Process for producing optimal current patterns for electrical impedance tomography

• US 5,588,429 A
• Filed: 10/05/1994
• Issued: 12/31/1996
• Est. Priority Date: 07/09/1991
• Status: Expired due to Term

• Current Assignee: Rensselaer Polytechnic Institute
• Original Assignee: Rensselaer Polytechnic Institute
• Inventors: Cheney, Margaret, Isaacson, David
• Primary Examiner(s): Sykes, Angela D.
• Application Number:
  US08/317,678
• Time in Patent Office: 818 Days
• Field of Search: 128/734, 128/741, 128/693, 364/413.02, 364/413.13, 364/413.15
• US Class Current: 600/547
• CPC Class Codes: A61B 5/0536 Impedance imaging, e.g. by tomography
• CPC Class Codes: G01R 27/26 Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables

Abstract

In electrical impedance tomography systems, the precision of voltage measurement is a critical factor in the results. Usually, the voltage values to be measured are limited by the necessity of limiting currents through the body to safe values. An effective method for increasing the apparent precision of the voltmeters is to use non-sinusoidal current patterns that produce the largest voltage variations in regions of most importance. This invention discloses several improvements in the methods by which the images resulting from any system of hardware that permits simultaneous injection of currents to all electrodes and voltage measurements at all electrodes, may be improved. One such improvement is a technique to find the shapes of the best current patterns to distinguish two different distributions of admittivity, conductivity, and permittivity in the region surrounded by electrodes. Another is a more complex procedure for finding the best shapes of the current patterns to best characterize an unknown pattern of admittivity, conductivity, or permittivity. Yet another is a procedure for calculating the values of voltages that would have been measured had sinusoidal sets of current been used, when actually using non-sinusoidal current patterns. This permits any standard reconstruction algorithm based on sinusoidal currents to be used with non-sinusoidal currents.

Claims

1. In an electrical impedance tomography system that includes a body with an array of electrodes on its surface, means for injecting spatial patterns of current into the electrodes consecutively, subject to a limitation on the maximum current in any electrode, and means for measuring the real and reactive voltage components at all the electrodes, an improved method for finding the values of an unknown distribution of permittivities from which images may be formed, comprising:

(a) selecting an arbitrary guessed permittivity distribution and applying an arbitrary guessed set of current patterns to the electrodes to generate a voltage pattern on the array for each current pattern;

(b) measuring all electrode voltages for each current pattern applied which are in quadrature with the current pattern;

(c) calculating values of all theoretical voltages that should have appeared on the electrodes for the body due to the arbitrary set of guessed current patterns and the arbitrary guessed permittivity distribution;

(d) calculating a new set of current patterns based on differences between the measured and calculated voltage values;

(e) calculating the differences between electrode currents of the arbitrary set of current patterns and the calculated new set of current patterns, to form current differences;

(f) if any of the current differences are greater than a selected tolerance, applying the new set of current patterns to the electrode array and repeating steps (b) to (e);

(g) when the current differences are smaller than the selected tolerance, testing whether any of the voltage differences between the measured and calculated values at the electrodes, using the last set of current patterns, are larger than a predetermined value;

(h) if so, computing a new permittivity distribution as a function of the new set of current patterns and the measured voltage values;

(i) repeating steps (b) through (f) using the new permittivity distribution to replace the previously assumed distribution in order to find a set of current patterns that better distinguishes the actual permittivity distribution from the new one; and

(j) repeating steps (g) through (i) as many times as are necessary to produce a calculated permittivity distribution whose calculated voltages are substantially identical to those measured.

2. A method according to claim 1, followed by the calculation and display of an image of the permittivity distribution.