IEE Colloquium on Advances in Electrical Tomography
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 Location: London, UK
 Conference date: 19 June 1996
 Conference number: 1996/143
 The following topics were dealt with: tissue characterisation using multifrequency electrical impedance tomography; high speed in vivo chest imaging; forward and inverse problems; integrated intelligent electrodes; electrical capacitance tomography; anisotropic problem; feature detection using neural networks; complex conductivity analysis; programmable DDS waveform generator; distributed pressure measurement; ionisation imaging; finite element based solvers; image reconstruction; entropic thresholding methods; ASIC processor; electromagnetic tomography instrumentation; industrial tomographic imaging
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A status report for medical impedance imaging
 Author(s): J.C. Newell
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Biomedical impedance imaging forms images of the interior of the human body based on the passive electrical properties of the body tissues. Because the electrical properties of different tissues art different, and some tissues' properties change with their functional state, clinical monitoring for abnormal function, particularly of the heart and lungs, may be possible. This paper reviews the current state of this technology, its possible medical applications, and speculates on future directions for the field. Impedance imaging is most highly developed for twodimensional, cylindrical applications. The ability of impedance imaging systems to show changes in conductivity is much greater than their ability to show absolute, or static conductivity values. Most present systems make a reference data set under some conditions, such as at endexpiration of a breath. They then form images of the changes that occur in later datasets, for example during inspiration. These images of changes are called dynamic images, and in this case would show areas such as the lungs, whose conductivity changed during inspiration. Image accuracy and resolution are reduced when the object studied is 3dimensional, or is not of circular crosssection. (3 pages)

Design of a VLSI mixed analogue and digital modular ASIC processor for capacitance tomography
 Author(s): K. Ullaland
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The increasing complexity of instrumentation in science and industry has lead to a growth in the use of Application Specific Integrated Circuits (ASICs). Solutions using ASICs are very cost efficient in high volume production. They are also very valuable when instrument volume, functionality, and reliability are the main issues. In the standard cell approach, the ASIC designer is allowed to use predefined functional building blocks as an aid in the design process. This is advantageous when using a highlevel design methodology for the digital circuits. Preverified models exist for the analogue and the digital standard cells, and thus the standard cell approach is very similar to using Field Programmable Gate Arrays (FPGAs). However, using standard cells, as opposed to full custom design techniques, leads to degraded flexibility and performance in terms of circuit density and speed. On the other hand one gains design time. (3 pages)

Performance evaluation of image reconstruction algorithms in an electromagnetic tomographic system
 Author(s): J. Velez and A.R. Borges
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The aim of electromagnetic tomography (EMT) or magnetic induction tomography is to estimate the electrical conductivity σ and the magnetic permeability μ distributions inside a given region of interest (ROI), by measuring the induced voltage at a number of detection coils placed around it. The induced voltages are nonlinearly related to the materials' electrical and magnetic properties since the magnetic flux paths are distorted by the unknown distributions. Therefore, EMT (such as EIT) is a nonlinear inverse problem that requires far more complex and computer intensive algorithms for image reconstruction than, for instance, Xray transmission tomography which is linear. However, if the distribution of materials inside the ROI consists of a small group of homogeneous objects which are separated by distances at least as large as their size, object interaction is not very strong and the nonlinear effects are minimised. For these cases, very fast algorithms which produce qualitative images can be used. Here, a performance evaluation of 3 of those algorithms is carried out. The presented results are based on real data which was collected on a EMT system developed at INESC (Aveiro, 1996). It is concluded that, if object interaction is not very strong, the inverse problem in electromagnetic tomography can be dealt with in a linear way (ART), or approximated to a linear problem (1 iteration NewtonRaphson). Qualitative images can then be generated in a very fast and efficient way which is very important for some industrial applications where real time imaging is required. (3 pages)

Developments in electromagnetic tomography instrumentation
 Author(s): G.M. Lyon ; Z.Z. Yu ; A.J. Peyton ; M.S. Beck
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A new EMT sensor and instrumentation is described which combines the best features of previous systems and has a modular structure to allow for future system expansion and development. (4 pages)

EIT for the condition monitoring of wood poles carrying overhead power lines
 Author(s): N. White and R. Waterfall
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Decay caused by the fungus Lentinus Lepideus is a major cause of failure in wood poles carrying overhead lines (Morris and Calver, 1985). The metabolism of the fungus produces ions, changing the impedance properties of the wood. The authors' aim is the development of a lowcost tomographic sensor to predict the loss in pole strength resulting from decay; the strength is defined as the pole's resistance to bending forces and is dependent on both the size and position of areas of decay. A previous lowcost approach to this problem used ultrasound (Tomikawa et al., 1986), but this is highly sensitive to transducer contact quality. Electrical impedance tomography (EIT) is an alternative method that can be used for this application. (3 pages)

Development of a variable density flowmeter for an industrial application using tomographic imaging
 Author(s): M. Young ; E. Pickup ; R. Deloughry ; T. Hartley ; S.A. Nixon ; L. Barratt
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A variable density flowmeter enables the mass flow of plastic nibs to be measured. The flowmeter is situated between the bunker discharge outlet valve and tanker. Measurement of the density distribution across the pipe using process tomography enables the mass flow into the tankers to be determined. A high frequency (1 MHz), eight electrode serial capacitance sensing system was developed. This was used primarily for imaging static objects and real time imaging up to 23 frames per second. To increase the imaging speed a parallel system (PT II) was developed. The PT II system consists of an eight electrode parallel capacitance sensing system, using a low frequency AC amplifier detector circuit. The use of a low frequency source has made the capacitance detector circuit stray immune and less sensitive to high frequency noise. A PC based, Texas TMS C40 parallel processing system is used to process data and provide the tomographic images. (3 pages)

Integrated intelligent electrodes for electrical capacitance tomography
 Author(s): P. Williams and T. York
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Preliminary investigations are described that consider the architecture for an electrical capacitance tomography system in which the processing circuitry for each `channel' is mounted directly onto each electrode. The work is motivated by the desire to improve signaltonoise by increasing the operating frequency. This, in turn, requires reduction of stray capacitance. Almost all of the electronic circuitry is included on a custom silicon integrated circuit that is implemented using a high voltage BiCMOS technology. The chip includes the frontend chargedischarge circuit with differential amplification, programmable gain and offset compensation, and analoguetodigital conversion. Data communication with the host is via a serial shift register and each electrode requires less than 10 electrical connections. (4 pages)

The anisotropic EIT problem
 Author(s): W.R.B. Lionheart
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It is now well known that an anisotropic conductivity is not uniquely determined by electrical data at the boundary. At least for the case of an analytic conductivity the only ambiguity is due to a possible distortion of the interior of the medium which would not change the boundary data. One needs then some additional data to determine the anisotropic conductivity uniquely from boundary data. If the conductivity is known to be isotropic then the unknown conductivity amounts to one unknown scalar field which is completely determined by the boundary data. One might expect that there are similar anisotropic problems where one unknown function can be determined from boundary data. (3 pages)

Feature detection using electromagnetic tomography and neural networks
 Author(s): J. Nuno Lau and A.R. Borges
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Electromagnetic tomography (EMT) or magnetic induction tomography is a recent imaging technique. Its purpose is to determine the distribution of electrical conductivity and magnetic permeability in a given regionofinterest (ROI). This distribution is computed from the measurements of the induced voltages at several coils placed around the ROI which are produced by the application of different excitation patterns. Usually images are obtained through inversion of the projection data. In this case, however, a different approach was used. The aim was to perform feature detection of anomalies of `a priori' known objects. Neural network (NN) techniques were used to process the data. It is concluded that neural networks (NNs) have shown to have good discrimination properties over the features for which they had been trained. Even in a not very precise environment where the positioning of the objects was done by hand, the networks proved to be good feature extractors. The estimation approach has lead to simpler and faster network training. For a similar resolution, the network complexity is also much lower than for the classification approach. The connection of EMT and NNs seems very much promising when one aims not to obtain an image, but to get information on the variability of a particular feature of a given object. Thus, it may prove to be very effective for nondestructive testing of metallic objects. Although the present work has only addressed surface defects, the authors believe that similar results could be obtained in other situations if the scanner frequency was lowered. (3 pages)

Analytic solution of complex conductivity in electrical impedance tomography
 Author(s): A.M. Shallof and D.C. Barber
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It is important to find the analytical solution of complex conductivity in order that can be used to the best possible advantage. In this paper the analytical solution of complex conductivity in electrical impedance tomography has been developed and has been applied to object imaging at the centre and at the boundary of different cases. (3 pages)

A programmable DDS waveform generator for electrical impedance tomography (EIT)
 Author(s): I.D. Schneider
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New forms of investigation with EIT require that the signals used for measurement are of an arbitrary form. Both multifrequency and nonsinusoidal signal forms extend the application of EIT to encompass further tissue characteristic information. Recently, multifrequency EITsystems have been constructed to characterise tissue on the basis of the bioimpedance spectrum. Ideally, the frequency response of tissue up to several MHz has to be obtained. A possibility is to measure simultaneously or successively the tissue impedance at different frequencies. Another approach is to use broadband signals like impulses or noise series. Maximum length sequences (MLS) area class of pseudorandom noise sequences which enable fast calculation of the impulse response by crosscorrelating the system's input and output signals. (3 pages)

Tissue characterisation using a multifrequency of electrical impedance tomography
 Author(s): A.M. Shallof and D.C. Barber
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A new method concerning multifrequency electrical impedance tomography has been developed and the full information contained in the complex tissue conductivity can be obtained. The results of the algorithm using a current distribution to maximise the structural information are presented. (4 pages)

Distributed pressure measurement using electrical impedance tomography: initial results
 Author(s): M.J. Booth and I. BasarabHorwath
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Reports initial results from a system which measures distributed planar pressure using electrical impedance tomography. A prototype pressure sensor has been developed; various objects were placed onto the surface of the sensor and measures of sensor performance calculated for each object. The objects varied in size and in the pressure they exerted on the sensor. The measures used to characterise the performance of the sensor for any given input are the fractional change in voltage and the visibility. The behaviour of the system to variations in the applied load is well behaved. (5 pages)

Tomographic imaging of ionisation in gases
 Author(s): C.J. Kotre
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A number of electrical tomographic imaging techniques have been developed in the last decade both in the fields of medical diagnostic imaging, where electrical impedance tomography (EIT) is the most common approach, and industrial tomography where a variety of capacitive and inductive regimes have also been investigated. Here, a related but novel approach to the problem of imaging a crosssectional distribution of ionisation within a volume of gas is described. The tomographic ionisation chamber consists of an insulating cylinder containing a volume of air and supporting a number of equally spaced electrodes surrounding the plane to be imaged. To produce a potential field pattern suitable for ionisation imaging, a potential difference of several hundred volts is applied at one electrode while the rest are maintained at a virtual earth potential. This gives the field pattern (shown here in a figure) which is identical to the familiar equipotential pattern used for EIT with adjacent electrode current drive, except that the equipotential lines and current streamlines are interchanged. To obtain sets of ionisation profiles from which an image can be reconstructed, the current passing through each of the virtually earthed electrodes is measured for each position of the high voltage drive electrode in turn. (4 pages)

A novel computer architecture for realtime solution of inverse problems
 Author(s): W.W. Loh and F.J. Dickin
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Present microcomputer technology, is not sufficiently powerful enough to perform realtime (approximately 100 iterations per second) reconstruction of quantitative tomographic electrical impedance tomography images. Specialised parallel computers designed to perform matrix manipulation efficiently have to be used for matrix manipulation problems, particularly sequential processes such as matrix multiplication, the wavefront array processor (WAP) is considered to be one of the most efficient parallel computer architectures. However, this configuration is not particularly efficient when the processing array is small (<3%) compared to the data matrix, and is therefore usually implemented in VLSI to keep the array large and cost low. A more superior architecture and combined matrix multiplication algorithm which can deliver up to five times more computing power than the WAP was developed by the authors. Together with this novel architecture, the paper also compares its efficiency with some of the more common commercial computers and coprocessor accelerator cards. (3 pages)

High speed in vivo chest impedance imaging with OXBACT Mark III
 Author(s): Y. Shi ; C.W. Denyer ; F.J. Lidgey ; W.R.B. Lionheart ; C.N. McLeod ; K.S. Paulson ; M.K. Pidcock
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Electrical impedance tomography (EIT) aims at imaging the internal conductivity distribution of one cross section of the human body. OXBACT Mark III is the third generation EIT system developed in Oxford for in vivo clinical studies. It is the only EIT system in Europe based on the adaptive current method. The preliminary in vivo results show that OXBACT III has a high precision data acquisition system due to the use high speed ADC, DSP sampling patterns and its novel calibration system. The first high speed in vivo images showing lung ventilation and cardiac function of subjects are encouraging. (3 pages)

A fast EIT system applied to a linear array
 Author(s): J.C. Simpson ; R.C. Tozer ; I.L. Freeston
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Electrical impedance tomography (EIT) is frequently used to image temporal changes in conductivity and in such applications measurement time is an important parameter. This paper describes a highspeed, injected current EIT system which uses a novel technique to reduce the measurement time. The total time required to perform the measurements necessary to produce an image is 2ms. The system uses separate sets of electrodes for injecting currents and sensing voltages and has been applied to a linear array of electrodes, rather than the more common encircling array, although it is not limited to such a geometry. (3 pages)

Forward and inverse problem solutions for three dimensional electrical impedance tomography
 Author(s): K. Kleinermann and N.J. Avis
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At present almost all measured boundary data in electrical impedance tomography (EIT) are collected via a single ring of electrodes placed around a three dimensional body. Despite the fact that these data originate from a three dimensional object, image reconstruction proceeds on the assumption that these data originated from a two dimensional object. In severe cases the complex three dimensional structure and resulting current flow lead to gross image distortions when images are reconstructed using existing two dimensional algorithms from these data. Since no effective means exist to collimate the data arising from a three dimensional object and hence provide a two dimensional image uncontaminated from offplane structure there appears to be little option but to address the third dimension aspects of EIT. This paper compares methods for generating the solution to the forward problem for a right circular cylinder with 16 drive electrodes evenly spaced in a plane around the cylinder, interleaved with 16 voltage measurement electrodes. It assesses the implications of these different methods on the resulting image reconstructions. (3 pages)

Finite element based threedimensional forward and inverse solvers for electrical impedance tomography
 Author(s): A. Binley ; P. Pinheiro ; F. Dickin
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Electrical impedance tomography (EIT) has been successfully employed to a wide range of problems including chemical process engineering, biomedical and environmental applications. In many cases information of a qualitative nature is satisfactory and to fulfil these aims techniques such as those based on backprojection methods have been used. In some cases quantitative information is needed, for example changes in true resistivity from some baseline case, rather than a `greyscale' change. Twodimensional (2D) methods have been used in a number of such cases although, since applications are rarely of a 2D nature, these studies cannot be classified as truly quantitative. In all cases the resultant tomogram reflects some change in electrical property, such as resistivity, but the scale of these changes cannot be quantified. Only with fully threedimensional (3D) forward and inverse models can a true quantitative image be produced. 3D modelling also permits more accurate treatment of the true shape of the body of the region under investigation. 3D analysis, however, is clearly much more computationally demanding than a 2D alternative. In addition, increasing the parameter dimension to accommodate 3D inversion demands larger measured datasets (and hence increased data acquisition time). The authors describe here a fully 3D EIT forward and inverse modelling procedure based on finite elements techniques that has been successfully employed for a number of applications. The authors' analysis refers specifically to resistive EIT in which the objective is to determine the 3D distribution of resistivity within the body of interest from a series of boundary measurements of resistance under different current injection configurations. (3 pages)

Electrical impedance tomography by finite elements using parameterisation of material structure
 Author(s): J.C. Appleby and M. Toft
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The majority of present tomographic reconstruction techniques are either computationally intensive or geometryspecific. In some materialsforming processes the internal structure is characterised by several regions of distinct electrical properties, and only the interface positions are unknown. The authors propose an algorithm which uses this knowledge to reduce significantly the number of degrees of freedom in the problem, thereby making possible the iterative use of the finite element method in an updating scheme for a realtime online manufacturing control tool. The authors present preliminary results which show that it can be effective in identifying exactly the location and size of simple features in a twophase material. (4 pages)