High-speed electric tomography

• US 5,544,662 A
• Filed: 10/06/1993
• Issued: 08/13/1996
• Est. Priority Date: 07/09/1991
• Status: Expired due to Term

• Current Assignee: Rensselaer Polytechnic Institute
• Original Assignee: Rensselaer Polytechnic Institute
• Inventors: Isaacson, David, Gisser, David G., Goble, John C., Cook, Raymond D., Saulnier, Gary J., Newell, Jonathan C.
• Primary Examiner(s): Smith, Ruth S.
• Application Number:
  US08/132,457
• Time in Patent Office: 1,042 Days
• Field of Search: 128/734, 364/413.13
• 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
• CPC Class Codes: G16H 40/40 for the management of medical equipment or devices, e.g. scheduling maintenance or upgrades
• CPC Class Codes: G16H 40/63 for local operation

Abstract

An apparatus for electrical impedance imaging of a body includes an array of multiple electrodes. Each is connected to an adjustable amplitude precision current source of fixed frequency and an oversampling type digital voltmeter. The current amplitudes are set to produce a series of spatial current patterns, all orthogonal to each other, and each applied for only a short time interval while voltages are measured. The voltage and current values are used to create real and reactive impedance images of the interior of the body.

Claims

1. An apparatus for electrical impedance imaging of a body, comprising:

a plurality of electrodes in an array and adapted to be in electrical contact with the body;

waveform means for supplying a plurality of voltage waveforms;

a plurality of voltage-to-current converters connected to the waveform means for receiving the voltage waveforms, each converter being separately connected to each electrode for applying a repetitive current waveform to each respective electrode, all of said repetitive current waveforms together forming a spatial current pattern for application to the array to generate voltage signals at the electrodes with respect to a common ground, for use in forming an electrical impedance image of at least part of a volume in the body;

control means for operating each of said voltage-to-current converters to apply the repetitive current waveforms to the current patterns in synchronism to said electrodes a multiplicity of times, said control means including said waveform means for generating, applying and removing a plurality of repetitive waveforms to and from all of said voltage-to-current converters simultaneously to power said plurality of electrodes simultaneously, said waveform means having a single master clock for use with all of said voltage-to-current converters and a common read-only memory with a digitally stored sinusoid for use in providing the repetitive waveforms;

separate voltmeter means connected to each electrode for measuring individual voltages at each electrode with respect to the common ground due to the multiplicity of concurrent patterns for use in imaging at least part of the volume of the body;

image means for receiving the voltages and forming an impedance image therefrom; and

digital-to-analog converter means connected to each voltage-to-current converter and forming part of a voltage reference source for each voltage-to-current converter, each digital-to-analog converter means comprising at least one high-speed digital-to-analog converter for determining a shape of the waveform according to values of a sequence of digital words applied consecutively at a uniform rate to an input of the voltage-to-current converter, and repeated for each cycle of the repetitive waveform, each high-speed digital-to-analog converter connected to an output of a low-speed digital-to-analog converter, the low-speed digital-to-analog converter for determining an amplitude and polarity of the waveform according to values of a separate digital word and forming a reference voltage at the output.

2. An apparatus according to claim 1, including synchronization means for applying and removing the repetitive waveforms to and from the plurality of electrodes at a peak of the repetitive waveform.

3. An apparatus according to claim 1, wherein said voltmeter means comprises a separate digital voltmeter for each electrode, each voltmeter acting as a matched filter for receiving voltage signals from each respective one of said plurality of electrodes.

4. An apparatus according to claim 3, wherein each digital voltmeter includes means for sampling and digitizing the electrode voltage a multiplicity of times during each cycle of a repetitive waveform and for a multiplicity of waveform cycles.

5. An apparatus according to claim 4, wherein the sampling and digitizing means takes samples at sampling points within each waveform cycle which are equally spaced but at different relative locations in successive waveform cycles, so that an aggregate of points sampled effectively constitutes a close and uniformly spaced sampling of a single cycle of the repetitive waveform.

6. An apparatus according to claim 5 including means for producing both real and quadrature voltage data from each aggregate of sampled points, resulting in a pair of images for each set of applied current patterns.

7. An apparatus according to claim 1, including a calibration voltage source connected to said control means, having a constant amplitude and acting as a reference, and switch means connected to the calibration voltage source for applying the reference to each of said electrodes separately for calibrating each separate voltmeter means separately.

8. An apparatus for electrical impedance imaging of a body, comprising:

a plurality of electrodes in an array and adapted to be in electrical contact with the body;

waveform means for supplying a plurality of voltage waveforms;

a plurality of voltage-to-current converters connected to the waveform means for receiving the voltage waveforms, each converter being separately connected to each electrode for applying a repetitive current waveform to each respective electrode, all of said repetitive current waveforms together forming a spatial current pattern for application to the array to generate voltage signals at the electrodes with respect to a common ground, for use in forming an electrical impedance image of at least part of a volume in the body;

control means for operating each of said voltage-to-current converters to apply the repetitive current waveforms to the current patterns in synchronism to said electrodes a multiplicity of times, said control means including said waveform means for generating, applying and removing a plurality of repetitive waveforms to and from all of said voltage-to-current converters simultaneously to power said plurality of electrodes simultaneously, said waveform means having a single master clock for use with all of said voltage-to-current converters and a common read-only memory with a digitally stored sinusoid for use in providing the repetitive waveforms;

separate voltmeter means connected to each electrode for measuring individual voltages at each electrode with respect to the common ground due to the multiplicity of concurrent patterns for use in imaging at least part of the volume of the body; and

image means for receiving the voltages and forming an impedance image therefrom;

each voltage-to-current converter including an output terminal, and means for introducing an adjustable negative resistance and an adjustable negative capacitance to the output terminal of each voltage-to-current converter, for establishing an effective high internal impedance for each voltage-to-current converter.

9. An apparatus according to claim 8, wherein each means for introducing adjustable negative resistance and negative capacitance to the output terminal of each voltage-to-current converter is a digitally adjustable and computer-controlled unit.

10. An apparatus according to claim 9, further comprising:

means for sensing a relative size and polarity of a real component and a quadrature component of an internal impedance of each voltage-to-current converter; and

means for using the sensed relative size and polarity of the real component and the quadrature component to maximize the internal impedance of each digitally adjustable, computer-controlled voltage-to-current converter.

11. An apparatus for electrical impedance imaging of a body, comprising:

a plurality of electrodes in an array and adapted to be in electrical contact with the body;

waveform means for supplying a plurality of voltage waveforms;

a plurality of voltage-to-current converters connected to the waveform means for receiving the voltage waveforms, each converter being separately connected to each electrode for applying a repetitive current waveform to each respective electrode, all of said repetitive current waveforms together forming a spatial current pattern for application to the array to generate voltage signals at the electrodes with respect to a common ground, for use in forming an electrical impedance image of at least part of a volume in the body;

control means for operating each of said voltage-to-current converters to apply the repetitive current waveforms to the current patterns in synchronism to said electrodes a multiplicity of times, said control means including said waveform means for generating, applying and removing a plurality of repetitive waveforms to and from all of said voltage-to-current converters simultaneously to power said plurality of electrodes simultaneously, said waveform means having a single master clock for use with all of said voltage-to-current converters and a common read-only memory with a digitally stored sinusoid for use in providing the repetitive waveforms;

separate voltmeter means connected to each electrode for measuring individual voltages at each electrode with respect to the common ground due to the multiplicity of concurrent patterns for use in imaging at least part of the volume of the body;

image means for receiving the voltages and forming an impedance image therefrom; and

a reference electrode separate from said plurality of electrodes forming said array, current-to-voltage resistor means connected to said reference electrode for measuring voltage at said reference electrode with respect to the common ground, a separate voltmeter connected to said current-to-voltage resistor for measuring a voltage proportional to the current in said reference electrode, and means for interrupting operation of each of the voltage-to-current converters upon detecting a current in the reference electrode exceeding a preset minimum value.

12. An apparatus according to claim 11, wherein said means for interrupting comprises a power supply relay for interrupting power to each of the voltage-to-current converters, a latch connected to said relay for operating the relay, and RMS signal detection means connected to said latch for processing the voltage proportional to the current in said reference electrode.

13. An apparatus for electrical impedance imaging of a body, comprising:

a plurality of electrodes in an array and adapted to be in electrical contact with the body;

waveform means for supplying a plurality of voltage waveforms;

a plurality of voltage-to-current converters connected to the waveform means for receiving the voltage waveforms, each converter being separately connected to each electrode for applying a repetitive current waveform to each respective electrode, all of said repetitive current waveforms together forming a spatial current pattern for application to the array to generate voltage signals at the electrodes with respect to a common ground, for use in forming an electrical impedance image of at least part of a volume in the body;

control means for operating each of said voltage-to-current converters to apply the repetitive current waveforms to the current patterns in synchronism to said electrodes a multiplicity of times, said control means including said waveform means for generating, applying and removing a plurality of repetitive waveforms to and from all of said voltage-to-current converters simultaneously to power said plurality of electrodes simultaneously, said waveform means having a single master clock for use with all of said voltage-to-current converters and a common read-only memory with a digitally stored sinusoid for use in providing the repetitive waveforms;

separate voltmeter means connected to each electrode for measuring individual voltages at each electrode with respect to the common ground due to the multiplicity of concurrent patterns for use in imaging at least part of the volume of the body;

image means for receiving the voltages and forming an impedance image therefrom; and

a calibration voltmeter, a current-to-voltage converter connected between said electrodes and the calibration voltmeter for converting current from each electrode into voltage to be measured by the calibration voltmeter, and switching mens connected to the current-to-voltage converter and the calibration voltmeter for calibrating each of said voltage-to-current converters separately.

14. An apparatus according to claim 13, wherein the switching means and calibration voltmeter are computer-controlled.