Localization of a wireless capsule endoscope finds many clinical applications from diagnostics to therapy. There are potentially two approaches of the electromagnetic waves based localization: a) signal propagation model based localization using a priori information about the persons dielectric channels, and b) recently developed microwave imaging based localization without using any a priori information about the persons dielectric channels. In this paper, we study the second approach in terms of a variety of frequencies and signal-to-noise ratios for localization accuracy. To this end, we select a 2-D anatomically realistic numerical phantom for microwave imaging at different frequencies. The selected frequencies are 13:56 MHz, 431:5 MHz, 920 MHz, and 2380 MHz that are typically considered for medical applications. Microwave imaging of a phantom will provide us with an electromagnetic model with electrical properties (relative permittivity and conductivity) of the internal parts of the body and can be useful as a foundation for localization of an in-body RF source. Low frequency imaging at 13:56 MHz provides a low resolution image with high contrast in the dielectric properties. However, at high frequencies, the imaging algorithm is able to image only the outer boundaries of the tissues due to low penetration depth as higher frequency means higher attenuation. Furthermore, recently developed localization method based on microwave imaging is used for estimating the localization accuracy at different frequencies and signal-to-noise ratios. Statistical evaluation of the localization error is performed using the cumulative distribution function (CDF). Based on our results, we conclude that the localization accuracy is minimally affected by the frequency or the noise. However, the choice of the frequency will become critical if the purpose of the method is to image the internal parts of the body for tumor and/or cancer detection.
It is widely acknowledged that tree roots and other forms of buried biomass have an adverse effect on the performance of ground-penetrating radars (GPRs). In this work we present experimental and theoretical work that quantifies that effect. Test sites containing extensive root infiltration at Eglin Air Force Base, FL were probed with a GPR. After completing the measurements, the sites were excavated, and the root structure and soil were thoroughly characterized. Supplemental GPR measurements of simple cylindrical objects in a laboratory setting were performed to investigate basic scattering behavior of buried roots. A numerical simulator based on the Discrete Dipole Approximation (DDA), an integral-equation-based method, was developed, validated and subsequently used to compute scattering from root structures modeled by an ensemble of buried cylinders. A comparison of the measurements and numerical calculations is presented that quantifies the potential for false alarms and increased clutter due to buried roots.
A numerical model for polarimetric signatures of surface-laid land mines is presented. The model simulates high-resolution images formed from Stokes parameters that describe thermal emission and reflection of sunlight and skylight. The temperature of the mine and its surroundings is computed as a function of time using a finite element solution of the heat transport equation. The effects of surface roughness are included via a two-scale model, in which the gross shape of the mine is represented by triangular facets (the surface facets of the finite element tetrahedra). Theoretical solutions for rough surface scattering are used to describe small-scale roughness on a facet. Multiply reflected contributions are neglected in the current implementation. An example is presented in which the role of each component is described and related to the observed image.
Previous modeling studies have indicated that a multi-frequency radiometer could prove advantageous for humanitarian demining due to the oscillatory patterns in brightness temperature versus frequency that would be observed in the presence of a sub-surface target. Initial experimental results are reported in this paper from a multi-frequency radiometer (MFRAD) system operating at 19 frequencies in the 2.1-6.5 GHz band. The basic design of MFRAD is reviewed, and the calibration and noise background removal procedures discussed. Experimental results with sub-surface metallic and styrofoam targets are then provided that demonstrate the predicted oscillatory behavior. An FFT-based detection algorithm is also described and applied to measured data. Further plans for experiments and tests with this system are also detailed.
A specular model has been used to predict the passive polarimetric infrared (IR) signature of surface-laid landmines. The signature depends on the temperature of the landmine and the sky radiance. The temperature of the landmine is measured using a thermocouple. The signature itself is measured using a polarimetric IR camera setup. The predictions are fit to the measurements using the refractive index as an optimization parameter. The obtained refractive indices of each landmine type are consistent, but for the PMN landmine much lower than determined in a previous indoor experiment. Throughout the measurement day, the average landmine polarimetric signature was higher than the average background signature. Moreover the polarimetric signature appears to be a more robust indicator of the shape of the landmine's top surface than the normal IR signature. A simulator of passive polarimetric imagery is also being developed. That work is based on a physical model for both the thermal and radiometric processes, and it includes a finite-element solution for the heat transfer problem, ray tracing to describe the incident sunlight and the effects of shadowing, and analytical models for the Mueller matrices of rough dielectric surfaces. Preliminary results from that model show substantial qualitative agreement with measured images.
A simple layered medium model for microwave thermal emission from a buried object shows that multiple frequency emission measurements can potentially provide an effective means for target detection. Object detection is obtained form a search for oscillatory features in multiple frequency brightness temperatures, which occur due to interference effects between the surface and buried object interfaces. Previous studies have considered simple homogeneous temperature and water content models of the soil medium, and show that oscillatory features versus frequency are not obtained in the absence of a target even with medium temperature or soil moisture variations. However, the more realistic case of non-constant temperature and water content versus depth was not considered in previous studies; these effects can potentially modify interference phenomena. In addition, subsurface objects have typically been modeled as layers whose horizontal dimensions are infinite; models including the effects of finite targets size are thus of interest.
Broadband electromagnetic induction (EMI) methods are promising in the detection and discrimination of subsurface metallic targets. We compute EMI responses from conducting and permeable spheroids by using a field expansion method which is based on the separation of variables in spheroidal coordinates. In addition to an exact formulation which utilizes the vector spheroidal wavefunctions inside the spheroid, we also develop an approximate theory known as the small penetration-depth approximation (SPA). For general permeability, SPA is applicable at high frequency and compliments the exact formulation which breaks down at high frequency. However, when the permeability of the spheroid is large enough, the SPA yields an accurate broadband response. Numerical results for the far-field frequency responses from prolate and oblate spheroids are presented. By neglecting mutual interactions between the spheroids, we also study the broadband EMI response from a collection of spheroids that are randomly oriented and have different sizes.
It has long been recognized that surface-laid land mines and other man-made objects tend to have different polarization characteristics than natural materials. This fact has been used to advantage in a number of mine detecting sensors developed over the last two decades. In this work we present the theoretical basis for this polarization dependence. The theory of scattering from randomly rough surfaces is employed to develop a model for scattering and emission from mines and natural surfaces. The emissivity seen by both polarized and unpolarized sensors is studied for smooth and rough surfaces. The polarized and unpolarized emissivities of rough surfaces are modeled using the solution of the reciprocal active scattering problem via the second order small perturbation method/small slope approximation(SPM/SSA). The theory is used to determine the most suitable angle for passive polarimetric IR detection of surface mines.
An analytical study of environmental and clutter effects on microwave radiometers used for the detection of buried objects is presented. To simplify the analysis, it is assumed that the soil/target medium has a constant physical temperature versus depth, so that Kirchhoff's law can be applied to determine emissivities, and a simple layered medium geometry is used to model a buried target. Changes in brightness temperatures which result due to the present of a buried target are illustrated for varying soil dielectric properties, radiometer frequencies, and target depths, and are contrasted with changes in brightness temperatures which can occur when no target is presented due to slight soil moisture or soil temperature variations. Brightness temperature clutter due to a small surface roughness is also analytically modeled, through application of the small slope approximation for the homogeneous medium case and the small perturbation method in the presence of a subsurface layer, and it is shown that surface clutter effects can be mitigated through proper choice of sensor polarization and observation angle. Particular attention is given to the relationship between passive and active microwave sensors; results demonstrate that these two can provide complementary information. Finally, the use of wideband radiometric measurements are discussed as a means for reducing environmental clutter effects and improving detection algorithms.
The detection of non-metallic anti-personnel landmines with ground penetrating radar (GPR) is complicated by low dielectric contrasts with the surrounding background medium. Previous studies have shown that the addition of water can improve dielectric contrasts but also increases loss so that target detectability is not necessarily improved. Previous studies have also shown that the addition of liquid nitrogen to wet soils can reduce background medium loss and restore target visibility. In this paper, further waveguide studies of target detection through a controlled depth of nitrogen penetration are reported, and it is shown that scattering from known depth targets can be significantly enhanced if an optimal amount of nitrogen is added. The procedure can also be generalized to unknown depth targets if measurements are taken as gradually increasing amounts of liquid nitrogen are added. Both analytical models and waveguide experiments are presented to illustrate these ideas. Finally, initial test of the soil modification techniques developed through waveguide experiments are reported with a dielectric rod GPR system; results indicate that these methods should be applicable to general GPR sensors.
Waveguide studies of the effectiveness of soil modification techniques for non-metallic mine detection with ground penetrating radar are described. Visibility improvements for a nylon target buried in sand are considered as varying amounts of water or liquid nitrogen are added to the sand. Experiments are performed in an S-band waveguide (2.6 - 3.8 GHz), and results show that increased water content improves shallow buried target visibility initially due to increased dielectric contrast between the target and background medium, but eventually obscures target responses due to increased loss. Initial tests of the addition of liquid nitrogen show reduced loss in the high moisture content case, so that targets can again be made visible. Analytical model results are also presented for the experimental configuration and found to be in good agreement with measured data, and further studies of soil modification effects with the analytical model are performed. A finite difference time domain (FDTD) electromagnetic model for more complicated geometries involving general target shapes and inhomogeneous water contents is also described.