Cardiac and respiratory related electrical impedance changes in the human thorax

Magnetic impedance tomography

Tissue can be characterized by its electrical impedance, especially if measurement can be extended over a range of frequencies. Recently, there has been a great deal of interest in imaging the distribution of electrical impedance through the technique of electrical impedance tomography (EIT). However, EIT has a number of practical problems relating to the placement of electrodes on the body. Such contacts are not required to collect magnetic field data around an object through which current is flowing and thus this approach may be more practical than EIT in the clinical environment. This paper describes the technique of magnetic impedance tomography (MIT), which allows reconstruction of the current distribution from magnetic field measurements. The reconstruction techniques used to generate the images and the prototype data collection system are described. Images produced using data collected from discrete and distributed current phantoms and the thorax during human respiration are presented.

Cerebral blood-flow monitor for use in neonatal intensive care units

The implementation of a real-time multichannel system for monitoring cerebral blood-flow is described. The instrument relies on a completely modular architecture and is based on the principle of measuring the electrical impedance between a number of periodically sensed electrode pairs positioned around the subject's head. The whole setup is controlled by a host computer that performs several functions, such as real-time acquisition, analysis, display and data logging. Two operating options can be chosen by the user: a normal mode that allows continuous monitoring and a triggered mode in which the measurement cycle is automatically started by the occurrence of a preset condition in some other circulatory signal, e.g. the permanently available ECG signal. The design is considerably user-friendly and embodies a number of special safety precautions to take account of the peculiar condition of patients, usually newborn infants hospitalized in intensive care units.

EITS changes following oleic acid induced lung water

We present the results of using electrical impedance tomographic spectroscopy (EITS) to follow the changes in lung water induced by oleic acid. Measurements were made on three goats before and after the injection of oleic acid. In addition to the EITs measurements, lung water was also measured using a double-indicator technique. Large falls in lung electrical impedance were seen as a result of the increase in lung water but the size of the fall was a function of the frequency at which the measurements were made. These changes have been modelled using the Cole equation. Four-electrode measurements were also made on two extracted porcine lungs and Cole equation modelling carried out following the introduction of saline into the lungs. Results were similar in the two sets of animal experiments.

Cardiac-related changes in lung resistivity as a function of frequency and location obtained from EITS images

ECG-gated electrical impedance tomographic spectroscopy (EITS) measurements of the lungs were taken on seven normal subjects in the frequency range 9.6 kHz to 614.4 kHz. The results show that in late systole the resistivity p' relative to the R-wave (i.e. p' = 1 at the R-wave) decreases consistently within the lung. In addition there arises an increase in p' in early systole towards the periphery of the lung. Frequency behaviour of p' changes with location. At all times after the R-wave, in the centre of the lung p' is higher at higher frequency f whereas in the periphery it is lower at higher f. The principal decrease in p' can be explained by increasing pulmonary blood volume due to cardiac contraction. The early systolic increase is presumably due to venous return to the left atrium locally leading blood output from the right ventricle which is delayed by the windkessel effect. Based on a model taking extracapillary and capillary blood volume increase into account, the change in frequency behaviour of p' is explained by regional variations in extracapillary blood vessel size determining the relative contributions of extracapillary blood volume and capillary blood volume change to p' at a certain frequency.

Improvement of cardiac imaging in electrical impedance tomography by means of a new electrode configuration

Until now, electrical impedance tomography (EIT) has been used for cardiac imaging with the electrodes attached transversally at the level of the fourth intercostal space at the anterior side. However, the results obtained with this electrode configuration have been disappointing. The aim of the present study was to improve the measurement design of EIT for cardiac imaging. Therefore, magnetic resonance imaging (MRI) scans were analysed in two healthy subjects to determine the optimum anatomical plane in which atria and ventricles are clearly visually separated. From these findings, we proposed a new oblique plane at the level of the ictus cordis anteriorly and 10 cm higher posteriorly. EIT pictures obtained in the oblique plane revealed a better visual separation between the ventricles and atria than with the electrodes attached in the transverse plane. Comparison between volume changes measured by means of MRI and impedance changes in different regions of interest measured with EIT were performed with the electrodes in the proposed oblique plane. Ventricular and atrial volume changes measured by MRI show the same pattern as do impedance changes measured by EIT. Furthermore, we assessed the reproducibility and validity of the oblique electrode configuration in ten healthy mate volunteers during rest and during exercise compared with the currently used transverse electrode configuration. The reproducibility coefficient assessed from repeated measurements with the electrodes attached in the oblique plane was 0.98 at rest and 0.85 during exercise. For the transverse plane the reproducibility coefficient was 0.96 at rest and 0.66 during exercise. The well-known increase in stroke volume during exercise is 40% in healthy subjects. The increase in impedance change during exercise compared with rest was 34 +/- 13% (20-59%) for the oblique plane and 68 +/- 57% (13-140%) for the transverse plane. From these results we infer that the stroke volume is assessed more accurately by using the oblique plane. From these findings, we conclude that the oblique plane improved the cardiac measurements, because (i) a better spatial separation of the heart compartments is obtained, (ii) the results are more reliable and (iii) measurements during exercise are more accurate with the electrodes attached in an oblique plane.

Deconvolved electrical impedance spectra track distinct cell morphology changes

A two-component Cole-Cole model was used to obtain statistically significant fits to 100-Hz-10-MHz impedance data for EMT-6 mouse tumors during the progressive histological changes induced by hyperthermia. The resulting fitting parameters were used to deconvolute and reconstruct the two dispersions which confer the predominant impedance features to this tissue. The time-dependent changes of these two dispersions were correlated with the concurrent, heat-induced morphological changes of the tumors' cells. The higher frequency dispersion (fc approximately 1 MHz) was identified with a Maxwell-Wagner relaxation process linked to the overall volume response of the cells. The lower frequency dispersion (fc approximately 10 kHz) represented an alpha-relaxation associated with the surface morphology and integrity of the plasma membranes. Thus, two aspects of the characteristic cellular damage sequence in these tumors were found to be separately discernable and trackable in real-time using the impedance data.

Data processing techniques for serial EIT spectroscopy images: a review of some preliminary results

Mutlifrequency EIT imaging should allow specific organs within the body to be identified by their impedance spectrum, and the use of parametric imaging should lead to a much greater freedom from movement artefacts. This will make EIT more attractive as a monitoring technique, but the data rate will require automated processing of the images. The application of dynamic regions of interest, generated on a frame by frame basis, is described, with examples from the imaging of neonatal lungs and adult stomach. The lung can be objectively identified on a single frame from the fRSC, SC and RC images, but the stomach could only be identified on the dynamic images.

Measured and expected Cole parameters from electrical impedance tomographic spectroscopy images of the human thorax

Electrical impedance tomographic spectroscopy (EITS) images have been recorded from a group of 12 normal subjects using frequencies from 9.6 kHz to 1.2 MHz. The impedance changes with frequency have been modelled on a pixel by pixel basis to produce parametric images as a means of characterizing tissue. The modelling was based on the Cole equation. The lungs are seen as areas of high characteristic frequency and low time constants SC and RS. The R/S images are much less uniform over the region of the lungs. Values characterizing the lung and cardiac regions are given. The results appear to be consistent with a model for the lungs whereby the model parameters can be related to alveolar structure and composition.