Surgery – Truss – Pad
Patent
1992-12-08
1994-05-17
Smith, Ruth S.
Surgery
Truss
Pad
A61B 505
Patent
active
053118782
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
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 (U.S. Pat. No. 4,617,939 corresponding to both the content of the U.S. patent being incorporated hereinto by reference) 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 only be 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.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a method of real time imaging using electrical impedance tomography, comprising: locations around a body to be investigated; electrodes; and multiplication of the measured data set with a matrix of weights, whereby the measurement noise is minimised by; electrodes simultaneously; and dividing the drive current into potential measurements before image reconstruction.
According to a second aspect of the invention there is provided apparatus for carrying out the method defined above, comprising: other electrodes; multiplication of the measured data set with a matrix of weights wherein there is further provided: electrodes simultaneously; measurement; and and to divide the drive current into the potential 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 .sqroot.13 (11.1 dB). 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 20 dB better than the earlier serial data collection system. The image reconstruction algorithm may be a non-iterative back-projection, or an iterative al
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Medical Physics, vol. 16, No. 2; D. C. Barber: "A review of image reconstruction techniques for electrical impedance tomography".
Barber David C.
Brown Brian H.
British Technology Group Limited
Smith Ruth S.
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