D W Armitage

Radiofrequency-induced hyperthermia: computer simulation of specific absorption rate distributions using realistic anatomical models.

A description is given of a computer simulation technique which predicts the specific absorption rate (SAR) distribution within the human body resulting from the application of radiofrequency electromagnetic energy. The method uses an extension to the principle of over-relaxation of electric potentials and the basis of the simulation is a realistic three-dimensional model derived from both dielectric and anatomical data. Two of the principal means of applying radiofrequency hyperthermia, namely the use of capacitive electrodes and inductive coils, have been provided for. The accuracy of the simulation has been favourably tested using an agar split-phantom and an infrared thermograph camera. The simulations can be used to assist the design and clinical use of radiofrequency applicators, and examples are given of the application of both an inductive coil and switched capacitive electrodes to heat the thorax.

Imaging cerebral haemorrhage with magnetic induction tomography: numerical modelling

Magnetic induction tomography (MIT) is a new electromagnetic imaging modality which has the potential to image changes in the electrical conductivity of the brain due to different pathologies. In this study the feasibility of detecting haemorrhagic cerebral stroke with a 16-channel MIT system operating at 10 MHz was investigated. The finite-element method combined with a realistic, multi-layer, head model comprising 12 different tissues, was used for the simulations in the commercial FE package, Comsol Multiphysics. The eddy-current problem was solved and the MIT signals computed for strokes of different volumes occurring at different locations in the brain. The results revealed that a large, peripheral stroke (volume 49 cm(3)) produced phase changes that would be detectable with our currently achievable instrumentation phase noise level (17 m degrees ) in 70 (27%) of the 256 exciter/sensor channel combinations. However, reconstructed images showed that a lower noise level than this, of 1 m degrees , was necessary to obtain good visualization of the strokes. The simulated MIT measurements were compared with those from an independent transmission-line-matrix model in order to give confidence in the results.

A Method to Solve the Forward Problem in Magnetic Induction Tomography Based on the Weakly Coupled Field Approximation

Magnetic induction tomography (MIT) is a noninvasive modality for imaging the complex conductivity ( ¿ = ¿+ j¿¿) or the magnetic permeability (¿) of a target under investigation. Because MIT employs noncontact coils for excitation and detection, MIT may be suitable for imaging biological tissues. In medical applications where high resolutions are sought, image reconstruction is a time and memory consuming task because the associated inverse problem is nonlinear and ill-posed. The time and memory constraints are mainly imposed by the solution of the forward problem within the iterative image reconstruction procedure. This paper investigates the application of a weakly coupled approximation to the solution of the forward problem and examines the accuracy against the computation time and memory gained in adopting this approximation. Initially, an analytical solution for mutual impedance change of a coil pair due to a large planar conductive object is presented based on a full wave theory and used to demonstrate a 10 MHz frequency excitation as an acceptable upper frequency limit under which the approximation is valid. Subsequently, a numerical impedance method adopting the approximation is presented. Here the impedance method is used to solve the forward problem, which employs electrical circuit analogues to mesh the target into a network that can be solved using circuit analysis and sparse matrix technique. The error due to the approximation is further estimated numerically with the impedance method against a commercial finite-element package (commercial FE solver, COMSOL) and results show at 10 MHz excitation a 0.4% of tolerance is achieved for conductivities in the range <0.5 S/m. The results also show the method can be applied for low conductivity medical applications and is computationally efficient compared to equivalent finite-element methods.

KNN classification of metallic targets using the magnetic polarizability tensor

Walk-through metal detectors are used at check points for preventing personnel and passengers from carrying threatening metallic objects, such as knives and guns, into a secure area. These systems are capable of detecting small metallic items, such as handcuff keys and blades, but are unable to distinguish accurately between threatening objects and innocuous items. This paper studies the extent to which a K-nearest-neighbour classifier can distinguish various kinds of metallic objects, such as knives, shoe shanks, belts and containers. The classifier uses features extracted from the magnetic polarizability tensor, which represents the electromagnetic properties of the object. The tests include distinguishing threatening objects from innocuous ones, classifying a set of objects into 13 classes, and distinguishing between several similar objects within an object class. A walk-through metal detection system is used as source for the test data, which consist of 835 scans and 67 objects. The results presented show a typical success rate of over 95% for recognizing threats, and over 85% for correct classification. In addition, we have shown that the system is capable of distinguishing between similar objects reliably. Overall, the method shows promise for the field of security screening and suggests the need for further research.

Seeing in the mist: real time video enhancement

Degradation of images due to atmospheric scattering is a phenomenon that causes problems in a number of imaging applications. By using knowledge of the scene geometry and a physical model of scattering, it is possible to apply a correction to remove the systematic effects of scattering. This paper describes a system that can perform atmospheric correction of colour PAL video in real time. Examples of the processed output are given for a static and an aircraft‐mounted camera, both in hazy conditions.

Non-contact multi-frequency magnetic induction spectroscopy system for industrial-scale bio-impedance measurement

Biological tissues have a complex impedance, or bio-impedance, profile which changes with respect to frequency. This is caused by dispersion mechanisms which govern how the electromagnetic field interacts with the tissue at the cellular and molecular level. Measuring the bio-impedance spectra of a biological sample can potentially provide insight into the samples properties and its cellular structure. This has obvious applications in the medical, pharmaceutical and food-based industrial domains. However, measuring the bio-impedance spectra non-destructively and in a way which is practical at an industrial scale presents substantial challenges. The low conductivity of the sample requires a highly sensitive instrument, while the demands of industrial-scale operation require a fast high-throughput sensor of rugged design. In this paper, we describe a multi-frequency magnetic induction spectroscopy (MIS) system suitable for industrial-scale, non-contact, spectroscopic bio-impedance measurement over a bandwidth of 156kHz-2.5MHz. The system sensitivity and performance are investigated using calibration and known reference samples. It is shown to yield rapid and consistently sensitive results with good long-term stability. The system is then used to obtain conductivity spectra of a number of biological test samples, including yeast suspensions of varying concentration and a range of agricultural produce, such as apples, pears, nectarines, kiwis, potatoes, oranges and tomatoes.

Measurement system for determining the magnetic polarizability tensor of small metal targets

This paper presents an apparatus to measure the spectroscopic magnetic response of small metallic objects and deduce the magnetic polarizability tensor. The measured transimpedances of a .222 Remington rifle cartridge and titanium cube are compared to simulated results and are found to match well providing verification of the method. The eigenvalues of the two objects are calculated and discussed highlighting the potential discriminatory aspect. The results support the proposed use of the eigenvalue spectra to provide subsurface classification and discrimination between landmines and clutter.

GPR combined with a positioning system to detect anti-personnel landmines

Landmines and explosive remnants of war (ERW) are a serious problem in many countries around the world. In 2012, there were, on average, ten casualties per day globally [1]. Anti-Personnel (AP) landmines have been developed to have only a small amount of metal content, thus making them harder to find with metal detectors. Often there is less metal in a landmine than in “clutter” metallic objects, which are also found during the de-mining process. One reason landmine removal is a time-consuming task using the conventional metal detector is that it is not possible to classify the objects very accurately; consequently all positive signals must be investigated with equal caution. However, the dielectric material (e.g. main charge and plastic case) surrounding the metal detonator components generally occurs in landmines, and less so in clutter. Metal objects detected by a dual sensor without an associated detection of plastic objects can be ignored unless they are metal AP mines. One way to detect the plastic is to add a Ground Penetrating Radar system (GPR) to the conventional metal detector. Developments over the last decade combining GPR with traditional inductive metal detection have demonstrated the value of this approach [2]. GPR data is generally analysed by collating a series of reflection responses (A-Scans), collected over an evenly spaced series of positions in a single direction. This data set is termed a B-Scan. On conventional geotechnical GPR systems, a position encoder is attached to one of the wheels, in order to capture the data at regular intervals. Metal-detector type AP landmine detection systems are handheld devices without wheels, so some other way of detecting position needs to be found other than the operators location. This paper describes an investigation into a positioning system for a handheld scanner. It describes how inertial measurements combined with a camera can determine the antenna position during the sensor sweep and how this information can be used to create the conventional B-Scan data sets. The paper compares the antenna-to-ground separation measurement extracted from both time and frequency domain GPR measurement systems to that given by an ultrasound distance sensor.

Determination of material and geometric properties of metallic objects using the magnetic polarisability tensor

A walk-through metal detector system has been used for measuring the magnetic polarisability tensor for a variety of metallic objects. We propose a method for classifying objects by their metallic composition using features of the tensor. Furthermore, we investigate the potential of using the tensor representation as an indication geometric properties of the object. The method used is shown to be accurate for classification of material composition. Furthermore, the results suggest that it is possible to use the tensor to distinguish between similar objects of different sizes in limited scenarios. These findings demonstrate the potential for this method, but also suggest the need for further studies.

Estimating magnetic polarizability tensor of buried metallic targets for land mine clearance

This paper addresses the problem of identifying metallic objects in buried landmines and discriminating them from clutter using low frequency electromagnetic induction (EMI) techniques. From dipolar fields, the magnetic polarizability tensor extracted from the target response can be used as a basis for identification. Here, a deterministic nonlinear optimization method is presented to estimate target polarizability matrix and location by fitting a dipole model to EMI data collected above target in a least squares sense. Using finite element simulated data with added synthetic low frequency noise (10 dB SNR), results show initial guess misestimating target position with few centimeters in the transversal (x, y) plane can be corrected very close to the true location. The method is also able to estimate the polarizability tensor to within 12 % error of the true tensor. Keywords-component; Electromagnetic Induction, landmines, UXO, Magnetic polarizability, nonlinear inverse problems.