Phantom Materials Used to Model Detection of Concealedweapons and Effects on Implant Devices in Metal Detectors*

We discuss the characterization of phantom materials that simulate the electromagnetic characteristics of various human tissues from 100 Hz to 10 MHz, and over a temperature range of 15 C to 40 C. The materials studied were: 1) a solution of potassium chloride in water, 2) a solution of propylene carbonate, ethylene carbonate, and salts, 3) a semi-solid material consisting of silicone and carbon black, 4) a semi-solid mixture of glycine, carrageenan, potassium chloride, and water. We found that to obtain time stability, the carbon black mixture must be temperature annealed. The silicone material has the advantage of being more rugged than the glycine mixture. The glycine material was jello-like in consistency and required refrigeration. INTRODUCTION The goal of this work is to develop a phantom material (PM) that simulates the relevant electromagnetic properties of the human body over the frequency range of 100 Hz to 10 MHz and a temperature range of 15 C to 40 C [1]. The development of these phantom materials is important for modeling the detection of weapons by metal detectors and understanding the interaction between medical implant devices and metal detector fields. Metal-detector magnetic fields interact with metallic objects by inducing eddy currents (J = ∇×H) in objects that modify the net magnetic field. Metal detector magnetic fields incident on the human body also generate eddy currents in the tissues, but to a much smaller degree than in metals. The incident magnetic fields attenuate as they pass into tissues. Characterization of phantom materials for study of metal detector response requires determination of the conductivity. Since the human body is electrically heterogeneous, PMs can be made that simulate the electrical response of a particular area of the human body. Most of the previously developed phantoms attempted to mimic both the conductivity and the permittivity of the human body. In this study we concentrate on the conductivity. Various candidate phantom materials have been studied by previous researchers [2—11]. The novel feature in our study was the comparison of four candidate materials. These were potassium chloride solutions (KCl); a liquid composed of ethylene carbonate, propylene carbonate, and tetraethyleammonium tetrafluoroborate (TEATFB) [3]; carbon black mixed into silicone (CBS), and a semisolid made form glycine, carrageenan, KCl, and water. All of these composites can be mixed to match the average conductivity of the various human tissues. There are advantages and disadvantages of each when used as PMs. CONDUCTIVITY MEASUREMENTS The conductivity measurements of PMs were made using an open-circuited coaxial holder and a dc conductivity meter [12]. We developed a full-mode model for the open-circuit termination and used it for conductivity determination. This model rigorously models the fringing capacitance at the open-circuit termination. The measurement data are the corrected reflection coefficients, Γ, from a network analyzer or LCR meter. In order to obtain an empirical estimate of our measurement uncertainties we measured liquid KCl reference standards. CONDUCTIVITY OF CANDIDATE LIQUIDS In this section we summarize the measurements on liquid PMs. In Figure 1, the conductivity of KCl in deionized water is plotted versus concentration for minor variations in temperature. The dependence of conductivity on KCl concentration is nearly linear. The required conductivities can be obtained by using the appropriate concentration of KCl. We also measured another liquid mixture which was studied by Broadhurst [3]. This liquid contains ethylene carbonate, propylene carbonate, and tetraethyleammonium tetrafluoroborate (salts). We found that for this liquid we could vary the conductivity over the range of human body tissue properties. ∗U.S. Government work not protected by U.S.copyright