A Regularized Boundary Element Formulation for Contactless SAR Evaluations within Homogeneous and Inhomogeneous Head Phantoms

This work presents a Boundary Element Method (BEM) formulation for contactless electromagnetic field assessments. The new scheme is based on a regularized BEM approach that requires the use of electric measurements only. The regularization is obtained by leveraging on an extension of Calderon techniques to rectangular systems leading to well-conditioned problems independent of the discretization density. This enables the use of highly discretized Huygens surfaces that can be consequently placed very near to the radiating source. In addition, the new regularized scheme is hybridized with both surfacic homogeneous and volumetric inhomogeneous forward BEM solvers accelerated with fast matrix-vector multiplication schemes. This allows for rapid and effective dosimetric assessments and permits the use of inhomogeneous and realistic head phantoms. Numerical results corroborate the theory and confirms the practical effectiveness of all newly proposed formulations

Numerically simulated exposure of children and adults to pulsed gradient fields in MRI

PurposeTo determine exposure to gradient switching fields of adults and children in a magnetic resonance imaging (MRI) scanner by evaluating internal electric fields within realistic models of adult male, adult female, and child inside transverse and longitudinal gradient coils, and to compare these results with compliance guidelines. Materials and MethodsPatients inside x-, y-, and z-gradient coils were simulated using anatomically realistic models of adult male, adult female, and child. The induced electric fields were computed for 1 kHz sinusoidal current with a magnitude of 1 A in the gradient coils. Rheobase electric fields were then calculated and compared to the International Commission on Non-Ionizing Radiation Protection (ICNIRP) 2004 and International Electrotechnical Commission (IEC) 2010 guidelines. The effect of the human body, coil type, and skin conductivity on the induced electric field was also investigated. ResultsThe internal electric fields are within the first level controlled operating mode of the guidelines and range from 2.7V m(-1) to 4.5V m(-1), except for the adult male inside the y-gradient coil (induced field reaches 5.4V m(-1)).The induced electric field is sensitive to the coil type (electric field in the skin of adult male: 4V m(-1), 4.6V m(-1), and 3.8V m(-1) for x-, y-, and z-gradient coils, respectively), the human body model (electric field in the skin inside y-gradient coil: 4.6V m(-1), 4.2V m(-1), and 3V m(-1) for adult male, adult female, and child, respectively), and the skin conductivity (electric field 2.35-4.29% higher for 0.1S m(-1) skin conductivity compared to 0.2S m(-1)). ConclusionThe y-gradient coil induced the largest fields in the patients. The highest levels of internal electric fields occurred for the adult male model. J. Magn. Reson. Imaging 2016;44:1360-1367

Relaxation-Time Determination from Continuous-Microwave Saturation of EPR Spectra

Based on the theories of Portis and of Castner 50 years ago, different continuous-wave measurement procedures for analyzing the microwave saturation power dependence of inhomogeneously broadened EPR lines were developed. Although these procedures have been refined, they still use only a few selected points on the saturation curve. A non-linear least-squares procedure for analyzing the microwave-power dependence of inhomogeneously broadened lines using all data points on a saturation curve has been developed. This procedure provides a simple alternative method to obtain magnetic relaxation data when the more direct pulse-saturation techniques are not available or are less suitable. The latter includes applications of quantitative EPR such as dosimetry. Then microwave saturation data should be obtained under conditions similar to those used in the quantitative measurements, which are usually made on first derivative spectra recorded using continuous-wave spectrometers. Selected applications to benchmark literature data and within the field of EPR dosimetry are discussed. The results obtained illustrate that relaxation times comparable to those yielded by various pulse-saturation EPR techniques can be obtained. It appears as a systematic feature that, whenever the pulse EPR data are fitted using bi-exponential functions, the shortest relaxation times obtained are those that correspond best to those measured using the current continuous-wave saturation method

Models and methods for computational electromagnetic dosimetry

The interaction between electromagnetic fields and the human body is a very complicated issue. In most cases it is not possible to measure accurately the electrical response of the human body to external sources. Because of this computational methods are used as an aid when determining the safety levels for human exposure to electromagnetic fields. In this thesis the field distribution caused by various sources is determined in different parts of the human body using detailed and anatomically correct human body models and computational methods. Both the distribution of electric currents induced by low-frequency magnetic fields and the absorption of radio-frequency fields are studied. The accuracy and reliability of the models and methods used is verified by comparing the acquired results to known closed-form solutions and calibration measurements. The obtained results can be utilised in the reliability analysis of computational methods used in electromagnetic dosimetry. Furthermore, some of the results are needed, for example, in the safety guidelines of medical personnel working close to magnetic resonance imaging scanners

Volume Integral Equation Methods for Forward and Inverse Bioelectromagnetic Approaches

L'abstract è presente nell'allegato / the abstract is in the attachmen

Modelling and characterisation of antennas and propagation for body-centric wireless communication

PhDBody-Centric Wireless Communication (BCWC) is a central point in the development of fourth generation mobile communications. The continuous miniaturisation of sensors, in addition to the advancement in wearable electronics, embedded software, digital signal processing and biomedical technologies, have led to a new concept of usercentric networks, where devices can be carried in the user’s pockets, attached to the user’s body or even implanted. Body-centric wireless networks take their place within the personal area networks, body area networks and body sensor networks which are all emerging technologies that have a broad range of applications such as healthcare and personal entertainment. The major difference between BCWC and conventional wireless systems is the radio channel over which the communication takes place. The human body is a hostile environment from radio propagation perspective and it is therefore important to understand and characterise the effect of the human body on the antenna elements, the radio channel parameters and hence the system performance. This is presented and highlighted in the thesis through a combination of experimental and electromagnetic numerical investigations, with a particular emphasis to the numerical analysis based on the finite-difference time-domain technique. The presented research work encapsulates the characteristics of the narrowband (2.4 GHz) and ultra wide-band (3-10 GHz) on-body radio channels with respect to different digital phantoms, body postures, and antenna types hence highlighting the effect of subject-specific modelling, static and dynamic environments and antenna performance on the overall body-centric network. The investigations covered extend further to include in-body communications where the radio channel for telemetry with medical implants is also analysed by considering the effect of different digital phantoms on the radio channel characteristics. The study supports the significance of developing powerful and reliable numerical modelling to be used in conjunction with measurement campaigns for a comprehensive understanding of the radio channel in body-centric wireless communication. It also emphasises the importance of considering subject-specific electromagnetic modelling to provide a reliable prediction of the network performance

HF-VHF帯における生体等価ファントムを用いた電磁波と人体相互作用の検証

研究科: 千葉大学大学院工学研究科学位:千大院工博甲第工185号博士(工学)千葉大

On a Low-Frequency and Contrast Stabilized Full-Wave Volume Integral Equation Solver for Lossy Media

In this article, we present a new regularized electric flux volume integral equation (D-VIE) for modeling high-contrast conductive dielectric objects in a broad frequency range. This new formulation is particularly suitable for modeling biological tissues at low frequencies, as it is required by brain epileptogenic area imaging, but also at higher ones, as it is required by several applications, including, but not limited to, deep brain stimulation (DBS). When modeling inhomogeneous objects with high complex permittivities at low frequencies, the traditional D-VIE is ill-conditioned and suffers from numerical instabilities that result in slower convergence and less accurate solutions. In this work, we address these shortcomings by leveraging a new set of volume quasi-Helmholtz projectors. Their scaling by the material permittivity matrix allows for the rebalancing of the equation when applied to inhomogeneous scatterers and, thereby, makes the proposed method accurate and stable even for high complex permittivity objects until arbitrarily low frequencies. Numerical results, canonical and realistic, corroborate the theory and confirm the stability and the accuracy of this new method both in the quasi-static regime and at higher frequencies

Radiotaajuisen säteilyn altistuksen arviointi kauneudenhoidon sovelluksissa

The aim of this Master’s Thesis was to assess the radiofrequency exposure of beauty care appliances and to be able to evaluate the safety of the devices according to the limits issued in the regulations for the exposure of electromagnetic fields. The treatments with radiofrequency beauty care appliances are usually associated with some degree of local tissue heating, thus the effects of excessive heating might cause some thermal damage in tissues. In the literature survey of this Thesis, the principles of radiofrequency (RF) radiation and its interaction mechanisms with biological tissue, the properties of human tissues, the structure and operation of RF beauty care appliances and different dosimetric assessment methods of radiofrequency radiation exposure are studied. To study the operation and output power of the RF beauty care appliances, a moveable power measurement set-up was developed. In this set-up the RF power, which connects to resistors representing human body and its impedance, was determined from the output signal with an oscilloscope. A model simulating a human forearm made of cylindrical container and tissue simulating liquid was fed with radiofrequency power of RF beauty care device under review. The temperature increase in the liquid was measured below the RF treatment electrode. An output power of the device, which was obtained from the temperature increase measurements, was used as an output power when assessing the exposure in numerical simulations with Finite Difference Time Domain (FDTD) method in homogeneous and heterogeneous human models. The numerical simulation model was successfully validated with the temperature increase measurements. The dosimetry of the RF exposure was based on simulations with heterogeneous model. The simulations showed that the distribution of the specific absorption rate (SAR) in the heterogeneous tissue model was really superficial, and maximum 10 g average SAR value might exceed the public exposure limit values. This value was determined to be 650 W/kg ± 38 % (k=2), meaning that when considering the public exposure limits, the treatment electrode can be held in one place for 1,1 seconds in head and trunk area and 2,2 seconds in limbs. The power measurement set-up can be used for getting more information on the appliances for surveillance use, but it still needs to be developed further to obtain more reliable estimations on the exposure of the device being measured