GB2246634A - Real-time electrical impedance tomography - Google Patents

Abstract

A method of, and apparatus for, real-time imaging are described employing a plurality of contact electrodes (1) located for example around the thorax (2) with simultaneous measurement of voltage differences at adjacent pairs of electrodes (1), digital demodulation of the voltages, and measurement and division of the drive current into the voltage measurements before image reconstruction by the use of transputers (20, 22, 24 and 26).

Description

REAL-TIME ELECTRICAL IMPEDANCE TOMOGRAPHY SYSTEM This invention relates to a real-time electrical impedance tomography (EIT) system to provide for example blood flow imaging.

Tomography systems are described in GB-PS 2 119 520B or GB-PS 2 160 323B (US-PS 4 617 939 corresponding to both) or WO 89/09564 on which the present applicants/inventors are named as inventors, and in which the impedance imaging system consists of a data collection system (the data being measured potentials between pairs of electrodes in a series of contact electrodes attached around a human or animal body, and in which another pair is a "drive" pair between which currents are caused to flow) and an image reconstruction system.

Frames of data could be collected serially by the data collection system. at twenty-four frames per second but image construction could onlybe carried out at approximately one frame per second. Whilst this is not a disadvantage in extracting certain slowly changing physiological data there are other data requirements where it is necessary to produce images much more quickly - for example, when observing blood flow in the body during the cardiac cycle.

In order to produce a system able to produce images much more quickly two developments are required. Firstly, a much faster digital processor or computer in order to implement the image construction algorithm rapidly.

Secondly, the data collected from the human body has to be improved in quality and in particular the noise level reduced.

In the previous system noise level could be reduced by averaging signals over several seconds before constructing an image. However, averaging is not possible in a system running rapidly in real-time and therefore the noise level must be reduced by other means.

According to a first aspect of the present invention, there is provided a method of real time imaging using electrical impedance tomography, comprising a) arranging a plurality of pairs of surface electrodes at spaced apart locations around a body to be investigated; b) applying constant current drive to selected electrodes; c) effecting high impedance voltage measurement on adjacent pairs of other electrodes; and d) forming images using an algorithm implemented as a single matrix multiplication of the measured data set with a matrix of "weights", characterised in that the measurement noise is minimised by; (i) measuring the adjacent pair voltage differences simultaneously; (ii) de-modulating the voltage measurement by digital correlation, and (iii) measuring the drive current in the same way as the voltage differences, and dividing the drive current into voltage measurements before image reconstruction.

According to a second aspect of the invention there is provided apparatus for carrying out the method defined above, comprising: a) a plurality of pairs of contact electrodes; b) means to apply constant current drive to selective electrodes; c) means to effect high impedance voltage measurements on adjacent pairs of other electrodes; d) means to form images using an algorithm implemented as a single matrix multiplication of the measured data set with a matrix of "weights" characterised in that there is further provided: (i) means to measure the adjacent-pair voltage differences simultaneously; (ii) digital correlators to demodulate the voltage measurement; and (iii) means to measure the drive current in the same way as the voltage differences and to divide the drive current into the voltage measurements before image reconstruction.

Thus, the required reduction in system noise is achieved by, inter alia, collecting the profiles of data in parallel instead of in series. Data is collected by first driving electrical current between a pair of electrodes and then recording the resulting voltages between all other adjacent pairs of electrodes. This set of voltages is referred to as a profile of measurements. By. recording all the voltages in one profile simultaneously each measurement can be allowed to take longer and hence can be made to a higher accuracy. For a system with sixteen electrodes the improvement to be expected by collecting profiles of measurements in parallel rather than in series is /13 Noise Noise reduction is also achieved by the use of Digital Signal Processing (DSP) technology to carry out all the signal demodulation digitally.A matched filter has been implemented using one DSP system for every four input signals; four DSP systems are used for a sixteen-electrode system.

Matched filters have previously been implemented in analogue electronics but the analogue multipliers involved are noisy devices and so the full benefit of the matched filter technique has not been achieved. By using a DSP system the measured noise performance is 20dB better than the earlier serial data collection system. The image reconstruction algorithm may be a non-iterative back-projection, or an iterative algorithm.

One application or adaptation of the invention is in the measurement of blood flow to organs such as the heart, lungs and brain.

The electrical resistivity of blood is approximately 1.6 Ohm metres. It varies in a well described manner with the haematocrit of the blood. If saline (0.9% solution) which has a much lower resistivity is introduced into the blood then the resistivity of the mixture will be less than that of the blood alone. For example, if 10ml of saline of resistivity 0.4 Ohm metres is introduced into a person with a blood volume of 5000ml and resistivity 1.6 Ohm metres then the resistivity of the mixture will fall by about 0.5%. If the time course of this change following an injection of saline into the venous system is measured then the blood flow to that organ can be calculated (Chinard F.P., Enns T and Nolan M.

Circulation Research. Vol X 473-491, 1962) The technique has been used to calculate cardiac output by injecting saline and then measuring the resulting resistivity change in an artery.

In the application or adaptation of the invention for this purpose, however, real-time impedance tomographical imaging will enable measurements to be made non-invasively.

The noise level from the real-time impedance imaging system is sufficiently low to allow changes of 0.5% to be observed relatively easily, and this gives rise to the possibility of obtaining accurate measurements of blood flow by impedance angiography.

Another application is to observe the normal variations in lung resistance during the respiratory and cardiac cycles. As air enters the lungs during respiration there is a proportional increase in tissue resistivity. By monitoring these changes in real-time it is possible for a clinician to extract data relevant to respiretory performance.

There are also changes in lung resistivity during the cardiac cycle as the volume of blood perfusing lung tissue changes.

These changes are small (typically 2%) but can be observed using the low noise capability of the system and so used to monitor conditions such as plumonary embolism.

The noise levels measured from the method and apparatus in accordance with the invention are surprisingly low and are now limited only by thermal noise.

The invention is illustrated in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a data acquisiton system for the method and apparatus of the invention; and Figure 2 is a block diagram of an image reconstruction processor.

As indicated in Figure 1, a plurality of surface contact electrodes 1 are arranged to circumscribe the thorax 2 of a body 3 to be investigated, each electrode 1 being connected by a lead 4 to a plurality of input amplifiers 5, and output 6 of which is to a DSP demodulator 7 incorporating in fact sixteen parallel demodulators. Power input to the input amplifiers 5 is by leads 8 from a current drive multiplexer 9, in turn supplied by lead 10 from a voltaged current converter 11, in turn connected by lead 12 to a timing device 13 for the whole system, the elements 1 and 4 to 13 constituting a data acquisition system (DAS) represented by box DAS in Figure 2.

Also indicated in Figure 1 are three additional electrodes 14, all of which are connected by leads 15 to an ECG gating electronics system 16 and the third of which is connected by an additional lead 17 to the input amplifier 5, although an ECG is not essential in for monitoring spikes, representing heart beats, and transmitting this data via a lead 18 to the DSP demodulator 7.

An output 19 from the demodulator 7 is to a control transputer 20 connected by lead 21 to a pre-processing and error checking/data laundering transputer 22, a lead 23 to an image construction transputer 24, and by a lead 25 to a display transputer 26, at which data can be captured for subsequent analysis and/or processing.

Claims (7)

1. A method of real-time imaging using electrical impedance tomography, comprising : a) arranging a plurality of pairs of surface electrodes at spaced apart locations around a body to be investigated; b) applying constant current drive to selected electrodes; c) effecting high impedance voltage measurement on adjacent pairs of other electrodes; and d) forming images using an algorithm implemented as a single matrix multiplication of the measured data set with a matrix of "weights", characterised in that the measurement noise is minimised by; (i) measuring the adjacent pair voltage differences simultaneously; (ii) de-modulating the voltage measurement by digital correlation, and (iii) measuring the drive current in the same way as the voltage differences, and dividing the drive current into voltage measurements before image reconstruction.

2. A method as claimed in Claim 1, wherein the algorithm is a non-iterative, back-projection algorithm.

3. Apparatus for carrying out the method of Claim 1, comprising: a) a plurality of pairs of contact electrodes; b) means to apply constant current drive to selective electrodes; c) means to effect high impedance voltage measurements on adjacent pairs of other electrodes; d) means to form images using an algorithm implemented as a single matrix multiplication of the measured data set with a matrix of "weights" characterised in that there is further provided: (i) means to measure the adjacent pair voltage differences simultaneously; (ii) digital correlators to demodulate the voltage measurement; and (iii) means to measure the drive current in the same way as the voltage differences and to divide the drive current into the voltage measurements before image reconstruction.

4. Apparatus as claimed in Claim 3, wherein the means to measure the adjacent-pair voltage differences simultaneously comprises a plurality of transputers.

5. Apparatus as claimed in Claim 4, wherein the transputers are a control transputer, a pre-processing and error checking transputer, an image construction transputer, and a display transputer.

6. Apparatus as claimed in any one of Claims 3 to 5, wherein the means to measure the drive current is a current drive multiplexer.

7. Apparatus for carrying out the method of Claim 1 substantially as hereinbefore described with reference to the accompar.ying drawing.