David G. Gisser

Electrode models for electric current computed tomography

A mathematical model for the physical properties of electrodes suitable for use in electric current computed tomography is discussed. 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, and 29.5 Omega -cm were studied. Values of effective contact impedance used in the numerical approximations were 58.0, 35.0, 15.0, and 7.5 Omega -cm/sup 2/, 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.<<ETX>>

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

Presents the design, implementation, and performance of Rensselaers 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.<<ETX>>

An electric current tomograph

A description is given of an instrument designed to acquire data for the construction of images of internal body structures based on measurements of electrical impedance made from a set of electrodes applied around the periphery of the body. The instrument applies currents at 15 kHz in any desired pattern to 32 electrodes and measures the resulting voltage at each electrode. The construction of a test phantom is also described and the results of initial studies showing the distinguishability of targets of differing sizes and conductivities placed in the phantom are reported. The system is able to distinguish the presence of 9-mm-diameter insulators or conductors placed in the center of a 30-cm-diameter circular tank of salt water. This system is capable of implementing an adaptive process of produce the best currents to distinguish the unknown conductivity from a homogeneous conductivity.<<ETX>>

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.

Detection and imaging of electric conductivity and permittivity at low frequency

The authors discuss low-frequency electrical impedance imaging, the process of constructing images of the electrical impedance of a bodys interior based on measurements of voltage and current made at the bodys surface. The electrical impedance accounts for both resistivity and permittivity. It is shown how permittivity can be exploited to improve the performance of an electrical impedance imaging system. It is shown that explicit use of the independent information in the data due to the permittivity will enhance a systems ability to distinguish objects in the interior of a body. In addition. the results of experiments performed using the Rensselaer ACT 2 system on a saline bath containing various objects are reported. These objects include both living tissue and metal conductors with oxide layers. The systems ability to distinguish these objects is demonstrated. and gray scale images of both their resistivity and permittivity distributions are given.<<ETX>>

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

It is shown that when a multiplicity of electrodes are attached to a bodys surface, the voltage data are most sensitive to changes in resistivity in the bodys interior when voltages are measured from all electrodes, including those carrying current. This assertion, which is true despite the presence of significant levels of skin impedance at the electrodes, is supported both theoretically and by experiment. Data were first taken for current and voltage at all electrodes. Then current was applied only at a pair of electrodes, with voltages measured on all other electrodes. 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 non-current-carrying electrodes had higher noise levels. It is concluded that in multiple electrode systems for electric-current-computed tomography, current should be applied and voltage should be measured from all available electrodes.<<ETX>>

Reactive effects in impedance imaging

The authors identify theoretical and practical benefits of including reactive effects in impedance imaging. Estimates of the size of the smallest inhomogeneity that can be detected by an impedance tomograph that uses reactive voltages show that an improvement in resolution can be expected. For physiologically meaningful data, a reduction of 25% in the radius of the smallest detectable inhomogeneity is feasible. Experiments demonstrate an improvement in detecting biological tissue in a saline bath. Design considerations for a system to achieve these goals are included.<<ETX>>

An automated data collection system for membrane transport experiments : Part I. Test of onsager reciprocity

Abstract This work presents a direct experimental examination of the linear phenomenological flux equations from the thermodynamics of irreversible processes for membrane transport. Results are given which confirm Onsager reciprocity in a NaClH2O—anion exchange membrane system with concentration and hydrostatic pressure differences. Data for isothermal, non-steady-state experiments are collected from a computerized and automated membrane transport apparatus. For the first time, the solvent and solute flux equations are solved simultaneously, and the four phenomenological transport coefficients are obtained from a single experiment using an ellipsoid algorithm for non-linear programming. It appears that the dependence of the L coefficients on the logarithmic mean transmembrane concentration, cse, is more important than the role of the water flux as an indicator of the limit of the linear region for membrane transport processes. Only in those experiments where cse was held nearly constant was Onsager reciprocity obtained.

On-Line Blood and Respiratory Gas Analysis

Data from a patient receiving ventilatory assistance are processed by computer to calculate pulmonary shunt, dead space/tidal volume ratio, oxygen uptake and delivery, and carbon dioxide elimination and delivery. The computations are based on routines described by Kelman and by Severinghaus, but modified to match limitations of testing imposed by the requirements for ventilatory assistance. Analyses are performed sequentially on arterial and venous blood samples and on respiratory gas samples. The output of the blood gas analyzers is fed on-line to a computer together with other data, such as patient identification, which are manually entered at a keyboard and with thumb-wheel switches. The computer processing begins with determining the concentration of oxygen and carbon dioxide in whole blood and of bicarbonate in plasma. The shunt equation is used to calculate a virtual shunt at therapeutic concentrations of inspired oxygen. Dead space/tidal volume ratios are corrected for mechanical dead space in the respiratory circuit. The analyzed results are returned to the operator within seconds via a video display. Since the data include blood samples from multiple patient sites, a cross-comparison is made by the computer and the operator is informed of unusually large differences in values.

Acute pulmonary edema assessed by electrical impedance imaging

Acute Iung injury, with decreased cardiac output, and arterid PO2 and increased pulmonary arterial pressure was produced by intravenous injection of oleic acid in dogs. During the next three hours, resistivity decreased in two regions of the chest. corresponding lo the lungs, as measured by elecuical impedance imaging. Overall resistivity in the enlire image decreased from 179 to 143 ohm-cm in 3 hours, while the peak lung resistivity decreascd from 248 to 207 ohm-cm. We conclude that impcdance imaging holds promise for clinical monitoring of acute pulmonary edema. Introduction Pulmonary edema and Adult Respiratory Discress Syndrome remain important complications following major surgery and multiple muma. Clinical diagnosis relies on physiological tests of gas exchange, including merial blood gas measurements. wilh calculation of intrapulmonary shunt, auscultation, and chest radiographs. The present study was conducted to determine if Electrical Impedance Imaging can also provide a clinically useful assessment of pulmonary edema. Electrical Impedance Imaging is a technique for forming images of the interior of the body based on electrical measurements made From electrodes applied to the skin. Known currencs are passed through the bcdy, and the voltages resulting on the electrodes are measured. A reconstruction algorithm then uses these data to solve the inverse problem and clsplay a map or image of the resistivity in the region surrounded by the electrodes. Since significant decreases in resistivity may occur in the lungs during edema formation, they might be detectabIe by Impedance Imaging. This hypothesis is an extension of an earlier study which showed decreases in overall thoracic resistivity in pulmonary edema patients [ I ] , We have studied dogs wilh pulmonary edema induced by intravenous infusion of oleic acid, a well-studied model of acule lung injury [2]. Images of the lungs showed progressive localized decreases in resistiviv throughout Ihe edema forma-