be able to detect any change of the product of temperature (T) and conductivity (σ), that is change of the product T*σ. However, in order to be able to study any specific brain area of interest using the proposed system, its focusing properties need to be improved. The present work investigates the use of dielectric materials as filling material inside the whole ellipsoid geometry or as matching layer around the head. Aim of both ideas is to improve the matching conditions on the skin-air interface. The results from the simulations verified the validity of this approach and also that focusing points appear to be sensitive to the spatial movement of the head. For the simulations, a commercial FDTD tool is used, along with an anatomically correct human head model developed from MRI scans. SAR distributions are also calculated inside the head revealing the potential implementation of hyperthermia treatment with the proposed system.

Determination of Electric Conductivity and Local SAR Via B1 Mapping

The electric conductivity can potentially be used as an additional diagnostic parameter, e.g., in tumor diagnosis. Moreover, the electric conductivity, in connection with the electric field, can be used to estimate the local SAR distribution during MR measurements. In this study, a new approach, called electric properties tomography (EPT) is presented. It derives the patient's electric conductivity, along with the corresponding electric fields, from the spatial sensitivity distributions of the applied RF coils, which are measured via MRI. Corresponding numerical simulations and initial experiments on a standard clinical MRI system underline the principal feasibility of EPT to determine the electric conductivity and the local SAR. In contrast to previous methods to measure the patient's electric properties, EPT does not apply externally mounted electrodes, currents, or RF probes, thus enhancing the practicality of the approach. Furthermore, in contrast to previous methods, EPT circumvents the solution of an inverse problem, which might lead to significantly higher spatial image resolution.

Influence of Use Conditions and Mobile Phone Categories on the Distribution of Specific Absorption Rate in Different Anatomical Parts in the Brain

SAR distributions in different anatomical parts of the brain were estimated for different use conditions and phone categories for the purpose of exposure assessment in epidemiological studies on RF exposure and brain cancer risk. The results suggested a variability of the exposure of the different parts depending on use conditions and mobile phone categories and highlighted the importance of evaluating the risk by connecting the tumor location to phone categories and use conditions. It was observed that the RF exposure of structures, particularly for those at the back of the brain, is sensitive to the phone shape and the antenna position relative to the head. The temporal lobe was generally the part with the highest exposure to RF, except for flip-type phones with the antenna on top extended. Occipital lobe and cerebellum were generally the second-highest exposed parts; exposure of parietal and frontal lobes were generally low, except for flip-type phones with the antenna in the center. It was observed that the exposure of the cerebellum cannot be neglected, whereas it was generally assumed to be low in previous epidemiological studies.

Assessment of SAR in the tissues near a cochlear implant exposed to radiofrequency electromagnetic fields

Cochlear implants (CI) are electronic devices used to restore partial hearing to people with severe hearing impairment. This paper aims to investigate if the introduction of a CI has an effect on SAR distribution in a head model exposed to radiofrequency electromagnetic fields (RF EMF) at mobile communication frequencies. The head model was obtained by image segmentation, the implant was modelled as a geometric structure and the exposure source was modelled as a uniform plane wave at 900 MHz, 1750 MHz and 1950 MHz, incident on the side of the head with the CI. Vertical and horizontal polarizations were simulated. Results show that the presence of a CI inside the cochlea produces negligible variations in the averaged SAR values, both in the head and in the cochlear tissues, although very localized differences in point SAR were found in the cochlea. Globally, these results suggest that finding harmful effects in the cochlear tissues will be unlikely.

Improving locoregional hyperthermia delivery using the 3-D controlled AMC-8 phased array hyperthermia system: A preclinical study

Background: The aim of this study is preclinical evaluation of our newly developed regional hyperthermia system providing 3-D SAR control: the AMC-8 phased array consisting of two rings, each with four 70 MHz waveguides. It was designed to achieve higher tumour temperatures and improve the clinical effectiveness of locoregional hyperthermia.Methods: The performance of the AMC-8 system was evaluated with simulations and measurements aiming at heating a centrally located target region in rectangular (30 × 30 × 110 cm) and elliptical (36 × 24 × 80 cm) homogeneous tissue equivalent phantoms. Three properties were evaluated and compared to its predecessor, the 2-D AMC-4 single ring four waveguide array: (1) spatial control and (2) size of the SAR focus, (3) the ratio between maximum SAR outside the target region and SAR in the focus. Distance and phase difference between the two rings were varied.Results: (1) Phase steering provides 3-D SAR control for the AMC-8 system. (2) The SAR focus is more elongated compared to the AMC-4 system, yielding a lower SAR level in the focus when using the same total power. This is counter-balanced by (3) a superficial SAR deposition which is half of that in the AMC-4 system, yielding a more favourable ratio between normal tissue and target SAR and allowing higher total power and up to 30% more SAR in the focus for 3 cm ring distance.Conclusion: The AMC-8 system is capable of 3-D SAR control and its SAR distribution is more favourable than for the 2-D AMC-4 system. This result promises improvement in clinical tumour temperatures.