Multifrequency bioimpedance determination

• US 20060004300A1
• Filed: 05/20/2005
• Published: 01/05/2006
• Est. Priority Date: 11/22/2002
• Status: Abandoned Application

• Current Assignee: ImpediMed Limited
• Original Assignee: ImpediMed Limited
• Inventors: Kennedy, James
• Application Number:
  US11/134,824
• Publication Number:
  US 20060004300A1
• Time in Patent Office: Days
• US Class Current: 600/547
• CPC Class Codes: A61B 5/053 Measuring electrical impedance or conductance of a portion of the body
• CPC Class Codes: A61B 5/0537 Measuring body composition by impedance, e.g. tissue hydration or fat content
• CPC Class Codes: A61B 5/4872 Body fat
• CPC Class Codes: A61B 5/7257 using Fourier transforms

Abstract

A method of determining the impedance of a subject is provided. This method involves applying an electrical signal representing a range of superimposed frequencies, and then determining the current flow through and voltage across the subject for a number of the frequencies within the range. The impedance of the subject is then determined at each of the number of frequencies. An apparatus and a processing system configured for use in impedance determination are also provided.

Claims

1. A method of determining the impedance of a subject, comprising:

applying an electrical signal representing a range of superimposed frequencies;

determining, for a number of frequencies within the range, the current flow through the subject and the voltage across the subject; and

determining the impedance of the subject at each of the number of frequencies.

2. The method of claim 1, further comprising

generating component signals, each component signal having a respective one of the number of frequencies;

and,

superpimosing the component signals to generate the electrical signal.

3. The method of claim 1, wherein the electrical signal is formed from white noise.

4. The method of claim 3, additionally comprising:

generating the white noise using a Linear Feedback Shift Register (LFSR) circuit to produce a pseudo-random digital sequence;

converting the pseudo-random digital sequence to an analog signal using a digital to analog (D/A) converter; and

,

applying the analog signal to the subject.

5. The method of claim 1, wherein determining the current flow comprises:

sampling the current of the electrical signal applied to the subject; and

converting the current signal to a digitized current signal.

6. The method of claim 5, wherein determining the voltage comprises:

obtaining a signal representing the voltage generated across the subject; and

converting the voltage signal to a digitized voltage signal.

7. The method of claim 6, additionally comprising digitizing the current and voltage signals by sampling the signals at a predetermined sample rate.

8. The method of claim 6, additionally comprising digitizing the current and voltage signals by sampling the signals with a predetermined sample length.

9. The method of claim 6, additionally comprising converting each of the digitized voltage and current signals into the frequency domain.

10. The method of claim 9, wherein the conversion is performed using a Fast Fourier Transform (FFT).

11. The method of claim 9, further comprising:

receiving the converted voltage and current signals; and

,

determining the impedance of the subject at each of the number of frequencies.

12. The method of claim 11, wherein the processing system is further adapted to determine the variation in the impedance with the frequency of the applied signal.

13. The method of claim 12, additionally comprising generating a graphical representation of the variation in the impedance with the frequency of the applied signal.

14. An apparatus for determining the impedance of a subject, comprising:

a signal generator configured to apply an electrical signal representing a range of superimposed frequencies;

a voltage detector configured to determine the voltage across the subject at a number of frequencies within the range;

a current detector configured to determine the current flow through the subject at a number of frequencies within the range; and

,

a processing system configured to determine the impedance of the subject at each of the number of frequencies.

15. The apparatus of claim 14, wherein the signal generator is adapted to:

generate component signals, each component signal having a respective one of the number of frequencies; and

,

superimpose the component signals to generate the electrical signal.

16. The apparatus of claim 14, wherein the electrical signal is formed from white noise.

17. The apparatus of claim 16, wherein the signal generator comprises:

a shift register circuit producing a pseudo-random digital sequence; and

a D/A converter converting the pseudo-random digital sequence to an analog signal.

18. The apparatus of claim 17, wherein the signal generator additionally comprises:

a shift register having an output coupled to the D/A converter; and

an exclusive OR (XOR) gate adapted to receive inputs from a number of predetermined locations in the first register, logically combine the inputs to generate an XOR output, and provide the XOR output to an input of the shift register;

19. The apparatus of claim 18, wherein the signal generator additionally comprises a second shift register, the second shift register being adapted to couple an output of the first shift register to an input of the D/A converter.

20. The apparatus of claim 14, wherein the current detector comprises:

a current sampler coupled to the signal generator for sampling the current flowing through the subject; and

a current analog to digital (A/D) converter for converting the sampled current to a digitized current signal.

21. The apparatus of claim 20, wherein the voltage detector comprises a voltage A/D converter coupled to the subject via a respective set of electrodes, the voltage A/D converter being adapted to generate a digitized voltage signal.

22. The apparatus of claim 21, wherein the current and voltage A/D converters are adapted to digitize the current and voltage signals by sampling the signals at a predetermined sample rate.

23. The apparatus of claim 21, wherein the current and voltage D/A converters are adapted to digitize the current and voltage signals by sampling the signals with a predetermined sample length.

24. The apparatus of claim 21, wherein the processing system is adapted to convert each of the digitized voltage and current signals into the frequency domain.

25. The apparatus of claim 24, wherein the conversion is performed using a FFT.

26. The apparatus of claim 24, wherein the processing system comprises processing electronics for performing the conversion.

27. The apparatus of claim 24, wherein the processing system is adapted to:

receive the converted voltage and current signals; and

determine the impedance of the subject at each of the number of frequencies.

28. The apparatus of claim 14, wherein the processing system is further adapted to determine the variation in the impedance with the frequency of the applied signal.

29. The apparatus of claim 14, wherein the processing system is further adapted to generate a graphical representation of the variation in the impedance with the frequency of the applied signal.

30. A processing system for use in an apparatus for determining the impedance of a subject, wherein the processing system is adapted to:

receive a digitized current signal representing the current flow through the subject at a number of frequencies, for an applied electrical signal representing a range of superimposed frequencies;

receive a digitized voltage signal representing the voltage across the subject at a number of frequencies within the range;

convert each digitized signals into the frequency domain; and

determine the impedance of the subject at each of the number of frequencies.

31. The processing system of claim 30, wherein the conversion is performed using a FFT.

32. The processing system of claim 30, wherein the processing system includes processing electronics for performing the conversion.

33. The processing system of claim 30, wherein the processing system includes a processor for determining the impedance.

34. The processing system of claim 33, wherein the processor is further adapted to determine the variation in the impedance with the frequency of the applied signal.

35. The processing system of claim 34, wherein the processing system includes a display, and wherein the processor is adapted to generating a graphical representation of the variation in the impedance with the frequency of the applied signal.

36. An apparatus for determining the impedance of a subject, comprising:

means for applying an electrical signal representing a range of superimposed frequencies;

means for determining the current flow through the subject for a number of frequencies within the range;

means for determining the voltage across the subject for the number of frequencies; and

means for depermining the impedance of the subject at each of the number of frequencies.

37. The apparatus of claim 36, wherein the means for applying an electrical signal representing a range of superimposed frequencies comprise a means for generating an electrical signal formed from the summation of a plurality of sine waves.