Gregory M. Noetscher

New VHP-Female v. 2.0 full-body computational phantom and its performance metrics using FEM simulator ANSYS HFSS

Simulation of the electromagnetic response of the human body relies heavily upon efficient computational models or phantoms. The first objective of this paper is to present a new platform-independent full-body electromagnetic computational model (computational phantom), the Visible Human Project® (VHP)-Female v. 2.0 and to describe its distinct features. The second objective is to report phantom simulation performance metrics using the commercial FEM electromagnetic solver ANSYS HFSS.

Continuous Wave Simulations on the Propagation of Electromagnetic Fields Through the Human Head

Characterizing the human head as a propagation medium is vital for the design of both on-body and implanted antennas and radio-frequency sensors. The following problem has been addressed: find the best radio-frequency path through the brain for a given receiver position - on the top of the sinus cavity. Two parameters, transmitter position and radiating frequency, should be optimized simultaneously such that 1) the propagation path through the brain is the longest; and 2) the received power is maximized. To solve this problem, we have performed a systematic and comprehensive study of the electromagnetic fields excited in the head by small on-body magnetic dipoles (small coil antennas). An anatomically accurate high-fidelity head mesh has been generated from the Visible Human Project data. The base radiator was constructed of two orthogonal magnetic dipoles in quadrature, which enables us to create a directive beam into the head. We have found at least one optimum solution. This solution implies that a distinct RF channel may be established in the brain at a certain frequency and transmitter location.

Directional in-quadrature orthogonal-coil antenna and an array thereof for localization within a human body in a Fresnel region

Body Area Networks (BANs) at a center frequency of 400MHz are not subject to the quasi-static limit, σ ≫ εω. This makes it possible to capitalize on established theoretical results for small antennas. Though the bulk of the body volume at 400MHz is still within the Fresnel region, major features of the classic far-field solutions may be readily applicable to BANs. In this study, we introduce the concept of a small directional receiving/transmitting antenna comprised of two small orthogonal coils (magnetic dipoles) driven/acquired in quadrature to create a highly-directional single-lobe beam into the body. An array of such radiators operating in the Fresnel region makes it possible to develop a simple yet effective localization algorithm within the body using RSS (Received Signal Strength) estimates for each individual element and superior to a receiving array of single-coil antennas.

Modeling Accuracy and Features of Body-Area Networks with Out-of-Body Antennas at 402 MHz

Voltage and power transfer functions for the path loss of a 402 MHz body-area network are reviewed. The corresponding expressions are valid in both the near- and far-fields of a transmitting antenna. It was shown that basic FDTD simulations for a homogeneous human-body model, implemented in MATLAB®, agreed quite well with the advanced FEM solver for an inhomogeneous accurate human-body model. Both methods modeled a voltage transfer function (path loss) for out-of-body dipole antennas at 402 MHz at different antenna locations, as close to the body as 15 mm. The reason for the good agreement was that the propagation path was mostly determined by a diffraction of the electromagnetic signal around the body, and not by propagation through the (inhomogeneous) body. Such an observation made it possible to use various homogeneous body meshes in order to study the effect of different body types and positions for out-of-body antennas. A method of creating such meshes using a three-dimensional body scanner is described. For a number of different white-male body meshes, the magnitudes of the received voltages matched exceptionally well when the antenna positions were measured from the top of the head.

VHP-Female full-body human CAD model for cross-platform FEM simulations — Recent development and validations

Simulation of the electromagnetic response of the human body relies heavily upon efficient computational models or phantoms. The first objective of this paper is to present an improved platform-independent full-body electromagnetic computational model (computational phantom), the Visible Human Project® (VHP)-Female v. 3.1 and to describe its distinct features and enhancements compared to VHP-Female v. 2.0. The second objective is to report phantom simulation for electric stimulation studies using the commercial FEM electromagnetic solver ANSYS MAXWELL.

Detecting in vivo changes of electrical properties of Cerebral Spinal Fluid using microwave signals from small coil antennas - numerical simulation

Dielectric properties of Cerebral Spinal Fluid (CSF) at microwave frequencies correlate with a higher level of glucose or protein observed at certain diseases, including early stages of Alzheimers disease. In this study, a simulation of in vivo monitoring dielectric properties of Cerebral Spinal Fluid (CSF) using small antennas precisely positioned around the human head is made. We use a realistic mesh model of the head and head organs obtained by fine segmentation of the Visible Human Project® data. An accurate source model with coincident phase centers is employed in the underlying Finite Difference Time-Domain (FDTD) algorithm. Simulation result strongly depends on the type and positioning of small antennas around the head, and on the mechanical accuracy. Once these factors are properly optimized, the changes in the relative dielectric constant on the order of 10-15% may be recorded using the phase shift of a second received pulse.

Comparative analysis of different hip implants within a realistic human model located inside a 1.5T MRI whole body RF coil.

Temperature rise in surrounding tissues of a large orthopedic metallic implant subject to MRI is a significant point of concern today. Numerical electromagnetic and thermal modeling offers a way to model this complex problem with a sufficient degree of accuracy. We developed a workflow for realistic implant modeling, which includes an MRI coil, a multi-tissue human model, and accurately registered hip implants. We also obtained differences in the power loss density rises generated due to the presence of three hip implants placed in a phantom or a realistic human model, located inside a 1.5 T coil.

A Simple Absolute Estimate of Peak Eddy Currents Induced by Transcranial Magnetic Stimulation Using the GR Model

Although rigorous numerical modeling is a major and very reliable tool of transcranial magnetic stimulation (TMS) analyses, it requires significant computational resources and time to complete. Therefore, in certain cases, a quick analytical estimate for the peak TMS currents induced by the procedure would be very useful. For instance, maximum exposure due to currents induced in an operators body, or eddy currents present in ancillary areas of the patient (e.g., the fetus of a pregnant woman) may be of interest. In such cases, a general simplified analytical solution that provides at least an upper absolute estimate of spatial eddy current distribution may be of value. In the present study, we suggest using an early model of Grandori and Ravazzani (GR model) related to an unbounded homogeneous space for that purpose. We formulate the model, implement it in software, and validate the model by comparison with a known numerical FEM solution. Finally, we apply the model in order to establish an upper absolute estimate for TMS eddy currents excited in a human body at various conditions.

Computational human model VHP-FEMALE derived from datasets of the national library of medicine

Simulation of the electromagnetic response of the human body relies upon efficient computational models. The objective of this paper is to describe a new platform-independent and computationally-efficient full-body electromagnetic model, the Visible Human Project® (VHP)-Female v.3.0 and to outline its distinct features. We also report model performance results using two leading commercial electromagnetic antenna simulation packages: ANSYS HFSS and CST MICROWAVE STUDIO®.

Accuracy of point source models with coincident phase centers in a cubic FDTD grid for arbitrary source orientation

Standard point source modeling in the Finite Difference Time Domain (FDTD) method has been accomplished by centering the source at the appropriate electric or magnetic field node in a given Yee cell. While this methodology is adequate for many applications, it produces a source whose Cartesian oriented field components have slightly different phase centers, which yields a visible propagation delay between these components. For phase sensitive applications, more accurate consideration of the source phase center may be required. In this study, we introduce a means of producing point sources with coincident phase centers. We will show that these sources provide good accuracy at close distances to the phase center while outperforming the standard sources in the near field when an arbitrary direction of the point source moment is required.