Electrical impedance tomography. Comments on reconstruction algorithms.
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Algorithms↗
Biomedical subjects
J C Newell
Publications and source records attributed to J C Newell.
Improving physical fitness and reducing injuries in physically demanding jobs.
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Accidents, Occupational↗
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.
Diagnostic Imaging↗
Oxygen delivery and consumption in head-injured and multiple trauma patients.
Critically ill patients often demonstrate that whole body oxygen consumption (VO2) is dependent on oxygen delivery (DO2). In this retrospective study, the relationship of VO2 to DO2 in patients with isolated head injury (HI, n = 18) was compared to that in patients with multiple trauma (MT, n = 60) without serious head injury. Mean pulmonary capillary wedge pressure, central venous pressure, arterial PCO2, cardiac index, and oxygen delivery were significantly lower in HI, but oxygen consumption was not different in the groups. In both groups, changes in DO2 (delta DO2) within each patient were significantly correlated with changes in VO2 (delta VO2) in that same patient. This relationship was not different between the HI patients, (delta VO2 = (0.20 +/- 0.02) delta DO2), and the MT patients (delta VO2 = (0.17 +/- 0.01) delta DO2). When these groups were further divided into those with high hematocrit (greater than 32%) and low hematocrit (less than 32%), HI patients with a low hematocrit demonstrated a steeper regression slope, with 26 +/- 3% of the DO2 change being reflected in the VO2 change. This was significantly greater than the slope in HI patients with high hematocrit (13 +/- 3%) and the MT patients at high (19 +/- 2%) or low (16 +/- 2%) hematocrits. These data show a correlation between changes in oxygen delivery and consumption that is similar in both head-injury patients and multiple trauma patients without serious head injury. This relationship was greatest in head-injured patients at low hematocrit. This relationship of VO2 and DO2 in both groups suggests an influence of neurohumoral factors rather than local tissue phenomena.(ABSTRACT TRUNCATED AT 250 WORDS)
Adolescent↗
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.
Algorithms↗
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.
Diagnostic Imaging↗
Patient-ventilator partitioning of the work of breathing during weaning.
The work of breathing and its division between the patient and the mechanical ventilator were studied during weaning of 5 post-operative surgical patients from Synchronized Intermittent Mandatory Ventilation. Work by the patient (WP) was estimated by integrating the product of flow and pressure over time during intervals when waveforms indicated patient effort; ventilator work (WV) was similarly estimated during positive pressure inspirations. The ratio of WP to the rate of work on the lungs (WL) increased progressively during weaning from 0.14 +/- 0.04 to 1.2 +/- 0.15 while WV/WL dropped from 1.31 +/- 0.08 to 0.13 +/- 0.11. Work on the lungs decreased during weaning. This was due in part to significant improvements in lung mechanics: resistance decreased from 9.9 +/- 0.9 to 6.1 +/- 1.6 cmH2O/1/s and compliance increased from 58 +/- 17 to 102 +/- 30 ml/cmH2O. The patient and ventilator work ratios, and the work of breathing quantify factors which may be directly useful to the clinician and to future systems to automate weaning.
Airway Resistance↗
Estimating lung mechanics of dogs with unilateral lung injury.
Extended least-squares algorithms using transpulmonary pressure and airway flow data from ventilatory waveforms were studied for their ability to track parameters of one- and two-compartment models of lung mechanics. A recursive extended least-squares algorithm with discounted measures estimated parameters of discrete-time models during synchronized intermittent mandatory ventilation. In tests on seven dogs developing oleic acid-induced unilateral hemorrhagic pulmonary edema, the one-compartment estimator responded rapidly and appropriately to changes in mechanics: compliance fell to 0.55 +/- 0.15 of its initial value and resistance rose by a factor of 1.8 +/- 0.5 in 3 h following injection of oleic acid. One-compartment parameter estimates revealed a difference between the airway resistance of inspiration and expiration. Two-compartment estimates were seldom physiologically plausible. The difference between inspiratory and expiratory resistance may have caused the two-compartment estimator to fail when applied to data from the entire respiratory cycle; when only expiratory data were used for estimation, the two-compartment estimates were meaningful. These estimates demonstrated increasing lung inhomogeneity after oleic acid was injected; at the end of 3 h, the ratio of the time constants of the two compartments ranged from 5 to 20 in six of the seven dogs. We conclude that the one- and two-compartment estimates may be combined to provide a meaningful assessment of lung mechanics.
Animals↗
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.
Computer Simulation↗
Effect of cardiac index and hematocrit changes on oxygen consumption in resuscitated patients.
Oxygen consumption (Vo2) has been found to depend on oxygen delivery (Do2) following resuscitation from hemorrhage in both humans and animals. The relative influence of blood flow and arterial oxygen (O2) content, the components of Do2, has not been separately assessed. To determine the relative contribution of content and flow, we determined Do2 and Vo2 while making systematic changes in cardiac index (CI) and hematocrit (HCT). Fourteen patients were studied within 36 hr of hypotension from which they were resuscitated to a HCT of 27.9 +/- 0.4% (mean +/- SEM). Following initial hemodynamic measurements, CI was manipulated by changing end expiratory pressure by increments of +/- 5 cm H2O and measurements were repeated. Patients were then transfused overnight to raise their HCT to 36.7 +/- 0.5% and measurements were repeated, varying CI in the same manner. The increase in HCT resulted in significant (P less than 0.05) increases in O2 delivery (+ 130 +/- 33 ml/min/m2), arterial O2 content (+ 3.9 +/- 0.3 vol%), and mixed venous O2 content (+ 3.7 +/- 0.4 vol%). O2 extraction decreased by 6 +/- 1% from 30 +/- 2%. The change in HCT did not alter Vo2 (143 +/- 7 ml/min/m2), CI (3.6 +/- 0.2 L/min/m2), or intrapulmonary shunt (18.1 +/- 1.7%). However, as CI was changed at both levels of HCT, there were changes in Vo2 directly dependent on Do2. We conclude that oxygen consumption in patients resuscitated from hemorrhage may be influenced by oxygen delivery and that this influence is related more to flow than to arterial content.
Adult↗
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.
Electricity↗
An electric current tomograph.
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Electric Conductivity↗
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.
Electric Conductivity↗
Influence of hematocrit on cardiopulmonary function after acute hemorrhage.
The 'optimal' hematocrit to which patients should be resuscitated after shock and trauma is controversial. To test the hypothesis that sufficient oxygen delivery can be provided at a lower hematocrit without impairing oxygen consumption or hemodynamic function, 25 patients were prospectively studied immediately following injury and/or acute hemorrhage. Patients were randomized to have their hematocrits (HCT) maintained near 30% (29.7 +/- 0.4% (M +/- SEM); n = 12) or 40% (38.4 +/- 0.6%, n = 13). Cardiopulmonary parameters were measured twice a day for 3 days. Statistical analysis used a repeated measures analysis of variance with patient age, and ventilator parameters (FIO2, PEEP, and ventilator mode) as covariates. Arterial and venous O2 saturations were not significantly different at different hematocrits, although arterial and venous O2 contents were lower at 30% HCT (a = 14.1 +/- 0.2 m10(2)/dl, v = 10.1 +/- 0.3 m10(2)/dl; vs. a = 17.4 +/- 0.4 m10(2)/dl, v = 13.6 +/- 0.6 m10(2)/dl; p less than 0.05). This resulted in a lower oxygen delivery at the lower HCT. Between the two groups, there also was no significant difference in cardiac index (overall mean, 3.64 +/- 0.16 ml/min/m2), heart rate (99 +/- 4 bpm), systemic vascular resistance (1,058 +/- 55 dyne-sec/cm5), or left ventricular stroke work index (4.3 +/- 0.3 X 10(6) dyne-cm/m2). Intrapulmonary shunt was higher with higher hematocrit (22.6 +/- 2.4% at 40% HCT vs. 14.6 +/- 1.6% at 30% HCT; p less than 0.05) with no difference in end-expiratory pressure.(ABSTRACT TRUNCATED AT 250 WORDS)
Acute Disease↗
Improvement in arterial oxygen control using multiple-model adaptive control procedures.
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Algorithms↗
Regression of calculated variables in the presence of shared measurement error.
To test hypotheses regarding relations between meaningful parameters, it is often necessary to calculate these parameters from other directly measured variables. For example, the relationship between O2 consumption and O2 delivery may be of interest, although these may be computed from measurements of cardiac output and blood O2 contents. If a measured variable is used in the calculation of two derived parameters, error in the measurement will couple the calculated parameters and introduce a bias, which can lead to incorrect conclusions. This paper presents a method of correcting for this bias in the linear regression coefficient and the Pearson correlation coefficient when calculations involve the nonlinear and linear combination of the measured variables. The general solution is obtained when the first two terms of a Taylor series expansion of the function can be used to represent the function, as in the case of multiplication. A significance test for the hypothesis that the regression coefficient is equal to zero is also presented. Physiological examples are provided demonstrating this technique, and the correction methods are also applied in simulations to verify the adequacy of the technique and to test for the magnitude of the coupling effect. In two previous studies of O2 consumption and delivery, the effect of coupled error is shown to be small when the range of O2 deliveries studied is large, and measurement errors are of reasonable size.
Cardiac Output↗
Pulmonary and cardiovascular consequences of immediate fixation or conservative management of long-bone fractures.
We randomly assigned patients with multiple trauma who had tibial or femoral fractures to one of two groups--one group received immediate fixation of all fractures, and the second group received conservative orthopedic management, consisting of traction or plaster casts. Studies were conducted twice each day for four days following injury. Mean cardiac index was 1.3 L/min/m2 higher and mean shunt was 5.2% lower in the immediate fixation group compared with the group receiving conservative treatment. Other pulmonary and systemic hemodynamic variables did not differ between the groups. The incidence of fat macroglobules in blood aspirated from the pulmonary capillaries was higher when compared with that in pulmonary arterial blood but was not significantly different between the two treatment groups. Platelet count was significantly lower and fibrinogen concentration was significantly higher in the group receiving immediate fixation. We found no diagnostic significance of the incidence of fat macroglobules in samples of blood aspirated from the pulmonary circulation. We conclude that patients receiving immediate fixation had less pulmonary dysfunction following multiple trauma and long-bone fractures.
Adolescent↗
Mathematical coupling of data: correction of a common error for linear calculations.
The need frequently arises in the scientific environment to investigate the relationship between quantities that are calculated from a common set of directly measured variables. However, the presence of error in the common set of measured variables distorts the relationship among the calculated quantities and can lead to incorrect conclusions. This article presents a method of correcting for such distortions in the Pearson correlation coefficient and in the linear regression coefficient for linear calculations involving two measured variables. The errors considered may be either independent of, or proportional to, the value of the variable being measured. Tests to determine whether these popular coefficients have values significantly different from zero are presented. An example from the physiology literature is presented to illustrate these techniques.
Mathematics↗