C.N. McLeod

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.

Generation of anisotropic-smoothness regularization filters for EIT

In the inverse conductivity problem, as in any ill-posed inverse problem, regularization techniques are necessary in order to stabilize inversion. A common way to implement regularization in electrical impedance tomography is to use Tikhonov regularization. The inverse problem is formulated as a minimization of two terms: the mismatch of the measurements against the model, and the regularization functional. Most commonly, differential operators are used as regularization functionals, leading to smooth solutions. Whenever the imaged region presents discontinuities in the conductivity distribution, such as interorgan boundaries, the smoothness prior is not consistent with the actual situation. In these cases, the reconstruction is enhanced by relaxing the smoothness constraints in the direction normal to the discontinuity. In this paper, we derive a method for generating Gaussian anisotropic regularization filters. The filters are generated on the basis of the prior structural information, allowing a better reconstruction of conductivity profiles matching these priors. When incorporating prior information into a reconstruction algorithm, the risk is of biasing the inverse solutions toward the assumed distributions. Simulations show that, with a careful selection of the regularization parameters, the reconstruction algorithm is still able to detect conductivities patterns that violate the prior information. A generalized singular-value decomposition analysis of the effects of the anisotropic filters on regularization is presented in the last sections of the paper.

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.

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.

A novel bipolar-drive circuit for medical applications

A novel drive circuit, useful for medical electronics, is capable of supplying a sample of human tissue, across which there should be zero direct voltage (dc), with a well-defined test current from a source having an output impedance exceeding 16 MOmega at 100 kHz.

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>>

A serial data acquisition architecture for continuous impedance imaging

Q.S. Zhu, MIEEE, C.N. McLeod, C.W. Denyer, F.J. Lidgey, MIEEE and W.R.B. Lionheart EIT Reseaxh Group School of Engineering, Oxford Brookes University, Oxford, OX3 OBP, UK. Abstract A serial data acquisition system architecture and its specifications for continuous impedance imaging are presented. The new system design avoids com lexity and high cast associated with the pade l approach which is normdy taken to obtain high speed in muhi-channel measurements. The scheme involves applications of a video speed A D C device and multiplexers, configured IO scan through a l l the channels simultaneously. Non-uniform over-sampling technique is implemented through precision control of digirishg thing. With the system architectuE. the multiple drive system is capable of achieving 16-bit data acquisition accuracy at a speed of 25 frames per second image production.

Electric Impedance Imaging

The sections in this article are 1 Physical Theory 2 Reconstruction Algorithms 3 Tissue Impedance 4 Electronic Systems 5 Application Areas

Descending aortic flow contribution to intrathoracic impedance-development and preliminary testing of a dual impedance model.

Impedance measurement of cardiac output has struggled to become established partly because there have been only a few attempts to establish a sound theoretical basis for this measurement. Our objective is to demonstrate that there is valuable aortic flow information available from an intrathoracic impedance signal which may eventually be useful in the measurement of cardiac output by impedance technology. A model, using dual impedance measurement electrodes and the change in impedance when blood flows, has been developed based on an intrathoracic impedance model of the descending aorta and esophagus. Using this model as the basis for measurement by an esophageal probe, we provide solutions to the velocity of blood flow in the descending aorta. Five patients were studied. Only three patients had suitable signals for analysis but the aortic flow profiles from these three patients were consistent and realistic. Aortic blood flow information may be obtained from the intrathoracic impedance signal using this dual impedance method.

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.