RESUMEN
This paper sets out to explore the effects of voxel resolution, from 2 mm down to 0.1 mm for Cartesian co-ordinates and the differences between Cartesian and spherical polar co-ordinates for a standardized test-bed model of the eye. This model was taken from the work of Yoriyaz et al (2005 Radiat. Prot. Dosim. 115 316-9) who have developed a detailed geometric description of the eye including choroid, retina, sclera, lens, cornea, anterior chamber, vitreous humour and optic nerve for ophthalmic brachytherapy. The spherical co-ordinate model has radial and angular steplengths of 0.1 mm and 0.25°, respectively. The current density averaged over 1 cm(2) and the 99th percentile value of the induced electric field have been calculated in the retina and central nervous system for uniform magnetic fields. The Cartesian co-ordinate calculations proceed in a sequence of grids at 2, 1, 0.5, 0.2 and 0.1 mm resolution with the potentials from the previous calculation at a coarser grid providing the boundary conditions on the finer grid. The 0.2 mm grid provides the boundary conditions for the spherical polar calculations. Comparisons are made with the International Commission on Non-Ionizing Radiation Protection reference levels.
Asunto(s)
Cabeza/anatomía & histología , Magnetismo , Modelos Anatómicos , Retina/anatomía & histología , Conductividad EléctricaRESUMEN
This paper presents finite-difference time-domain (FDTD) calculations of SAR in the University of Florida newborn female model. The newborn model is based upon a surface representation of the organs of the body, using non-uniform rational B-spline surfaces (NURBS). The surface model can then be converted into voxels at any resolution required. This flexibility allows the preparation of voxel models at 2, 1 and 0.5 mm to investigate the effect of resolution on dispersion and the choice of algorithms to calculate SAR in the Yee cell as the frequency increases up to 6 GHz. The added advantage of the newborn model is that it is relatively small and so FDTD calculations can be made tractable at a very fine resolution of 0.5 mm. A comparison is made between the calculated external electric fields required to produce the basic restriction on whole-body-averaged SAR and the ICNIRP reference levels for public exposure. At 250 MHz, the whole body resonance, the ICNIRP reference level does not provide a conservative estimate of the whole-body-averaged SAR restriction. The reference level is also breached in the range 700-2450 MHz by all of the irradiation geometries considered.
Asunto(s)
Fantasmas de Imagen , Dosis de Radiación , Radiometría/instrumentación , Universidades , Algoritmos , Impedancia Eléctrica , Electricidad , Femenino , Florida , Humanos , Recién Nacido , Propiedades de SuperficieRESUMEN
This paper presents calculations of the induced current density in the body at 50 Hz from applied electric and magnetic fields. An extensive ensemble of 25 voxel models has been used to investigate the effects of body posture, anatomy, age and pregnancy. This set includes six adult models, eight child models and seven pregnant female models at various stages of gestation. The four postures investigated in the HPA adult model, NORMAN, were the standard position with the arms at the side, with the arms vertically above the head, the arms horizontally to the side and sitting.
Asunto(s)
Envejecimiento/fisiología , Tamaño Corporal/fisiología , Electricidad , Modelos Biológicos , Postura/fisiología , Embarazo/fisiología , Recuento Corporal Total/métodos , Adulto , Carga Corporal (Radioterapia) , Niño , Simulación por Computador , Campos Electromagnéticos , Femenino , Humanos , Masculino , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Adulto JovenRESUMEN
This paper compares the foetal SAR in the HPA hybrid mathematical phantoms with the 26-week foetal model developed at the National Institute of Information and Communications Technology, Tokyo, and the set of 13-, 26- and 38-week boundary representation models produced at Rensselaer Polytechnic Institute. FDTD calculations are performed at a resolution of 2 mm for a plane wave with a vertically aligned electric field incident upon the body from the front, back and two sides from 20 MHz to 3 GHz under isolated conditions. The external electric field values required to produce the ICNIRP public exposure localized restriction of 2 W kg(-1) when averaged over 10 g of the foetus are compared with the ICNIRP reference levels.
Asunto(s)
Feto/efectos de la radiación , Modelos Biológicos , Absorción , Femenino , Feto/fisiología , Edad Gestacional , Humanos , Agencias Internacionales/normas , Madres , Fantasmas de Imagen , Embarazo , Protección Radiológica/normas , Valores de Referencia , TemperaturaRESUMEN
This paper provides an intercomparison of the HPA male and female models, NORMAN and NAOMI with the National Institute of Information and Communications Technology (NICT) male and female models, TARO and HANAKO. The calculations of the whole-body SAR in these four phantoms were performed at the HPA, at NICT and at the Nagoya Institute of Technology (NIT). These were for a plane wave with a vertically aligned electric field incident upon the front of the body from 30 MHz to 3 GHz for isolated conditions. As well as investigating the general differences through this frequency range, particular emphasis was placed on the assumptions of how dielectric properties are assigned to tissues (particularly skin and fat) and the consequence of using different algorithms for calculating SAR at the higher frequencies.
Asunto(s)
Carga Corporal (Radioterapia) , Modelos Biológicos , Programas Informáticos , Recuento Corporal Total/métodos , Simulación por Computador , Europa (Continente) , Japón , Dosis de Radiación , Efectividad Biológica Relativa , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Validación de Programas de Computación , Recuento Corporal Total/normasRESUMEN
This paper identifies uncertainties and problems in the practical application of the ICNIRP low frequency basic restriction on current density. This quantity should be averaged over a cross-section of 1 cm(2) perpendicular to the current direction. The rationale and the sensitivity of the current density average are investigated. There are difficulties in finding a square centimetre of spinal cord over which to average. The consequences of including neighbouring tissues in the averages are investigated in the male and female voxel models NORMAN and NAOMI for applied uniform electric and magnetic fields at 50 Hz. Also the case of the non-uniform magnetic field from a horizontal current carrying conductor adjacent to the back of the body is investigated. The maximum and 99th percentile current density values are compared with the 1 cm(2) average in the derivation of external field reference levels. For more information on this article, see medicalphysicsweb.org.
Asunto(s)
Protección Radiológica/normas , Adulto , Algoritmos , Fenómenos Biofísicos , Biofisica , Campos Electromagnéticos/efectos adversos , Femenino , Física Sanitaria , Humanos , Agencias Internacionales , Masculino , Modelos Biológicos , Protección Radiológica/estadística & datos numéricos , Sensibilidad y Especificidad , Médula Espinal/anatomía & histología , Médula Espinal/efectos de la radiaciónRESUMEN
The University of Florida (UF) Series B paediatric phantoms were developed for medical and radiation protection photon dosimetry. The series includes a 9 month male, a 4 year female, an 8 year female, an 11 year male and a 14 year male. In this paper they have been adapted to calculate the whole-body-averaged specific energy absorption rate (SAR) in children for plane wave exposure from 50 MHz to 4 GHz. The consideration of children is important in the application of the ICNIRP public exposure reference levels above approximately 1 GHz. The uniformly scaled models of NORMAN and NAOMI suggest that the ICNIRP reference level does not provide a conservative estimate of the whole-body-averaged SAR restriction for 5 year and 1 year old models. Comparison is made with the previous linearly scaled versions of NORMAN and NAOMI for calculations at 2 mm resolution. Further FDTD calculations were performed at resolutions of 1 and 0.7 mm above 900 MHz to elucidate the effects of grid resolution on SAR. A comparison is made between the calculated external electric fields required to produce the basic restriction on the whole-body-averaged SAR and the ICNIRP reference levels for public exposure.
Asunto(s)
Fantasmas de Imagen , Ondas de Radio , Absorción , Niño , Preescolar , Femenino , Humanos , Lactante , Masculino , Recuento Corporal Total/métodosRESUMEN
This paper describes the finite-difference time-domain calculation of SAR from 20 MHz to 3 GHz in hybrid voxel-mathematical models of the pregnant female. Mathematical models of the developing foetus at 8-, 13-, 26- and 38-week gestation were converted into voxels and combined with the reference adult female model, NAOMI at a resolution of 2 mm. Whole-body averaged SAR in the mother is presented as well as the average over the foetus, over the foetal brain and in 10 g of the foetus. The electric field values required to produce the ICNIRP public exposure restriction of 2 W kg(-1) when averaged over 10 g of the foetus were calculated. Comparison suggests that the ICNIRP public reference level is a conservative predictor of local SAR in the foetus.
Asunto(s)
Feto/efectos de la radiación , Ondas de Radio , Encéfalo/metabolismo , Simulación por Computador , Campos Electromagnéticos , Femenino , Edad Gestacional , Humanos , Modelos Teóricos , Fantasmas de Imagen , Embarazo , Dosis de Radiación , Radiometría , Factores de Tiempo , Imagen de Cuerpo EnteroRESUMEN
This paper describes the development of 2 mm resolution hybrid voxel-mathematical models of the pregnant female. Mathematical models of the developing foetus at 8-, 13-, 26- and 38-weeks of gestation were converted into voxels and combined with the adult female model, NAOMI. This set of models was used to calculate induced current densities and electric fields in the foetus from applied 50 Hz magnetic and electric fields. The influence of foetal tissue conductivities was investigated and implications for electromagnetic field guidelines discussed.
Asunto(s)
Electricidad , Campos Electromagnéticos , Feto/fisiología , Modelos Biológicos , Embarazo/fisiología , Protección Radiológica/métodos , Radiometría/métodos , Carga Corporal (Radioterapia) , Simulación por Computador , Femenino , Humanos , Imagenología Tridimensional/métodos , Especificidad de Órganos , Dosis de Radiación , Efectividad Biológica Relativa , Dispersión de RadiaciónRESUMEN
Finite-difference time-domain (FDTD) calculations have been performed of the whole-body averaged specific energy absorption rate (SAR) in a female voxel model, NAOMI, under isolated and grounded conditions from 10 MHz to 3 GHz. The 2 mm resolution voxel model, NAOMI, was scaled to a height of 1.63 m and a mass of 60 kg, the dimensions of the ICRP reference adult female. Comparison was made with SAR values from a reference male voxel model, NORMAN. A broad SAR resonance in the NAOMI values was found around 900 MHz and a resulting enhancement, up to 25%, over the values for the male voxel model, NORMAN. This latter result confirmed previously reported higher values in a female model. The effect of differences in anatomy was investigated by comparing values for 10-, 5- and 1-year-old phantoms rescaled to the ICRP reference values of height and mass which are the same for both sexes. The broad resonance in the NAOMI child values around 1 GHz is still a strong feature. A comparison has been made with ICNIRP guidelines. The ICNIRP occupational reference level provides a conservative estimate of the whole-body averaged SAR restriction. The linear scaling of the adult phantom using different factors in longitudinal and transverse directions, in order to match the ICRP stature and weight, does not exactly reproduce the anatomy of children. However, for public exposure the calculations with scaled child models indicate that the ICNIRP reference level may not provide a conservative estimate of the whole-body averaged SAR restriction, above 1.2 GHz for scaled 5- and 1-year-old female models, although any underestimate is by less than 20%.
Asunto(s)
Algoritmos , Campos Electromagnéticos , Modelos Biológicos , Protección Radiológica/métodos , Radiometría/métodos , Medición de Riesgo/métodos , Recuento Corporal Total/métodos , Absorción , Adulto , Factores de Edad , Carga Corporal (Radioterapia) , Niño , Preescolar , Simulación por Computador , Femenino , Humanos , Lactante , Transferencia Lineal de Energía/fisiología , Masculino , Dosis de Radiación , Efectividad Biológica Relativa , Reproducibilidad de los Resultados , Factores de Riesgo , Sensibilidad y EspecificidadRESUMEN
This paper outlines the development of a 2 mm resolution voxel model, NAOMI (aNAtOMIcal model), designed to be representative of the average adult female. The primary medical imaging data were derived from a high-resolution MRI scan of a 1.65 m tall, 23 year old female subject with a mass of 58 kg. The model was rescaled to a height of 1.63 m and a mass of 60 kg, the dimensions of the International Commission on Radiological Protection reference adult female. There are 41 tissue types in the model. The application of NAOMI to the calculations of induced current densities and electric fields from applied low frequency magnetic and electric fields is described. Comparisons are made with values from the male voxel model, NORMAN. The calculations were extended from 50 Hz up to 10 MHz. External field reference levels are compared with the ICNIRP guidelines.
Asunto(s)
Campos Electromagnéticos , Imagen por Resonancia Magnética/métodos , Modelos Biológicos , Radiometría/métodos , Radiometría/normas , Recuento Corporal Total/métodos , Recuento Corporal Total/normas , Adulto , Carga Corporal (Radioterapia) , Simulación por Computador , Femenino , Humanos , Internacionalidad , Imagen por Resonancia Magnética/normas , Dosis de Radiación , Estándares de Referencia , Efectividad Biológica RelativaRESUMEN
Finite-difference time-domain (FDTD) calculations of the specific energy absorption rate (SAR) from a representative TETRA handset have been performed in an anatomically realistic model of the head. TETRA (Terrestrial Trunked Radio) is a modern digital private mobile radio system designed to meet the requirements of professional users, such as the police and fire brigade. The current frequency allocations in the UK are 380-385 MHz and 390-395 MHz for the public sector network. A comprehensive set of calculations of SAR in the head was performed for positions of the handset in front of the face and at both sides of the head. The representative TETRA handset considered. operating at 1 W in normal use, will show compliance with both the ICNIRP occupational and public exposure restrictions. The handset with a monopole antenna operating at 3 W in normal use will show compliance with both the ICNIRP occupational and public exposure restrictions. The handset with a helical antenna operating at 3 W in normal use will show compliance with the ICNIRP occupational exposure restriction but will be over the public exposure restriction by up to approximately 50% if kept in the position of maximum SAR for 6 min continuously.