Antennas implanted in a human body are largely applicable to hyperthermia and biotelemetry. To make practical use of antennas inside a human body, resonance characteristics of the implanted antennas and their radiation signature outside the body must be evaluated through numerical analysis and measurement setup. Most importantly, the antenna must be designed with an in-depth consideration given to its surrounding environment. In this paper, the spherical dyadic Green's function (DGF) expansions and finite-difference time-domain (FDTD) code are applied to analyze the electromagnetic characteristics of dipole antennas and low-profile patch antennas implanted in the human head and body. All studies to characterize and design the implanted antennas are performed at the biomedical frequency band of 402-405 MHz. By comparing the results from two numerical methodologies, the accuracy of the spherical DGF application for a dipole antenna at the center of the head is evaluated. We also consider how much impact a shoulder has on the performance of the dipole inside the head using FDTD. For the ease of the design of implanted low-profile antennas, simplified planar geometries based on a real human body are proposed. Two types of low-profile antennas, i.e., a spiral microstrip antenna and a planar inverted-F antenna, with superstrate dielectric layers are initially designed for medical devices implanted in the chest of the human body using FDTD simulations. The radiation performances of the designed low-profile antennas are estimated in terms of radiation patterns, radiation efficiency, and specific absorption rate. Maximum available power calculated to characterize the performance of a communication link between the designed antennas and an exterior antenna show how sensitive receivers are required to build a reliable telemetry link.

Implanted antennas inside a human body: simulations, designs, and characterizations

Purpose We develop a realistic and flexible 4-D digital mouse phantom and investigate its usefulness in molecular imaging research. Methods Organ shapes were modeled with non-uniform rational B-spline (NURBS) surfaces based on high-resolution 3-D magnetic resonance microscopy (MRM) data. Cardiac and respiratory motions were modeled based on gated magnetic resonance imaging (MRI) data obtained from normal mice. Pilot simulation studies in single-photon emission computed tomography (SPECT) and X-ray computed tomography (CT) were performed to demonstrate the utility of the phantom. Results NURBS are an efficient and flexible way to accurately model the anatomy and cardiac and respiratory motions for a realistic 4-D digital mouse phantom. The phantom is capable of producing realistic molecular imaging data from which imaging devices and techniques can be evaluated. Conclusion The phantom provides a unique and useful tool in molecular imaging research. It can be used in the development of new imaging instrumentation, image acquisition strategies, and image processing and reconstruction methods.

Development of a 4-D digital mouse phantom for molecular imaging research.

The authors develop a unique CT simulation tool based on the 4D extended cardiac-torso (XCAT) phantom, a whole-body computer model of the human anatomy and physiology based on NURBS surfaces Unlike current phantoms in CT based on simple mathematical primitives, the 4D XCAT provides an accurate representation of the complex human anatomy and has the advantage, due to its design, that its organ shapes can be changed to realistically model anatomical variations and patient motion A disadvantage to the NURBS basis of the XCAT, however, is that the mathematical complexity of the surfaces makes the calculation of line integrals through the phantom difficult They have to be calculated using iterative procedures; therefore, the calculation of CT projections is much slower than for simpler mathematical phantoms To overcome this limitation, the authors used efficient ray tracing techniques from computer graphics, to develop a fast analytic projection algorithm to accurately calculate CT projections directly from the surface definition of the XCAT phantom given parameters defining the CT scanner and geometry Using this tool, realistic high-resolution 3D and 4D projection images can be simulated and reconstructed from the XCAT within a reasonable amount of time In comparison with other simulators with geometrically defined organs, the XCAT-based algorithm was found to be only three times slower in generating a projection data set of the same anatomical structures using a single 32 GHz processor To overcome this decrease in speed would, therefore, only require running the projection algorithm in parallel over three processors With the ever decreasing cost of computers and the rise of faster processors and multi-processor systems and clusters, this slowdown is basically inconsequential, especially given the vast improvement the XCAT offers in terms of realism and the ability to generate 3D and 4D data from anatomically diverse patients As such, the authors conclude that the efficient XCAT-based CT simulator developed in this work will have applications in a broad range of CT imaging research

/pdf/realistic-ct-simulation-using-the-4d-xcat-phantom-1yin3pt6o7.pdf

Realistic CT simulation using the 4D XCAT phantom

The antenna radiation pattern and other characteristics are significantly altered by the presence of the human body. This interaction as well as the resultant deposition of microwave power in the body (specific absorption rate-SAR) are of particular interest for cellular telephones and similar communication devices. This paper builds on and extends the previous analyses of parameters that influence the antenna-user interaction. Computer tomography (CT) and magnetic resonance imaging (MRI)-derived, high-resolution models of the human head are used. The numerical analysis is performed with the finite-difference time-domain (FDTD) method. The specific findings are: 1) a box model of a human head provides grossly distorted and unreliable results for the antenna radiation pattern; 2) a spherical model of the human head provides results that are relatively close to those obtained with a relatively simple, but more realistic, head model; 3) the SAR values obtained with spherical or simplified head models, that do not include the ear, are greater than those for a realistic head model that includes the ear; and 4) a hand holding the handset absorbs significant amount of antenna output power, which can be considerably decreased by modifying the geometry of the handset metal box.

A study of the handset antenna and human body interaction

Reliable attenuation correction methods for quantitative emission CT (ECT) require accurate delineation of the body contour and often necessitate knowledge of internal anatomic structure. Two broad classes of methods have been used to calculate the attenuation map: transmission-less and transmission-based attenuation correction techniques. Whereas calculated attenuation correction belonging to the first class of methods is appropriate for brain studies, more adequate methods must be performed in clinical applications, where the attenuation coefficient distribution is not known a priori, and for areas of inhomogeneous attenuation such as the chest. Measured attenuation correction overcomes this problem and uses different approaches to determine this map, including transmission scanning, segmented magnetic resonance images, or appropriately scaled CT scans acquired either independently on separate or simultaneously on multimodality imaging systems. Combination of data acquired from different imagers suffers from the usual problems of working with multimodality images--namely, accurate co-registration from the different modalities and assignment of attenuation coefficients. A current trend in ECT is to use transmission scanning to reconstruct the attenuation map. Combined ECT/CT imaging is an interesting approach; however, it considerably complicates both the scanner design and the data acquisition and processing protocols. Moreover, the cost of such systems may be prohibitive for small nuclear medicine departments. A dramatic simplification could be made if the attenuation map could be obtained directly from the emission projections, without the use of a transmission scan. This is being investigated either using a statistical model of emission data or applying the consistency conditions that allow one to identify the operator of the problem and, thus, to reconstruct the attenuation map. This article presents the physical and methodologic basis of attenuation correction and summarizes recent developments in algorithms used to compute the attenuation map in ECT. Other potential applications are also discussed.

https://jnm.snmjournals.org/content/jnumed/44/2/291.full.pdf

Determination of the Attenuation Map in Emission Tomography

Manual segmentation of 129 x-ray CT transverse slices of a living male human has been done and a computerized 3-dimensional volume array modeling all major internal structures of the body has been created. Each voxel of the volume contains a index number designating it as belonging to a given organ or internal structure. The original x-ray CT images were reconstructed in a 512×512 matrix with a resolution of 1 mm in the x,y plane. The z-axis resolution is 1 cm from neck to midthigh and 0.5 cm from neck to crown of the head. This volume array represents a high resolution model of the human anatomy and can serve as a voxel-based anthropomorphic phantom suitable for many computer-based modeling and simulation calculations.

Computerized three‐dimensional segmented human anatomy

Due to the availability of digitally stored human anatomy images, 3-dimensional surfaces of internal structures of the body can be stored in computer volume arrays. Such a volume based software phantom delineates internal human organs with millimeter resolution and lends itself to fully 3-dimensional Monte Carlo simulations. Our simulation models 45 internal human organs (each with an associated radioisotope concentration and attenuation coefficient), calculates gamma radiation histories through these structures, and accepts gamma events onto a collimated planar camera. Variance reduction techniques are applied to decrease the time required to compute a given number of events at the detector. Stratification and two implementations of forced detection variance reduction techniques are compared to ”brute force” calculations for their efficiency speed-ups in this heterogeneous geometry. Simulated clinical images of the liver are shown.

Voxel based Monte Carlo calculations of nuclear medicine images and applied variance reduction techniques

A digital voxel phantom which closely resembles a typical male anatomy has been created. Organ outlines were manually drawn with 1-mm resolution in each of 78 transverses of the human torso. Such an anthropomorphic three-dimensional phantom has several interesting applications in the radiological sciences. Monte Carlo simulations can yield diagnostically realistic images of new investigational radiopharmaceuticals. Uptake characteristics approximated from small animal experiments can be used to test the imaging characteristics in human geometries. The resulting images can serve as ideal projection data of known source and attenuator distributions. This can lead to a better understanding of the image formation process for clinically realistic models, and can prove especially interesting in testing and improving tomographic reconstruction algorithms. Dose calculations for internal and external radiation sources using this phantom can give new insights in the field of health physics and therapy. >

Charles R. Harrell

Papers

Computerized three‐dimensional segmented human anatomy

Voxel based Monte Carlo calculations of nuclear medicine images and applied variance reduction techniques

High resolution anthropomorphic phantom for Monte Carlo analysis of internal radiation sources