Jianxiang Shen

The Virtual Family—development of surface-based anatomical models of two adults and two children for dosimetric simulations

The objective of this study was to develop anatomically correct whole body human models of an adult male (34 years old), an adult female (26 years old) and two children (an 11-year-old girl and a six-year-old boy) for the optimized evaluation of electromagnetic exposure. These four models are referred to as the Virtual Family. They are based on high resolution magnetic resonance (MR) images of healthy volunteers. More than 80 different tissue types were distinguished during the segmentation. To improve the accuracy and the effectiveness of the segmentation, a novel semi-automated tool was used to analyze and segment the data. All tissues and organs were reconstructed as three-dimensional (3D) unstructured triangulated surface objects, yielding high precision images of individual features of the body. This greatly enhances the meshing flexibility and the accuracy with respect to thin tissue layers and small organs in comparison with the traditional voxel-based representation of anatomical models. Conformal computational techniques were also applied. The techniques and tools developed in this study can be used to more effectively develop future models and further improve the accuracy of the models for various applications. For research purposes, the four models are provided for free to the scientific community.

Numerical investigations of MRI RF field induced heating for external fixation devices

BackgroundThe magnetic resonance imaging (MRI) radio frequency (RF) field induced heating on external fixation devices can be very high in the vicinity of device screws. Such induced RF heating is related to device constructs, device placements, as well as the device insertion depth into human subjects. In this study, computational modeling is performed to determine factors associated with such induced heating.MethodsNumerical modeling, based on the finite-difference time-domain (FDTD) method, is used to evaluate the temperature rises near external device screw tips inside the ASTM phantom for both 1.5-T and 3-T MRI systems. The modeling approach consists of 1) the development of RF coils for 1.5-T and 3-T, 2) the electromagnetic simulations of energy deposition near the screw tips of external fixation devices, and 3) the thermal simulations of temperature rises near the tips of these devices.ResultsIt is found that changing insertion depth and screw spacing could largely affect the heating of these devices. In 1.5-T MRI system, smaller insertion depth and larger pin spacing will lead to higher temperature rise. However, for 3-T MRI system, the relation is not very clear when insertion depth is larger than 5 cm or when pin spacing became larger than 20 cm. The effect of connection bar material on device heating is also studied and the heating mechanism of the device is analysed.ConclusionsNumerical simulation is used to study RF heating for external fixation devices in both 1.5-T and 3-T MRI coils. Typically, shallower insertion depth and larger pin spacing with conductive bar lead to higher RF heating. The heating mechanism is explained using induced current along the device and power decay inside ASTM phantom.

Prediction of Effective Permittivity of Diphasic Dielectrics as a Function of Frequency

An analytical model based on an equivalent impedance circuit for effective permittivity of a composite dielectric as a function of frequency with complex-shaped inclusions is presented. The geometry of the capacitor containing this composite dielectric is discretized into partial impedance elements, the total equivalent impedance is calculated, and the effective permittivity of the composite dielectric is obtained from this equivalent impedance. An example application using this method is given for an individual cell of a diphasic dielectric consisting of a high-permittivity spherical inclusion enclosed in a low-permittivity parallelepiped. The capacitance and resistance for individual discretized elements in the composite cell are modeled as a function of an inclusion radius. The proposed approach is then extended to a periodic three-dimensional structure comprised of multiple individual cells. The equivalent impedance model is valid for both static and alternating applied electric fields, over the entire range of volume fraction of inclusions. The equivalent impedance model has a few advantages over existing effective medium theories, including no limitations on the shape of inclusions or their separation distance.

Computational study of external fixation devices surface heating in MRI RF environment

Heating effect by external fixation devices under MRI RF field was studied numerically for both 1.5-T and 3-T MRI systems. It is found that changing insertion depth and pin spacing could largely affect the surface heating level. In 1.5-T MRI, smaller insertion depth and larger pin spacing will produce larger temperature rise. However, for 3T system, the relation is not very clear when insertion depth became larger than 5cm or when pin spacing became larger than 20cm. Effect of connection bar material on external fixator is also studied and the heating mechanism of the device is analysed.

Analysis of via impedance variations with a Polynomial Chaos method

In this paper, we propose a systematic framework for the optimization and analysis of the equivalent characteristic impedance of practical via structures. The framework consists of (a) optimizing via structures for impedance matching using a Genetic algorithm, and (b) numerically characterize, by Polynomial Chaos (PC) method, the sensitivity of the equivalent characteristic impedance to the manufacturing uncertainties in the various geometrical parameters of a via structure. The PC method can be effectively used to compute important statistical information, such as moments, probabilities and sensitivities with respect to the design variables. The PC method is straightforward to implement, and can be orders of magnitude faster than the traditional Monte Carlo (MC) method. The proposed framework naturally leads to a rigorous methodology for EM design/control in the presence of multiple sources of uncertainty.

An efficient polynomial chaos method for uncertainty quantification in electromagnetic simulations

Realistic EM simulations involve input parameters that have variability associated with them can have substantial impact on the simulation outputs; thus the capability of quantifying these impacts becomes of great importance. In the stochastic modeling of most EM problem, computational efficiency is a concern in the selection of the method because of the intensive nature of electromagnetic simulations. In addition to the efficiency, a certain level of accuracy is also desired. This paper will address these requirements by introducing an inexpensive stochastic method; it is used to statistically characterize the effective permittivity of composite structures.

Electromagnetic compatibility issues between vehicular mounted antennas and implantable medical devices

Numerical investigations are performed to investigate the interactions between vehicular mounted antennas and an implantable medical device in the vicinity of the vehicular. From the studies, it is shown that higher electromagnetic energy deposition was found in the vicinity of the devices. In addition, it is observed that for higher frequency, emission from vehicular antenna will have stronger interference on implantable device.

Design of composite materials using a genetic algorithm

The ability to design dielectric composite material exhibiting maximum loss at a specific frequency is of interest in telecommunications and radar absorption. In this paper, an optimization procedure based on genetic algorithm is presented for the design of composite material with maximum loss at multiple frequencies using multiphase and multilayer of spherical filler particles. This approach can also be used to design composite materials having approximately constant maximum loss in a wideband.

Design and characterization of composite materials for EMC applications

In this paper, an optimization procedure is proposed to design the maximum loss for composite material at multiple frequencies using multiphase spherical particles. This approach is based on a genetic algorithm together with the Maxwell-Garnett formulas. Once initial designs are completed, more accurate finite-difference method will be used to evaluate the designs. This approach can also be used to design composite materials having approximately constant maximum loss in a frequency band.

Stochastic analysis of electromagnetic properties variations for composite mixtures

A sparse grid technique is used to accelerate the evaluation of the effects on inclusion electrical parameter variations on the overall mixture homogenized electrical properties. This technique can significantly reduce the CPU time and yet maintain the accuracy of the stochastic analysis.