ACT3: a high-speed, high-precision electrical impedance tomograph.

This paper presents the design, implementation, and performance of Rensselaer's third-generation Adaptive Current Tomograph, ACT3. This system uses 32 current sources and 32 phase-sensitive voltmeters to make a 32-electrode system that is capable of applying arbitrary spatial patterns of current. The instrumentation provides 16 b precision on both the current values and the real and reactive voltage readings and can collect the data for a single image in 133 ms. Additionally, the instrument is able to automatically calibrate its voltmeters and current sources and adjust the current source output impedance under computer control. The major system components are discussed in detail and performance results are given. Images obtained using stationary agar targets and a moving pendulum in a phantom as well as in vivo resistivity profiles showing human respiration are shown.

Detection and imaging of electric conductivity and permittivity at low frequency.

This paper is concerned with low frequency electrical impedance imaging, which is the process of constructing images of the electrical impedance of a body's interior based upon measurements of voltage and current made at the body's surface. The electrical impedance accounts for both resistivity and permittivity. This paper shows how permittivity can be exploited to improve the performance of an electrical impedance imaging system. We show that explicit use of the independent information in the data due to the permittivity will ehance a system's ability to distinguish objects in the interior of a body. In addition, we report the results of experiments performed using the Rensselaer ACT 2 system on a saline bath containing various objects. These objects include both living tissue and metal conductors with oxide layers. We demonstrate the system's ability to distinguish these objects, and we exhibit gray scale images of both their resistivity and permittivity distributions.

Rapid assessment of electrode characteristics for impedance imaging.

Electrical impedance imaging is the technique for producing images of the resistivity of internal body structures based on measurements of voltage and current from electrodes applied to the body's surface. When a multiplicity of electrodes are applied in one or more rows around a body structure such as the thorax or limb, it is useful to be able to rapidly assess the general status of the electrode-body interface to determine if the skin has been suitably prepared, and that electrode and skin impedance are suitably low. In addition, assessment of the impedance of individual electrodes should precede acquisition of data for image formation. This communication presents techniques for assessing the overall skin and electrode impedances relative to the impedance of the body interior, and for assessing the integrity of each electrode's contact impedance.

Errors due to measuring voltage on current-carrying electrodes in electric current computed tomography.

Electric current computed tomography is a process for determining the distribution of electrical conductivity inside a body based upon measurements of voltage or current made at the body's surface. Most such systems use different electrodes for the application of current and the measurement of voltage. This paper shows that when a multiplicity of electrodes are attached to a body's surface, the voltage data are most sensitive to changes in resistivity in the body's interior when voltages are measured from all electrodes, including those carrying current. This assertion is true despite the presence of significant levels of skin impedance at the electrodes. This conclusion is supported both theoretically and by experiment. Data were first taken using all electrodes for current and voltage. Then current was applied only at a pair of electrodes, with voltages measured on all other electrodes. We then constructed the second data set by calculation from the first. Targets could be detected with better signal-to-noise ratio by using the reconstructed data than by using the directly measured voltages on noncurrent-carrying electrodes. Images made from voltage data using only noncurrent-carrying electrodes had higher noise levels and were less able to accurately locate targets. We conclude that in multiple electrode systems for electric current computed tomography, current should be applied and voltage should be measured from all available electrodes.

Electrode models for electric current computed tomography.

This paper develops a mathematical model for the physical properties of electrodes suitable for use in electric current computed tomography (ECCT). The model includes the effects of discretization, shunt, and contact impedance. The complete model was validated by experiment. Bath resistivities of 284.0, 139.7, 62.3, 29.5 omega.cm were studied. Values of "effective" contact impedance zeta used in the numerical approximations were 58.0, 35.0, 15.0, and 7.5 omega.cm2, respectively. Agreement between the calculated and experimentally measured values was excellent throughout the range of bath conductivities studied. It is desirable in electrical impedance imaging systems to model the observed voltages to the same precision as they are measured in order to be able to make the highest resolution reconstructions of the internal conductivity that the measurement precision allows. The complete electrode model, which includes the effects of discretization of the current pattern, the shunt effect due to the highly conductive electrode material, and the effect of an "effective" contact impedance, allows calculation of the voltages due to any current pattern applied to a homogeneous resistivity field.

Theory and performance of an adaptive current tomography system.

It has been shown that there exists an optimum set of current patterns for distinguishing one conductivity distribution from another. Since the optimum set of current patterns depends on the conductivity distribution being imaged it must be determined for each object being imaged. This paper describes how these current patterns may be determined and describes a system for achieving this in practice.

Current topics in impedance imaging.

We introduce a definition of 'best' currents to apply to an electrode array on the surface of a body in order to distinguish between the conductivity inside the body and a conjectured conductivity. Using these 'best' currents, we illustrate with a simple example the general fact that a single current applied between a pair of electrodes, loses its ability to distinguish between different conductivities as the size of the region over which the current is applied goes to zero. We next introduce approximations to the best currents on systems having L electrodes, and calculate the ability of these systems to distinguish between conductivities as L goes to infinity and the electrode size goes to zero. We conclude with a simple example that illustrates a process for producing the 'best' currents without a previous knowledge of what is inside the body.

A mobile physiological data acquisition system for clinical research.

The S.R. Powers Trauma Research Center at Albany Medical Center makes a wide variety of standard and specially designed physiological measurements on injured patients. To overcome the disadvantages of admitting patients to a single bed research unit, a mobile data acquisition system was designed to study patients in the Intensive Care Unit, Burn Unit, Emergency Room, and Recovery Room. This mobile system acquires data using microcomputer controlled data acquisition and communicates via modem with a larger computer located in the Trauma Unit Research Center. The laboratory computer performs data analysis and returns results to the instrument cart at the bedside. A dry spirometer, which is reset on inspiration by the mechanical ventilators, was designed, built, and incorporated into the cart for measuring expired gas volumes and flows.

A system to measure functional residual capacity in critically ill patients.

The use of continuous positive airway pressure (CPAP) and intermittent mandatory ventilation (IMV) in spontaneously breathing, intubated patients has prompted the development of new procedures for measuring functional residual capacity (FRC). The authors have developed a system for measuring FRC by the multiple breath nitrogen washout technique, which is suitable for use on intubated patients breathing with CPAP, IMV, or intermittent positive pressure ventilation (CONTROL) and on nonintubated patients. This system uses a pair of synchronized volume ventilators to permit a step change in inspired N2 fraction while providing therapeutic ventilatory support. A rapid-response nitrogen analyzer and a modified bellows spirometer are used for continuous measurement of airway nitrogen concentration and expired gas flow rate. FRC is calculated on-line by a digital computer. The system accuracy was tested on a mechanical lung simulator in the CPAP and CONTROL modes. The measured volume was found to agree within 58 +/- 52 ml of the actual volume in the CONTROL mode and within 104 +/- 22 ml in the CPAP mode. The system was also tested for repeatability by making duplicate FRC determinations in patients with respiratory insufficiency. In the 18 patients studied, the correlation coefficient of these duplicate measurements was r = 0.987 and the mean difference between measurements was 49 +/- 24 ml. This noninvasive system also provides data used to calculate anatomical deadspace by Fowler's method (VSDS) and uniformity of ventilation (V/V) for multicompartment lung models.

An automatic sigh feature for animal ventilators.

A device is described that provides a reliable automatic deep breath to an animal being ventilated with a typical laboratory ventilator. The deep breath is provided by closing a solenoid valve in the expiratory tubing causing the animal to inspire two, three, or four consecutive tidal volumes without expiration. The interval between deep breaths is selected to be every 1, 2, 4, 6, 8, or 10 min. Control of this interval is provided by simple integrated circuits. Total parts cost of this device is approximately $90.

Techniques of EEG frequency analysis for evaluation of uremic encephalopathy.

In an effort to provide nephrologists with practical, objective, and quantitative methods for evaluating uremic encephalopathy and the severity of uremia, we investigated five techniques for EEG frequency analysis, the selection of EEG samples for analysis, and the normal values for dominant frequency and the percent of EEG power from one through six Hz. The five techniques consisted of 1. handcounting, 2. use of a tape recorder and sonic analysis system to determine EEG power versus frequency, 3. use of the tape recorder and sonic analysis system to determine EEG voltage versus frequency, 4. on-line measurement of % EEG voltage from one through six Hz, and 5. use of base line crossovers to count the number of waves occurring at each frequency. All techniques were found to be satisfactory. The most significant difference between techniques depended on whether wave amplitude influenced the analysis (techniques 2, 3, and 4) or did not (techniques 1 and 5); in the former case slow wave activity was more evident than in the latter case. Our experience indicated that the determination of relative EEG power from each frequency (EEG power versus frequency or power spectral density) was the most practical and revealing mode of analysis, but any of these techniques would be valid and useful if employed with awareness of the difference in the normal range obtained by different techniques.