Biomedical subjects

P J Dimbylow

Publications and source records attributed to P J Dimbylow.

At least 19 recordsLinked to original sources

PubMed2006

Variations in calculated SAR with distance to the perfectly matched layer boundary for a human voxel model.

The purpose of this work was to investigate the dependence of whole-body averaged specific energy absorption rate (SAR), calculated using the finite-difference time-domain (FDTD) method, on the width of the free space region between a numerical phantom and the perfectly matched layer (pml) absorbing boundary. Results show that an increase in this width from 2 cells to 70 cells caused variations in the calculated whole-body averaged SAR of less than 2% for the FDTD code employing split-field pmls. Similarly, an increase in the width of the pml layer had little effect on the whole-body SAR values.

Biophysical Phenomena↗

PubMed2006

FDTD calculations of specific energy absorption rate in a seated voxel model of the human body from 10 MHz to 3 GHz.

Finite-difference time-domain (FDTD) calculations have been performed to investigate the frequency dependence of the specific energy absorption rate (SAR) in a seated voxel model of the human body. The seated model was derived from NORMAN (NORmalized MAN), an anatomically realistic voxel phantom in the standing posture with arms to the side. Exposure conditions included both vertically and horizontally polarized plane wave electric fields between 10 MHz and 3 GHz. The resolution of the voxel model was 4 mm for frequencies up to 360 MHz and 2 mm for calculations in the higher frequency range. The reduction in voxel size permitted the calculation of SAR at these higher frequencies using the FDTD method. SAR values have been calculated for the seated adult phantom and scaled versions representing 10-, 5- and 1-year-old children under isolated and grounded conditions. These scaled models do not exactly reproduce the dimensions and anatomy of children, but represent good geometric information for a seated child. Results show that, when the field is vertically polarized, the sitting position causes a second, smaller resonance condition not seen in resonance curves for the phantom in the standing posture. This occurs at approximately 130 MHz for the adult model when grounded. Partial-body SAR calculations indicate that the upper and lower regions of the body have their own resonant frequency at approximately 120 MHz and approximately 160 MHz, respectively, when the grounded adult model is orientated in the sitting position. These combine to produce this second resonance peak in the whole-body averaged SAR values calculated. Two resonance peaks also occur for the sitting posture when the incident electric field is horizontally polarized. For the adult model, the peaks in the whole-body averaged SAR occur at approximately 180 and approximately 600 MHz. These peaks are due to resonance in the arms and feet, respectively. Layer absorption plots and colour images of SAR in individual voxels show the specific regions in which the seated human body absorbs the incident field. External electric field values required to produce the ICNIRP basic restrictions were derived from SAR calculations and compared with ICNIRP reference levels. This comparison shows that the reference levels provide a conservative estimate of the ICNIRP whole-body averaged SAR restriction, with the exception of the region above 1.4 GHz for the scaled 1-year-old model.

Adult↗

PubMed2006

The calculation of SAR from limb current in the female voxel phantom, NAOMI.

Calculations of localised SAR in the legs and arms have been performed in a female voxel phantom, NAOMI. A finite difference method was used to solve the quasistatic potential equation from 0.1 to 80 MHz for a unit current injected into a limb. The relationship between limb current and SAR has been investigated. The values of localised SAR in the leg as a function of applied plane wave electric field are also predicted from the knowledge of limb currents calculated by the finite-difference time-domain (FDTD) method for whole-body exposure. Comparisons are made with the results of previous work for the normalised male model, NORMAN, and the implications for electromagnetic guidelines are discussed.

Adult↗

PubMed2005

Effects of posture on FDTD calculations of specific absorption rate in a voxel model of the human body.

A change in the posture of the human body can significantly affect the way in which it absorbs radiofrequency electromagnetic radiation. To study this, an anatomically realistic model of the body has been modified to develop new voxel models in postures other than the standard standing position with arms to the side. These postures were sitting, arms stretched out horizontally to the side and vertically above the head. Finite-difference time-domain (FDTD) calculations of the whole-body averaged specific energy absorption rate (SAR) have been performed from 10 MHz to 300 MHz at a resolution of 4 mm. Calculations show that the effect of a raised arm above the head posture was to increase the value of the whole-body averaged SAR at resonance by up to 35% when compared to the standard, arms by the side position. SAR values, both whole-body averaged and localized in the ankle, were used to derive the external electric field values required to produce the SAR basic restrictions of the ICNIRP guidelines. It was found that, in certain postures, external electric field reference levels alone would not provide a conservative estimate of localized SAR exposure and it would be necessary to invoke secondary reference levels on limb currents to provide compliance with restrictions.

Biophysics↗

PubMed2002

Fine resolution calculations of SAR in the human body for frequencies up to 3 GHz.

Finite-difference time-domain (FDTD) calculations of whole-body averaged specific energy absorption rate (SAR) have been performed from 100 MHz to 3 GHz at the basic 2 mm resolution of the voxel (volume pixel) model NORMAN without any rescaling to larger cell sizes. The reduction in the voxel size from previous work allows SAR to be calculated at higher frequencies. Additionally, the calculations have been extended down to 10 MHz, covering the whole-body resonance regions at a resolution of 4 mm. As well as for the adult phantom, SAR values are calculated for scaled versions representing 10-, 5- and 1-year-old children for both grounded and isolated conditions. External electric field levels are derived from limits of whole-body averaged SAR and localized SAR in the ankle, and compared with NRPB investigation levels and ICNIRP reference levels. The ICNIRP field reference levels alone would not provide a conservative estimate of the localized SAR exposure in the leg for grounded conditions. It would be necessary to invoke the secondary reference level on limb current to provide compliance with basic restrictions on localized SAR averaged over 10 g.

Absorption↗

PubMed2002

Modelling fields induced in humans by 50/60 Hz magnetic fields: reliability of the results and effects of model variations.

This paper presents a comparison of anatomically realistic human models and numerical codes in the dosimetry of power frequency magnetic fields. The groups at the University of Victoria and the National Radiological Protection Board have calculated the induced electric fields in both their 'UVic and 'NORMAN' models using independently developed codes. A detailed evaluation has been performed for a uniform magnetic field at 60 Hz. Comparisons of all dosimetric metrics computed in each particular model agree within 2% or less. Since in situ measurements cannot be performed in humans, and achievable accuracy of measurements in models and animals is not likely to be better than 10-15%, the comparisons presented should provide confidence limits on computational dosimetry. An evaluation of the effect of model size, shape and resolution has also been performed and further illuminated the reasons for differences in induced electric fields for various human body models.

Algorithms↗

PubMed2001

The relationship between localised SAR in the arm and wrist current.

Calculations are presented of the specific energy absorption rate, SAR, in the lower arm of the NRPB anatomically realistic voxel model. NORMAN, for induced currents from 100 kHz to 80 MHz. The wrist region has a narrow cross section and contains little high conductivity muscle, comprising mainly low conductivity bone, tendon and fat. Consequently there is a channelling of the current through the high conductivity muscle, which produces high, localised values of the SAR. Values averaged over 10 g and 100 g of tissue are calculated as a function of the current flowing through the wrist.

Absorption↗

PubMed2000

Current densities in a 2 mm resolution anatomically realistic model of the body induced by low frequency electric fields.

Current density distributions in a fine resolution (2 mm) anatomically realistic voxel model of the human body have been calculated for uniform, low frequency vertically aligned electric fields for a body grounded and isolated from 50 Hz to 10 MHz. The voxel phantom NORMAN is used which has a height of 1.76 m and a mass of 73 kg. There are 8.3 million voxels in the body differentiated into 37 tissue types. Both finite-difference potential and time-domain methods were used. Results are presented for the current density averaged over 1 cm2 in muscle, heart, brain and retina. Electric field values required to reach the NRPB and ICNIRP basic restrictions on current density are derived and compared with the external field guidelines from these standards.

Computer Simulation↗

PubMed1998

Induced current densities from low-frequency magnetic fields in a 2 mm resolution, anatomically realistic model of the body.

This paper presents calculations of current density in a fine-resolution (2 mm) anatomically realistic voxel model of the human body for uniform magnetic fields incident from the front, side and top of the body for frequencies from 50 Hz to 10 MHz. The voxel phantom, NORMAN, has a height of 1.76 m and a mass of 73 kg. There are 8.3 million voxels in the body differentiated into 37 tissue types. Both the impedance method and the scalar potential finite difference method were used to provide mutual corroboration. Results are presented for the current density averaged over 1 cm2 in muscle, heart, brain and retina.

Electromagnetic Fields↗

PubMed1997

FDTD calculations of the whole-body averaged SAR in an anatomically realistic voxel model of the human body from 1 MHz to 1 GHz.

This paper presents finite-difference time-domain (FDTD) calculations of the whole-body averaged SAR in an anatomically realistic voxel model of the human body. This model, NORMAN, consists of approximately 9 million voxels, of 2 mm dimension in the adult phantom, segmented into 37 tissue types. SAR values are presented for an adult phantom and for scaled 10, 5 and 1 year old models, grounded and isolated in air from 1 MHz to 1 GHz for plane wave exposure. External electric field values corresponding to a whole-body averaged SAR of 0.4 W kg-1 are also presented.

Absorption↗

PubMed1996

Computation of electromagnetic fields for high-frequency magnetic resonance imaging applications.

A numerical method is presented to compute electromagnetic fields inside a 2 mm high resolution, anatomically detailed model of a human head for high-frequency magnetic resonance imaging (MRI) applications. The method uses the biconjugate gradient algorithm in combination with the fast Fourier transform to solve a matrix equation resulting from the discretization of an integrodifferential equation representing the original physical problem. Given the current distribution in an MRI coil, the method can compute both the electric field (thus the specific energy absorption rate (SAR)) and the magnetic field, also known as the B1 field. Results for the SAR and B1 field distribution, excited by a linear and a quadrature birdcage coil, are calculated and presented at 64 MHz, 128 MHz and 256 MHz, corresponding to the operating frequencies of the 1.5 T, 3 T and 6 T MRI systems. It is shown that compared with that at 64 MHz, the SAR at 128 MHz is increased by a factor over 5 and the SAR at 256 MHz is increased by a factor over 10, assuming the same current strength in the coil. Furthermore, compared with the linear excitation, the average SAR for the quadrature excitation is reduced by a factor over 2 and the maximum SAR is reduced by a factor over 3. It is also shown that the B1 field at high frequencies exhibits a strong inhomogeneity, which is attributed to dielectric resonance.

Algorithms↗

PubMed1994

SAR calculations in an anatomically realistic model of the head for mobile communication transceivers at 900 MHz and 1.8 GHz.

A new mathematical model of the head has been constructed from a set of serial MRI slices from one subject. Finite-difference time-domain (FDTD) calculations of the specific energy absorption rate (SAR) have been performed on this model with a 2 mm resolution for a generic mobile communication transceiver represented by a quarter-wavelength monopole on a metal box. The antenna was mounted either at the centre or corner of the top face of the box. The frequencies considered were 900 MHz and 1.8 GHz. Three irradiation geometries were considered, a vertical handset in front of the eye and vertical and horizontal orientations at the side of the ear. The effect of a hand grasping the handset was considered. The head model was scaled to represent the head of an infant and a subset of calculations was performed to verify that the SAR deposited in the infant head did not exceed that in the adult. Results are also presented for a half-wavelength dipole. The maximum SAR values produced by the generic transceiver for the horizontal orientation at the side of the head which is the most typical position, averaged over 10 g of tissue at 900 MHz and 1.8 GHz, are 2.1 and 3.0 W kg(-1) per W of radiated power. The corresponding values over 1 g of tissue are 2.3 and 4.8 W kg(-1) per W at 900 MHz and 1.8 GHz. However, if one were to consider all possible operational conditions, the placement of the transceiver in front of the eye will give 3.1 and 4.6 W kg(-1) per W averaged over 10 g of tissue and 4.7 and 7.7 W kg(-1) per W over 1 g of tissue at 900 MHz and 1.8 GHz, respectively.

Adult↗

PubMed1993

FDTD calculations of the SAR for a dipole closely coupled to the head at 900 MHz and 1.9 GHz.

This paper presents finite-different time-domain (FDTD) calculations of the specific absorption rate (SAR) averaged over the mass of the eye, and over 1 g and 100 g of tissue in a realistic model of the head from a closely coupled half-wavelength dipole source. The SAR is predicted as a function of the separation between the vertical dipole and the surface of the eye. The feed point of the dipole is on the axis defined by the centre of the eye. Phantoms representing an adult and a one-year-old child have been considered for irradiation at 900 MHz and 1.9 GHz.

Absorption↗

PubMed1991

Finite-difference time-domain calculations of SAR in a realistic heterogeneous model of the head for plane-wave exposure from 600 MHz to 3 GHz.

This paper presents finite-difference time-domain (FDTD) calculations of the specific energy absorption rate (SAR) in a fine-scaled, heterogeneous, realistic model of the head for frequencies ranging from 600 MHz to 3 GHz. The phantom has been derived from an atlas of cross-sectional anatomy. The cell size is 3.2 mm which results in a 120,000 cell head model comprising brain, bone/fat, muscle, skin, blood, air and eye humour, lens and sclera. Irradiation from the front and side for plane-wave exposure of an adult and an infant are considered.

Adult↗

PubMed1991

Finite-difference time-domain calculations of absorbed power in the ankle for 10-100 MHz plane wave exposure.

This paper presents finite-difference time-domain (FDTD) calculations of the short-circuit current and the specific energy absorption rate (SAR) in the ankle. Plane wave exposures from 10 to 100 MHz for an adult, a 10-year old and 5-year old are considered. The calculations are performed in two parts. Firstly, the coupling between a homogeneous whole body phantom and the applied field is calculated. The region around a lower leg is then expanded, the leg being described by a realistic fine-scale, heterogeneous model. Tangential electric field components from the first part are used as boundary conditions on the reduced domain of the second part. Electric field values based on a maximum SAR of 20 W . kg-1 are presented.

Adult↗

PubMed1988

The calculation of induced currents and absorbed power in a realistic, heterogeneous model of the lower leg for applied electric fields from 60 Hz to 30 MHz.

The ankle consists mainly of bone and tendon with little muscle. Currents will tend to preferentially flow through the high-conductivity muscle and this can result in very high local values of the specific energy-absorption rate (SAR). This paper presents a finite-difference method to calculate SAR in a realistic, heterogeneous model of the leg below the knee. The structure of the leg is defined by cross-sectional slices from an anatomical atlas which are converted into a 3D model of over 14,000 cells. Four types of tissue are included in the model--muscle, cortical bone, trabecular bone and connective tissue. Displacement as well as ionic currents are considered in a complex potential representation. The current to be injected into the limb model is obtained from the computed coupling of an applied vertical electric field with a 1.8 m tall, homogeneous phantom. Values of the maximum current density and SAR in the ankle from 60 Hz to 30 MHz are presented. Field limits based on a maximum SAR of 20 W kg-1 averaged over 1 g of tissue are given. Sensitivity analyses with regard to the range of tissue electric properties and the ankle cross-sectional area are performed.

Ankle↗

PubMed1987

Finite difference calculations of current densities in a homogeneous model of a man exposed to extremely low frequency electric fields.

This paper presents three-dimensional finite difference calculations of induced current densities in a grounded, homogeneous, realistically human-shaped phantom. Comparison is made with published experimental values of current density at 60 Hz, measured in conducting saline manikins with their arms down by the side. The congruence between calculation and experiment gives confidence in the applicability of the numerical method and phantom shape to other configurations. The effect of raising both arms above the head is to reduce the current densities in the head and neck by approximately 50% and to increase those from the thorax downwards by 20-30%. A sensitivity analysis was performed on the shape and dimensions of the phantom, from a 45-kg, 1.5-m-tall person to a 140-kg, 1.9-m-tall person. When the phantom is grounded through both feet the current densities range from 50 to 90 microAm-2 in the head (all values for a 60-Hz, 1-kVm-1, vertical applied field), 70 to 140 microAm-2 in the thorax, 150 to 440 microAm-2 at the crotch, and 500 to 2,230 microAm-2 in the ankle. When grounded through only one foot the current densities at the crotch range from 400 to 1,000 microAm-2 and from 1,000 to 4,400 microAm-2 in the ankle of the grounded leg. Scale transformations of the short-circuit current with phantom height, weight, and surface area are confirmed.

Animals↗

PubMed1985

Radon diffusion modelling.

A mathematical model has been developed that examines the ingress of radon into houses, through a vertical crack in an otherwise impervious concrete floor. Initially, the model considered the diffusive flow of radon from its soil source and this simulation has highlighted the dependency of the flux of radon into the house on the magnitude of various parameters, such as the diffusion coefficient of radon in soil. A preliminary investigation of the modelling of pressure-driven flow into a building is presented, and the potential of this type of analysis is discussed.

Diffusion↗