Biomedical subjects

W R Lionheart

Publications and source records attributed to W R Lionheart.

6 recordsLinked to original sources

PubMed2001

Generalized optimal current patterns and electrical safety in EIT.

There are a number of constraints which limit the current and voltages which can be applied on a multiple drive electrical imaging system. One obvious constraint is to limit the maximum ohmic power dissipated in the body. Current patterns optimizing distinguishability with respect to this constraint are singular functions of the difference of transconductance matrices with respect to the power norm (the optimal currents of Isaacson). If one constrains the total current (L1 norm) the optimal patterns are pair drives. On the other hand if one constrains the maximum current on each drive electrode (an L(infinity) norm), the optimal patterns have each drive channel set to the maximum source or sink current value. In this paper we consider appropriate safety constraints and discuss how to find the optimal current patterns with those constraints.

Electric Impedance↗

PubMed2001

A MATLAB package for the EIDORS project to reconstruct two-dimensional EIT images.

The EIDORS (electrical impedance and diffuse optical reconstruction software) project aims to produce a software system for reconstructing images from electrical or diffuse optical data. MATLAB is a software that is used in the EIDORS project for rapid prototyping, graphical user interface construction and image display. We have written a MATLAB package (http://venda.uku.fi/ vauhkon/) which can be used for two-dimensional mesh generation, solving the forward problem and reconstructing and displaying the reconstructed images (resistivity or admittivity). In this paper we briefly describe the mathematical theory on which the codes are based on and also give some examples of the capabilities of the package.

Algorithms↗

PubMed2001

Time series of EIT chest images using singular value decomposition and Fourier transform.

The aim of this study is to propose a useful method for exploring regional ventilation and perfusion in the chest. The paper describes two methods based on singular value decomposition (SVD) and Fourier transform (FT) respectively. This work shows that power spectral density (PSD) and phase images (derived from the Fourier transform) are easier to interpret and more useful tools for exploiting in vivo EIT data in healthy volunteers in order to explore the cardiovascular and respiratory systems.

Electric Impedance↗

PubMed2000

Sensitivity matrix and reconstruction algorithm for EIT assuming axial uniformity.

In electrical impedance tomography (EIT) two-dimensional models continue to be applied despite their known inability to provide correct reconstruction. In this paper, a reconstruction algorithm that assumes a translationally invariant conductivity distribution is described. A more precise forward solver is obtained by taking off-slice currents into consideration. An appropriate sensitivity matrix is derived. Numerical evidence for the improvement in precision compared to two-dimensional reconstruction is given.

Algorithms↗

PubMed1994

Development of a real-time adaptive current tomograph.

Following the successful development of a multiple-drive electrical impedance tomography system OXPACT-II featuring a voltage-driven current method for in vitro studies, research work currently being undertaken at the EIT research group in Oxford is aimed at developing a real-time multiple-drive adaptive system, called the Oxford Brookes Adaptive Current Tomograph Mark-III (OXBACT-III) which will operate at several frequencies in between 10-160 kHz. The objective of this system development is to enable EIT clinical studies to be undertaken based on the adaptive current method. One of the most important issues addressed in the new system design is to achieve high data acquisition speed while maintaining sufficient system accuracy. This paper will describe the overall data acquisition system structure and relevant system performance specifications.

Data Display↗

PubMed1993

An adaptive current tomography using voltage sources.

This paper describes the development of an adaptive electric current tomography system which contains a novel front-end analog architecture. Programmable voltage sources have been 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.

Electrodes↗