A D Leathard

Cardiac and respiratory related electrical impedance changes in the human thorax

Electrical impedance measurements have been made from the human trunk over the frequency range 9.6 kHz to 614 kHz. Measurements have been made from 12 normal subjects and the amplitude of the impedance changes associated with the cardiac and respiratory cycles have been recorded. It was found that the real part of the impedance fell to 64.0% of its low frequency value over the measured range of frequencies and that the changes associated with respiration fell in a similar manner. However, the cardiac related changes fell more rapidly with increasing frequency to 28.2% of the low frequency value. The origin of the measured changes is discussed with a view to understanding why the cardiac related changes fall more rapidly. It is not possible to relate in any simple way the frequency dispersion of a single component to that of the whole trunk. However, the results are consistent with the lungs being the major origin of both the cardiac and respiratory related components. The origin of the cardiac related impedance changes could be the pulsatile volume changes in the pulmonary tree. These could be shunted by nonpulsatile lung tissue that has decreasing impedance at high frequency and thus decreases the relative magnitude of the cardiac related changes. This hypothesis needs to be tested using localized measurements from the thorax and 3D modeling of the trunk.<<ETX>>

Multi-frequency imaging and modelling of respiratory related electrical impedance changes

Two studies concerning multi-frequency impedance measurements are presented. The first uses tetrapolar measurements made on the thorax and the second electrical impedance tomography images, also made from the thorax. The way in which the impedance and the changes in impedance with ventilation depend upon frequency are investigated using Cole-Cole modelling and also a physiological model of lung tissue. There is an excellent fit to the Cole-Cole model, and the results show that it should be possible to identify tissue on the basis of the impedance spectrum and the spectrum of the changes in impedance during breathing.

Blood flow imaging using electrical impedance tomography

It is shown that a real-time electrical impedance tomography (EIT) system can be used to image the flow of saline through the human vascular system. A 10 ml bolus of 0.9% saline injected intravenously distal to an EIT imaging plane allows venous flow to be observed. Measurements on a cylindrical tank with flow along axial conductive tubes have been used to establish that the area under a concentration against time curve can be obtained from the EIT images and used to determine the flow rate down the tube. In vivo results show that flow images of the venous system in a limb can be obtained and that there is adequate sensitivity to follow the passage of a saline bolus though the cardiac chambers.

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.

A fast parametric modelling algorithm with the Powell method

This paper presents a model that comprises only two parameters (R/S, fr) and the application of three function minimization algorithms (simplex, Powell and modified Powell) to this model to obtain parametric images. Comparisons among the three algorithms in terms of efficiency and reliability were carried out. It was found that, with proper initialization by taking the shape of the modelled data into consideration, the minimization function can be approximated by a quadratic function near the minimum point, therefore the iteration times can be minimized in the modified Powell method. The results show that with the modified Powell method a substantial reduction of computation time can be achieved in the parametric imaging. This makes it possible to obtain a 16 x 16 parametric image in 1 s.

Transport of gastric contents (electric impedance imaging)

A planar array of electrodes has been used to provide a longitudinal section of the stomach. Impedance changes at the gastric frequency of 0.05 Hz can be detected. The changes are mainly located around the periphery of the stomach image, suggesting that they are the result of movement of the stomach wall. The generation of a vector histogram of wall movement gives a noninvasive method which appears to quantify the peristaltic waves which produce transport in the stomach.

Modelling of cardiac-related changes in lung resistivity measured with EITS.

Resistivity data from 9.6 kHZ to 1.2 MHz were recorded from eight normal subjects using an electrical impedance tomographic spectroscopy (EITS) system and then averaged to a mean cardiac cycle using the ECG gating technique. The Cole-Cole model, that is, extracellular resistance R connected in parallel with intracellular resistance S and membrane capacitance C in series, with a distribution parameter a, was applied to model the frequency characteristics and to produce parametric images. During systole, SC and RC were found to decrease and FR increase. The changes in R/S were not consistent among the subjects. We estimated the peak changes in R, S and C to be -2.5%, -3.3% and -7.6% respectively. The results can be explained by considering the blood vessels as spheres of different sizes with blood inside them. The decrease in R during systole might be caused by the increased blood content in relatively large vessels, whereas that in S by the increased blood volume in relatively small vessels. The capacitance of blood is normally smaller than that of lung tissue, whereas FR blood is higher than that of lung tissue. Hence, as blood content increases, C should decrease and FR increase.

Identifying oesophageal contents using electrical impedance tomography

Investigations have been carried out using the Sheffield mark II real-time EIT system, to look at changes in conductivity associated with swallowing. A ring of 16 electrodes was placed around the neck of 10 subjects, who then performed swallows with four liquids of different conductivities, ranging from water (sigma = 0.03 mS cm-1) to salty soup (sigma = 35.8 mS cm-1). Results showed that the conductive and non-conductive liquids could be distinguished. Bolus transit times were calculated from region of interest curves, and the average transit time for the 10 subjects was found to be 320 +/- 100 ms.

Noise equalization within EIT images

We first describe experiments designed to measure the spatial distribution of noise within an electrical impedance tomographic image of a saline-filled tank. These experiments show that the noise increases by a factor of up to 30 from the periphery towards the centre of the image. We then propose a method of noise equalization that depends upon spending longer collecting the smaller potentials than the larger potentials. The method is tested and shown to give an improvement in noise uniformity of about 16 dB.