Specific Absorption Rate Distribution Research Articles

Waveguide Applicator System for Head and Neck Hyperthermia Treatment

The main purpose of this article is a complex hyperthermia applicator system design for treatment of head and neck region. The applicator system is composed of four waveguides with a stripline horn aperture and circular water bolus. The specific absorption rate (SAR) and temperature distribution from this applicator in various numerical phantom models was investigated. For used targets, the treatment planning based on the optimization process made through the SEMCAD X software is added to show the steering possibilities of SAR and thereby temperature distribution. Using treatment planning software, we proved that the SAR and temperature distribution can be effectively controlled (by amplitude and phase changing) improving the SAR and temperature target coverage approximately by 20 %. For the proposed applicator system analysis and quantitative evaluation of two parameters 25 % iso-SAR and 41°C iso-temperature contours in the treatment area with the respect to sensitive structures in treatment area were defined. To verify our simulation results, the real measurement of reflectivity coefficient as well as the temperature distribution in a homogenous phantom were performed.

Direct SAR mapping by thermoacoustic imaging: A feasibility study.

To develop a new method capable of directly measuring specific absorption rate (SAR) deposited in tissue using the thermoacoustic signal induced by short radiofrequency (RF) pulse excitation. A detailed model based on the thermoacoustic wave generation and propagation is presented. We propose a new concept for direct measurement of SAR, to be used as a safety assessment/monitoring tool for MRI. The concept involves the use of short bursts of RF energy and the measurement of the resulting thermoacoustic excitation pattern by an array of ultrasound transducers, followed by image reconstruction to yield the 3D SAR distribution. We developed a simulation framework to model this thermoacoustic SAR mapping concept and verified the concept in vitro. Simulations show good agreement between reconstructed and original SAR distributions with an error of 4.2, 7.2, and 8.4% of the mean SAR values in axial, sagittal, and coronal planes and support the feasibility of direct experimental mapping of SAR distributions in vivo. The in vitro experiments show good agreement with theory (r2 = 0.52). A novel thermoacoustic method for in vivo mapping of local SAR patterns in MRI has been proposed and verified in simulation and in a phantom experiment. Magn Reson Med 78:1599-1606, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Dental Implants and Mobile-Phone Use: How implant presence and position affect antenna parameters, specific absorption rate, and current density

In this article, the effect of the existence and position of dental implants on antenna parameters, specific absorption rate (SAR), and current density induced by the metal part in the human head is investigated. A SAM phantom, belonging to the Computer Simulation Technology (CST) Microwave Studio, and a dipole antenna radiating at 900 and 1,800 MHz are used in this investigation. SAR and current density distribution are compared in a head model with different positions of implants.

A New High-Resolution Electromagnetic Human Head Model: A useful resource for a new specific-absorption-rate assessment model

A high-resolution realistic human head model for the human exposure assessment of the specific absorption rate (SAR) is presented in this article. To our best knowledge, this model is the first Chinese electromagnetic (EM) human head model (CMODEL). The model is built using the first Chinese Visible Human (CVH) data set containing 3,640 serial axial anatomical images. In particular, the model has a voxel resolution of 0.16 ? 0.16 ? 0.25 mm3 for the head part and 0.16 ? 0.16 ? 0.5 mm3 for the shoulder part. Such a high-resolution model allows for an accurate simulation of 49 different tissues and organs for the SAR assessment. A numerical investigation concerning the detailed SAR distribution of the standard specific anthropomorphic mannequin (SAM) model, the popular but low-resolution HUGO model, and this CMODEL is conducted and involves exposing the three models to radiation from the same dual-band antenna, and this investigation demonstrates the usefulness of a high-resolution EM human head model. For the sake of comparison, the eye models that are extracted from the CMODEL and HUGO models, which are illuminated by a one-half wavelength dipole antenna, are employed to demonstrate the advantage of the CMODEL for the SAR assessment.

Temperature distribution and Specific Absorption Rate inside a child’s head

This paper represents the numerical analysis of Specific Absorption Rate (SAR) and temperature distribution within a real child head model exposed to mobile phone radiation at the frequency of f=900MHz. In this research the SAR and temperature distribution are obtained by numerical solutions of the equation of electromagnetic waves propagation and by bioheat equation, respectively, and are shown inside different biological tissues and organs during exposure to electromagnetic radiation from a mobile phone. As electromagnetic properties of tissues depend on the electromagnetic waves frequency, the value of SAR and temperature will be different for different tissues and organs. The maximum absorption of electromagnetic energy is in the surface layers of the model, whereby this value is greater than the maximum allowed value defined by standards. Furthermore, the increase in temperature is the highest in those biological tissues and organs that are closest to the source of radiation i.e. a mobile phone. Moving away from a mobile phone, the temperature decreases, but more slowly than the SAR values. In the analysis of the temperature rise resulting from tissues and organs heating due to the effects of electromagnetic fields on a child’s head, special attention will be given to the maximum temperature increase in the brain.

Influence of dissolved gases in coupling waterbolus on superficial hyperthermia and evaluation of a water conditioning system with inline degassing

Purpose. A water conditioning system with inline degassing is presented for real time removal of the dissolved gases to improve heat delivery during superficial hyperthermia. Methodology. Coupled electromagnetic (EM) and thermal models are used to study the influence of air bubbles on the tissue specific absorption rate (SAR) and temperature distributions for a 915 MHz spiral antenna. The design and characterization of the water conditioning system with inline degassing, and evaluation on tissue phantoms are presented. Results. Perturbations in the simulated tissue SAR and temperature distributions for small air volume fractions in the bolus (1%–2%) were confirmed with phantom measurements. The water conditioning system maintained the bolus thickness and temperature within ±0.5% at a flow rate of 1 l min−1 and ±0.2 °C respectively, and degassed the circulating water at an exponential rate with respect to time. The power coupled during phantom heating was more than 90% using the proposed system, and fluctuated between 31% and 90% for circulation of the pre-degassed water. Phantom heating experiments for the 39 °C bolus recorded a 3 °C rise in temperature at 5 mm depth with inline degassing, and 1.9 °C–2.4 °C without inline degassing. Conclusion. The inline bolus water degassing system is simple and cost effective, has shorter treatment pre-preparation time and could improve superficial hyperthermia treatment.

Simultaneous Quantitative Imaging of Electrical Properties and Proton Density From B1 Maps Using MRI.

Electrical conductivity and permittivity of biological tissues are important diagnostic parameters and are useful for calculating subject-specific specific absorption rate distribution. On the other hand, water proton density also has clinical relevance for diagnosis purposes. These two kinds of tissue properties are inevitably associated in the technique of electrical properties tomography (EPT), which can be used to map in vivo electrical properties based on the measured B1 field distribution at Larmor frequency using magnetic resonance imaging (MRI). The signal magnitude in MR images is locally proportional to both the proton density of tissue and the receive B1 field; this is a source of artifact in receive B1-based EPT reconstruction because these two quantities cannot easily be disentangled. In this study, a new method was proposed for simultaneously extracting quantitative conductivity, permittivity and proton density from the measured magnitude of transmit B1 field, proton density-weighted receive B1 field, and transceiver phase, in a multi-channel radiofrequency (RF) coil using MRI, without specific assumptions to derive the proton density distribution. We evaluated the spatial resolution, sensitivity to contrast, and accuracy of the method using numerical simulations of B1 field in a phantom and in a realistic human head model. Using the proposed method, conductivity, permittivity and proton density were then experimentally obtained ex vivo in a pork tissue sample on a 7T MRI scanner equipped with a 16-channel microstrip transceiver RF coil.

Microwave ablation of ex vivo bovine tissues using a dual slot antenna with a floating metallic sleeve

Purpose: To study the efficiency of a dual slot antenna with a floating metallic sleeve on the ablation of different ex vivo bovine tissues.Materials and methods: COMSOL Multiphysics® version 4.4 (Stockholm, Sweden), which is based on finite element methods (FEM), was used to design and simulate monopole and dual slot with sleeve antennas. Power, specific absorption rate (SAR), temperature and necrosis distributions in the selected tissues were determined using these antennas. Monopole and dual slot with sleeve antennas were designed, simulated, constructed and applied in this study based on a semi-rigid coaxial cable. Ex vivo experiments were performed on liver, lung, muscle and heart of bovine obtained from a public animal slaughter house. The microwave energy was delivered using a 2.45 GHz solid-state microwave generator at 40 W for 3, 5 and 10 min. Aspect ratio, ablation length and ablation diameter were also determined on ablated tissues and compared with simulated results. Student’s t-test was used to compare the statistically significant difference between the performance of the two antennas.Results: The dual slot antenna with sleeve produces localised microwave energy better than the monopole antenna in all ablated tissues using simulation and experimental validation methods. There were significant differences in ablation diameter and aspect ratio between the sleeve antenna and monopole antenna. Additionally, there were no significant differences between the simulation and experimental results.Conclusions: This study demonstrated that the dual slot antenna with sleeve produced larger ablation zones and higher sphericity index in ex vivo bovine tissues with minimal backward heating when compared with the monopole antenna.

Investigation on improved water-loaded diagonal horn applicators for hyperthermia

This paper describes simulation, theoretical, and/or experimental studies of specific absorption rate (SAR) distribution in biological phantom (muscle)/tri-layered bio-media without and with irregular-shaped tumors in direct contact with improved water-loaded metal diagonal horns designed at 2450 and 915 MHz. Further, thermal simulation results based on Pennes’ bio-heat equation are also provided for each of the proposed applicators, which is in direct contact with realistic tri-layered bio-media without and with irregular-shaped tumor embedded within the muscle layer. The proposed horns are improved versions of the respective conventional metal diagonal horns in which the aperture fields, which are modified by introducing four conducting pins at appropriate locations near each antenna aperture, correspond to fundamental and higher order modes. The theoretical aperture field distributions of the proposed horns designed at 2450 and 915 MHz are compared with the corresponding simulated ones. Further, the theoretical SAR parameters (penetration depth (PD) and effective field size (EFS)) in the phantom muscle medium due to the proposed horns designed at 2450 and 915 MHz are compared with the corresponding simulated and/or experimental results as well as with those obtained due to the corresponding conventional horns of identical dimensions. Moreover, effects on PD, EFS/EFS25, and temperature distribution profiles due to presence of irregular-shaped tumor in the tri-layered bio-media at 2450 and 915 MHz are also investigated. It is demonstrated that desired temperature range 41–45 °C can be achieved and maintained for 60 min with input power of 4/10 W at the frequencies of 2450/915 MHz for treating superficial tumors.

Temperature induced in the testicular and related tissues due to electromagnetic fields exposure at 900 MHz and 1800 MHz

Increased testicular temperature adversely affects the reproductive system in the male. Environmental conditions, namely high ambient temperature and electromagnetic fields (EMFs), influence the temperature increase in the human body. This study considers the computationally determined specific absorption rate (SAR) and the heat transfer in a piecewise-homogeneous human model of the male reproductive organs and upper thigh exposed to an electric dipole antenna. The study focuses on increases in testicular temperature due to EMF absorption. Much attention is paid to the effects of the operating frequency and exposure time on the SAR and temperature increases induced by exposure to a near-field EMF. The electric field, SAR and temperature distributions in various tissues during exposure to EMFs are obtained by numerical simulation of EM wave propagation and an unsteady bioheat transfer model. This study indicated that when the model is exposed to EMFs at the frequencies of 900 and 1800MHz, the highest SAR values are obtained in the scrotum. In the testis, which is the most sensitive part of the male reproductive system, the SAR value of the 900MHz frequency is significantly higher than that of 1800MHz, while there are no significant differences in the temperature increases between the two operating frequencies. The obtained results may be of assistance in determining exposure limits for the power output of wireless transmitters, and their operating frequency for use with humans. Significant of this workThe study focuses on increases in testicular temperature due to EMF absorption. Much attention is paid to the effects of the operating frequency and exposure time on the SAR and temperature increases induced by exposure to a near-field EMF. The electric field, SAR and temperature distributions in various tissues during exposure to EMFs are obtained by numerical simulation of EM wave propagation and an unsteady bioheat transfer model. The obtained results may be of assistance in determining exposure limits for the power output of wireless transmitters, and their operating frequency for use with humans.

In vivo setup characterization for pulsed electromagnetic field exposure at 3 GHz

An in vivo setup for pulsed electric field exposure at 3 GHz is proposed and characterized in this work. The exposure system allows far field, whole-body exposure of six animals placed in Plexiglas cages with a circular antenna. Chronic exposures under 18 W incident average power (1–4 kW peak power) and acute exposures under 56 W incident average power (4.7 kW peak power) were considered. Numerical and experimental dosimetry of the setup allowed the accurate calculation of specific absorption rate (SAR) distributions under various exposure conditions. From rat model numerical simulations, the whole-body mean SAR values were 1.3 W kg−1 under chronic exposures and 4.1 W kg−1 under acute exposure. The brain-averaged SAR value was 1.8 W kg−1 and 5.7 W kg−1 under chronic and acute exposure, respectively. Under acute exposure conditions, a 10 g specific absorption of 1.8 ± 1.1 mJ · kg−1 value was obtained. With temperature rises below 0.8 °C, as measured or simulated on a gel phantom under typical in vivo exposures, this exposure system provides adequate conditions for in vivo experimental investigations under non-thermal conditions.

SIMULATION STUDY OF AN EBG-M APPLICATOR TOWARDS NON-INVASIVE BREAST HYPERTHERMIA CANCER PROCEDURE

Interest in the development of hyperthermia for cancer procedure is growth explosively all over the years as it is able to damage and kill the cancer cell non-invasively. An EBG-M antenna is designed with FDTD simulation packages, which is then applied towards treated cancerous area, where for the purpose of this simulation studies, breast is developed as the targeted cancer area to be treated. The radiation absorption is presented as the results obtained to be compared with previous works and also to be analyzed. It has been recognized that the designed antenna or also called as the applicator is able to offer depth radiation absorption and also improved the focusing, which is represented by the specific absorption rate (SAR) distribution towards breast hyperthermia cancer procedure.

Numerical Implementation of Representative Mobile Phone Models for Epidemiological Studies

This paper describes an implementation method and the results of numerical mobile phone models representing real phone models that have been released on the Korean market since 2002. The aim is to estimate the electromagnetic absorption in the human brain for casecontrol studies to investigate health risks related to mobile phone use. Specific absorption rate (SAR) compliance test reports about commercial phone models were collected and classified in terms of elements such as the external body shape, the antenna, and the frequency band. The design criteria of a numerical phone model representing each type of phone group are as follows. The outer dimensions of the phone body are equal to the average dimensions of all commercial models with the same shape. The distance and direction of the maximum SAR from the earpiece and the area above –3 dB of the maximum SAR are fitted to achieve the average obtained by measuring the SAR distributions of the corresponding commercial models in a flat phantom. Spatial peak 1-g SAR values in the cheek and tilt positions against the specific anthropomorphic mannequin phantom agree with average data on all of the same type of commercial models. Second criterion was applied to only a few types of models because not many commercial models were available. The results show that, with the exception of one model, the implemented numerical phone models meet criteria within 30%.

B1-based SAR reconstruction using contrast source inversion\u2013electric properties tomography (CSI-EPT)

Specific absorption rate (SAR) assessment is essential for safety purposes during MR acquisition. Online SAR assessment is not trivial and requires, in addition, knowledge of the electric tissue properties and the electric fields in the human anatomy. In this study, the potential of the recently developed CSI-EPT method to reconstruct SAR distributions is investigated. This method is based on integral representations for the electromagnetic field and attempts to reconstruct the tissue parameters and the electric field strength based on B_{1}^{ + } field data only. Full three-dimensional FDTD simulations using a female pelvis model are used to validate two-dimensional CSI reconstruction results in the central transverse plane of a 3T body coil. Numerical experiments demonstrate that the reconstructed SAR distributions are in good agreement with the SAR distributions as determined via 3D FDTD simulations and show that these distributions can be computed very efficiently in the central transverse plane of a body coil with the two-dimensional approach of CSI-EPT.

SAR reduction in the modelled human head for the mobile phone using different material shields

AbstractEvery mobile phone emits radio frequency electromagnetic energy. The amount of this energy absorbed by the human head is measured by the specific absorption rate (SAR). There are standard limits, according to which phones sold should be below certain SAR level. To maintain these limits, shields can be used for the mobile phones. In this paper using ANSYS HFSS, modelling of the human head is done. The modern mobile phone design is used for simulation. SAR distribution is measured for different layers of the head due to exposure to radiation from the mobile phone operating at GSM-1800 frequency band. The performance of the mobile phone antenna due to different shields is observed. Proper shielding material properties are found.

Flow and heat transfer in biological tissue due to electromagnetic near-field exposure effects

Electromagnetic fields (EMF) have been a vital part of our daily life over the past decade. Therefore, people are continuously exposed to electromagnetic (EM) sources in their vicinity generated by electronic devices such as those emitted by Wi-Fi, mobile phones, portable wireless router and other wireless services. This study aims to investigate the SAR, fluid flow and heat transfer in biological tissue due to EM near-field exposure. In a tissue model, the effects of distance to an EMF source and tissue permeability on natural convection in the biological tissue are systematically investigated. The specific absorption rate (SAR), fluid flow and the temperature distributions in the tissue during exposure to EM fields are obtained by numerical simulation of EM wave propagation and heat transfer equations. The EM wave propagation is expressed mathematically by Maxwell’s equations. The heat transfer model used in this study is developed based on bioheat model and porous media model. By using the porous media model, the distribution patterns of temperature are quite different from the bioheat model by the strong blood dissipation effect of the porous media model. The exposure distance significantly influences the SAR, velocity field and temperature distribution. Moreover, the tissue permeability also affects the temperature distribution patterns within the tissue. The obtained results may be of assistance in determining exposure limits for the power output of the wireless transmitter, and its distance from the human body. The results can also be used as a guideline to clinical practitioners in EM relates the interaction of the radiated waves with the human body.

Dosimetric Characteristics of an EMF Delivery System Based on a Real-Time Impedance Measurement Device

In this paper, the dosimetric characterization of an EMF exposure setup compatible with real-time impedance measurements of adherent biological cells is proposed. The EMF are directly delivered to the 16-well format plate used by the commercial xCELLigence apparatus. Experiments and numerical simulations were carried out for the dosimetric analysis. The reflection coefficient was less than -10dB up to 180MHz and this exposure system can be matched at higher frequencies up to 900 and 1800MHz. The specific absorption rate (SAR) distribution within the wells containing the biological medium was calculated by numerical finite-difference time domain simulations and results were verified by temperature measurements at 13.56MHz. Numerical SAR values were obtained at the microelectrode level where the biological cells were exposed to EMF including 13.56, 900, and 1800MHz. At 13.56MHz, the SAR values, within the cell layer and the 270-μL volume of medium, are 1.9e3 and 3.5W/kg/incident mW, respectively.

Simulation and experimental evaluation of selective heating characteristics of 13.56 MHz radiofrequency hyperthermia in phantom models

Hyperthermia with a radiofrequency electric field can treat a cancer by delivering energy to heat and kill cancer cells. For a deeply located tumor, focused energy to raise its local temperature selectively is crucial for effective cancer treatment with minimal injury to normal tissues. We evaluated the selective heating characteristics of hyperthermia with 13.56 MHz radiofrequency energy in phantom models by experimental and computational approaches. Phantom materials with different electrical properties such as distilled water, normal saline, egg white and porcine meat were used. Temperature rise in depthwise during heating at 75 W output power was compared. The temperature of the egg white increased selectively in the distilled water but not significantly in the saline. Numerical simulation was performed using SEMCAD X software to calculate the distributions of specific absorption rate (SAR) and temperature, which enabled us to elucidate the selective heating mechanism in those phantom models.

A Wideband Microwave Exposure Setup for Suspended Cells Cultures: Numerical and Experimental EM Characterization

An in vitro exposure setup developed to expose suspended cells in Petri dishes on a large frequency band up to a few gigahertz was studied. This system was based on far-field exposure with a horn antenna in an anechoic chamber. A specific incubator was designed with a water-cooling system for constant temperature inside the setup. Four exposed Petri dishes presented a similar specific absorption rate (SAR) distribution with a whole-volume SAR of $0.56 \pm 0.40~\hbox{W/kg/W}_{\rm inc}$ at 2.45 GHz. The incubator was equipped with a stirring mechanism allowing an homogenous distribution of temperature in the biological solution. The SAR was assessed over the 1.5–3 GHz frequency range. Using the temperature control and stirring, an increase of 0.4 $^\circ {\rm C}$ in temperature was observed after 2 h exposure at 2.45 GHz, 22 W incident power. This dosimetric study demonstrated the ability of this system to expose cells in suspension at different frequencies in a controlled environment .

MRI-Based Electrical Property Retrieval by Applying the Finite-Element Method (FEM)

In this study, the finite-element method is applied to reconstruct the electrical properties (conductivity and permittivity) maps of media with 1-D, 2-D, and 3-D inhomogeneities based on the RF magnetic field distributions in a magnetic resonance imaging (MRI) system. The proposed method handles the discontinuity of electrical properties well. It therefore shows high accuracy at the boundary compared to retrieval methods in the literature. The method has successfully been validated using both analytical and numerical data. Moreover, the noise sensitivity of the proposed method is studied. Good accuracy of retrievals can be obtained when proper coils are chosen (e.g., transverse electromagnetic coils). It is an effective approach showing one step further towards accurate electrical property mapping based on MRI data. It helps to pave the way to accurate calculations of specific absorption rate distributions and temperature distributions of human body under MRI scans, which are crucial for the evaluation of the safety of MRI systems. Moreover, the increased accuracy of the electrical property maps of human body show their potential to be a diagnostic tool for tumors and cancers, especially at their early stages. Following-up research is expected to make the proposed method robust and practical for clinical applications.