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.

Measurement of the activating function of magnetic stimulation using combined electrical and magnetic stimuli.

The technique of combined magnetic and electrical stimulation of a peripheral nerve was used to determine the effectiveness of the combined stimulus and its dependence on the relative positioning of electrodes and stimulator coil along the axis of the nerve. The results were used to determine the magnetic stimulation-activating function of a long, straight nerve in the arm, and are shown to be consistent with published theoretical models constructed under conditions of simplified tissue geometry. With appropriate positioning of the two stimulators and similar tissue current waveforms, both enhancement and inhibition of an electrical stimulus were demonstrated and the maximum amplitude of the combined stimulus approached the arithmetic sum of that produced by each stimulator individually.

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.

A microcomputer-controlled cardiac stimulator.

Detailed investigation of abnormal heart rhythms requires electrical stimulators that can deliver sophisticated sequences of stimuli to the heart under controlled laboratory conditions. A dual-channel stimulator that provides an appropriate hardware interface between a controlling microcomputer and the patient is described. The computer gives the system power and flexibility and, most importantly, provides a suitable user interface. The hardware interface is designed to have an ergonomic division between set-up and run-time tasks. Both software and hardware are discussed, and clinical examples of typical usage of the stimulator are given.

Magnetic nerve stimulation: the effect of waveform on efficiency, determination of neural membrane time constants and the measurement of stimulator output.

We describe the first investigation into the effect on stimulation efficiency of varying the output of a commercial magnetic stimulator based on our original clinical design. Over the range of magnetic field waveforms considered, it is shown that the stored energy required to achieve stimulation, both cortically and in the periphery, varies by approximately 2:1. Greater efficiency is obtained by using shorter risetime magnetic fields. This results in more effective stimuli for the same stored energy, or, for the same stimulus, a decrease in energy storage, power dissipation and peak currents, thus simplifying hardware design. A novel method of processing the data obtained from different waveforms is presented which enables neural membrane time constant to be calculated. Data from normal subjects is presented showing both peripheral and neural time constants to be of order 150 microseconds. The cortical measurements represent the first non-invasive determination of cortical membrane time constant in man. Time constant measurements using magnetic stimulation may be clinically useful because they give information concerning the electrical properties of the nervous system not available from present techniques. Finally a method of quantifying the output of magnetic stimulators and coils is described which enables laboratory comparisons to be made, and takes into account magnetic field waveforms and coil geometry. The proposed symbol for this new measurement is Et150 with units volt seconds/meter.

Magnetic stimulation of the human brain and peripheral nervous system: an introduction and the results of an initial clinical evaluation.

This report describes a novel method of stimulating the motor cortex and deep peripheral nerves in humans. The technique, developed in the Department of Medical Physics of Sheffield University, uses a large pulse of magnetic field to induce currents within the body and is painless. The basic principles of magnetic stimulation are described, and the technique is compared with conventional electrical stimulation. Safety aspects are discussed with reference to established clinical electrical and magnetic procedures. The results of the first clinical study using magnetic stimulation are described and show clear central motor pathway slowing in multiple sclerosis patients.

Impedance imaging using linear electrode arrays.

The characteristics of using a linear array of electrodes for impedance imaging are examined. The advantages of a linear array include known electrode position, localised resolution, and mobility, but there are some disadvantages. The problems of the choice of data set, and of the range of measurements are examined. The results of investigations into resolution and sensitivity are presented, and also of some preliminary in vivo measurements. It is concluded that a linear array may have possible clinical applications.

Electrical stimulation at the wrist as an aid for the profoundly deaf.

The development and evaluation of a sensory electrical substitution aid for the profoundly deaf is described. The aid is a small wrist-worn device which converts sound into an electrical stimulus at the wrist. The initial aim was to give profoundly deaf people useful information from ambient sounds. Evaluation of the Mark 1 aid on 15 hearing and five profoundly deaf people indicated that such a device might be of use to some deaf people. Investigation of the sensitivity of the peripheral nervous system to electrical pulse stimulation showed that information could be transmitted in a more efficient manner so a Mark 2 aid was developed which incorporated circuitry to transmit pitch information, potentially useful in lipreading. The Mark 2 aid was shown to give improved results over the Mark 1 aid in transmitting voice intonation, but gave no help in an isolated word lipreading test. A portable Mark 2 aid was developed to enable clinical trials to be carried out on profoundly deaf people.