I.L. Freeston

A real-time electrical impedance tomography system for clinical use-design and preliminary results

An instrument is described which produces images of the electrical impedance distribution within the body at a rate of 25 frames per second, allowing lung ventilation and lung perfusion to be observed in real time. The instrument makes impedance measurements using an array of 16 electrodes on the surface of the body, and reconstructs the images using a weighted backprojection technique. The design of the data acquisition electronics and the reconstruction and display processor are described. Some preliminary in vitro and in vivo results from the system are presented.<<ETX>>

Determination of the Distribution of Conduction Velocities in Human Nerve Trunks

A technique is described which enables the distribution of conduction velocities within the alpha fiber group of an in situ human peripheral nerve trunk to be determined. The technique is based on the analysis of the shape of compound nerve action potentials recorded from surface electrodes and is non-invasive. Velocity distributions calculated for a group of normal adults are presented and represent the first known measurements of this parameter.

Design of a phase-sensitive detector to maximize signal-to-noise ratio in the presence of Gaussian wideband noise

A design is presented for a phase-sensitive detector (PSD) based on matched filter theory, which is implemented using digital signal processing (DSP). The signal-to-noise ratio (SNR) improvement given by the new design is equal to that of the matched filter for the signal under consideration, and hence is maximal when the noise is Gaussian and additive. The theory of operation of analogue phase-sensitive detectors is discussed, and the SNR improvement obtained by using an ideal PSD is derived, along with the specific conditions under which this SNR can be expected. The limitations of real PSDs are then discussed. The new design is then presented in detail and its performance is compared to the analogue PSD. Experimental results are given which support the theoretical model of the demodulator, and an example of the use of the demodulator in a real application is given.

Modeling of an Active Nerve Fiber in a Finite Volume Conductor and Its Application to the Calculation of Surface Action Potentials

Potentials within and on the surface of a finite cylindrical volume conductor due to a single active nerve fiber along its center have been calculated by solving Laplaces equation using a relaxation model. The results have enabled the variation of the potential that would be recorded from a surface electrode to be estimated for differing nerve depths and conduction velocities.

Electrical impedance tomography applied to semiconductor wafer characterization

A new method is described for determining the distribution of resistivity of semiconductor wafers and thin conducting films. It uses peripheral electrodes to inject small direct currents into the wafer or film and to measure the resultant potential differences, from which the distribution of resistivity is calculated. The usable area of the wafer is not degraded and, because the wafer is scanned electrically, the method is much faster than conventional methods. Results obtained on silicon wafers and conducting films are presented, which demonstrate the sensitivity and reproducibility of the method.

Impedance Imaging Using Induced Currents

The principal features of an electrical impedance tomography system using induced currents are described. Images of the distribution of conductivity in a two-dimensional phantom are obtained using an algorithm based on a sensitivity matrix. Results are also presented which demonstrate the separation of conductivity and permittivity images from measurements of the complex peripheral voltage, the formation of unreferenced permittivity images, and the use of capacitively coupled electrodes in a measuring system without direct electrical contact.

Real And Imaginary Impedance Images Using Induced Currents

It is shotwt that 5y utilising phase information in an impedance imaging system using induced currents, images of permittivity distribution c8n be reconstructed. Results are presented which show good agreement between simulated and measured data. Furthermore the results demonstrate that permittivity images can reveal objects that are almost invisible in a conductivity image.

Digital demodulator for electrical impedance imaging

Possible applications of a real-time electrical impedance tomography system are discussed, and ways are considered in which the required signal-to-noise (SNR) ratio can be attained. It is shown that a digital demodulator based on matched filter theory provides an SNR improvement equal to that of its analog counterpart. In addition, the digital filter is more versatile than its analog equivalent. For example, the measurement time can be varied in software, to allow for different frame rates. Experimental results show that the filter operates as expected.<<ETX>>

Real Time Electrical Impedance Imaging

We present the design of an electrical impedance tomography system, capable of 25 frame s-l. The main problem which had to be overcome was making the impedance measurements with sufficient accuracy to image lung perfusion. We present invitro results which show that the required specification has been met. and preliminary invivo results have been obtained.

Impedance imaging of 3D objects using magnetically induced currents

The technique of impedance tomography using magnetically induced currents is shown to be capable of reconstructing two dimensional images from a three dimensional phantom.