Kevin Paulson

Electrode modelling in electrical impedance tomography

In electrical impedance tomography (EIT), measurements of an applied electrical current and the corresponding electrical potential are made on a finite number of electrodes placed on the boundary of an object. These measurements are then used to reconstruct the electrical conductivity distribution in the interior of the object. Iterative solutions of this inverse problem involve frequent solution of the forward problem, and it is therefore important to be able to model the current flow through the electrodes. This paper discusses one such model and describes how the related boundary value problem can be solved using semi-analytical and numerical techniques. Some conclusions regarding the proportion of the boundary that should be covered by electrodes are also drawn.

Optimal experiments in electrical impedance tomography

Electrical impedance tomography (EIT) is a noninvasive imaging technique which aims to image the impedance within a body from electrical measurements made on the surface. The reconstruction of impedance images is a ill-posed problem which is both extremely sensitive to noise and highly computationally intensive. The authors define an experimental measurement in EIT and calculate optimal experiments which maximize the distinguishability between the region to be imaged and a best-estimate conductivity distribution. These optimal experiments can be derived from measurements made on the boundary. The analysis clarifies the properties of different voltage measurement schemes. A reconstruction algorithm based on the use of optimal experiments is derived. It is shown to be many times faster than standard Newton-based reconstruction algorithms, and results from synthetic data indicate that the images that it produces are comparable.

An adaptive current tomograph using voltage sources

An adaptive electric current tomography system that contains a novel front-end analog architecture was developed. Programmable voltage sources were used to deliver currents into the study object and to avoid the difficulties of obtaining high-quality current sources. Through inverting an admittance matrix, the system is capable of achieving a desired current drive pattern by applying a computed voltage pattern. The tomograph, operating at 9.6 kHz, comprises 32 driving electrodes and 32 voltage measurement electrodes. The study of system noise performance shows high SNR in the data acquisition which is enhanced by a digital demodulation scheme. In vitro reconstruction images have been obtained with the data collected by the tomograph.<<ETX>>

POMPUS: an optimized EIT reconstruction algorithm

Electrical impedance tomography (EIT) is a non-invasive imaging technique which aims to image the impedance of material within a test volume from electrical measurements made on the surface. The reconstruction of impedance images is an ill-posed problem which is both extremely sensitive to noise and highly computationally intensive. This paper defines an experimental measurement in EIT and calculates optimal experiments which maximize the distinguishability between the region to be imaged and a best estimate conductivity distribution. These optimal experiments can be derived from measurements made on the boundary. We describe a reconstruction algorithm, known as POMPUS, which is based on the use of optimal experiments. We have shown that, given some mild constraints, if POMPUS converges, it converges to a stationary point of our objective function. It is demonstrated to be many times faster than standard, Newton based, reconstruction algorithms. Results using synthetic data indicate that the images produced by POMPUS are comparable to those produced by these standard algorithms.

A probe for organ impedance measurement

In this paper, we describe the theory and practical implementation of an electrical impedance probe for making in vivo measurements of the electrical admittance of living tissue. The probe uses concentric annular electrodes and is shown to sample a more localized, yet greater, volume of tissue than the standard four-electrode probe. We have developed a mathematical model for the conduction of current between the probe electrodes assuming that we are investigating a uniform, isotropic, semi-infinite region and taking into account the contact impedance between the electrodes and the organ. The electric fields produced by the probe have been calculated by solving a weakly singular Fredholm integral equation of the second kind. The size and position of the probe electrodes have been optimized to maximize both the accuracy in the admittance measurement and insensitivity to contact impedance. A probe and driving hardware have been constructed and experimental results are provided showing the accuracy of admittance measurements at 50 and 640 KHz.

A hybrid phantom for electrical impedance tomography

One of the design considerations for electrical impedance tomography phantoms is that they must be easy to model accurately. This paper describes a phantom with this property. Experimental results from its evaluation and testing are given.

Optimal measurements in electrical impedance tomography

We develop a definition of an experiment in electrical impedance tomography. From this definition optimal current patterns and measurement patterns are derived which yield the ‘best’ possible set of measurements for Err reconstruction. A reconstruction algorithm based on the use of these optimal patterns is compared to standard, Newton based, reconstruction.

Fast, non-linear inversion for electrical impedance tomography

Abstract Electrical impedance tomography (EIT) is a non-invasive imaging technique which aims to image the impedance within a test volume from electrical measurements made on the surface. The reconstruction of impedance images is an illposed problem which is both extremely sensitive to noise and highly computationally intensive. This paper defines an experimental measurement in EIT, and calculates optimal experiments which maximize the distinguishability between the region to be imaged and a best estimate conductivity distribution. These optimal experiments can be derived from measurements made on the boundary. A reconstruction algorithm, known as POMPUS, based on the use of optimal experiments is derived. It is proved to converge given some mild constraints, and is demonstrated to be many times faster than standard, Newton-based reconstruction algorithms. Results using synthetic data indicate that the images produced by POMPUS are comparable to those produced by these standard algorithms.

High speed in vivo chest imaging with OXBACT III

In vivo electrical impedance chest images are presented, showing cardiac and respiratory changes during breathing and physiological manoeuvres.

Fast, Non-Linear Inversion for Electrical Impedance Tomography

Electrical Impedance Tomography, (EIT), is a non-invasive imaging technique which aims to image the impedance within a test volume from electrical measurements made on the surface. The reconstruction of impedance images is an ill-posed problem which is both extremely sensitive to noise and highly computationally intensive. This paper defines an experimental measurement in EIT and calculates optimal experiments which maximise the distinguishability between the region to be imaged and a best estimate conductivity distribution. These optimal experiments can be derived from measurements made on the boundary. A reconstruction algorithm, known as POMPUS, based on the use of optimal experiments is derived. It is proved to converge given some mild constraints and is demonstrated to be many times faster than standard, Newton based, reconstruction algorithms. Results using synthetic data indicate that the images produced by POMPUS are comparable to those produced by these standard algorithms.