Spectral analysis for a more reliable determination of physiological parameters

• US 20080027299A1
• Filed: 02/16/2007
• Published: 01/31/2008
• Est. Priority Date: 02/20/2006
• Status: Active Grant

• Current Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V.
• Original Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V.
• Inventors: Moersdorf, Hans-Joachim, Tobola, Andreas, Vogl, Ulrich
• Granted Patent: US 8,295,901 B2
• Time in Patent Office: Days
• US Class Current: 600/323
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7207 of noise induced by motion artifacts
• CPC Class Codes: A61B 5/7257 using Fourier transforms

Abstract

An apparatus for determining a spectral ratio between a first signal having a first spectrum which depends on a biological quantity, and a second signal having a second spectrum which depends on a biological quantity, the first signal and the second signal having a plurality of harmonics of a periodic signal, the apparatus including a computer for computing a first wave ratio between a spectral value of the first spectrum which has a frequency of a harmonic of the plurality of harmonics, and a spectral value of the second spectrum which has the frequency of the harmonic; and for computing a second wave ratio between a spectral value of the first spectrum which has a frequency of another harmonic of the plurality of harmonics, and a spectral value of the second spectrum which has the frequency of the other harmonic. In addition, the apparatus includes an optimizer for determining the spectral ratio while using the first and second wave ratios, the optimizer being configured to determine the spectral ratio such that it differs from the first and second wave ratios and meets an optimization criterion.

Claims

1. An apparatus (100) for determining a spectral ratio between a first signal having a first spectrum which depends on a biological quantity, and a second signal having a second spectrum which depends on a biological quantity, the first signal and the second signal having a plurality of harmonics of a periodic signal, the apparatus comprising:

a computer (110) for computing a first wave ratio between a spectral value of the first spectrum which has a frequency of a harmonic of the plurality of harmonics, and a spectral value of the second spectrum which has the frequency of the harmonic;

and for computing a second wave ratio between a spectral value of the first spectrum which has a frequency of another harmonic of the plurality of harmonics, and a spectral value of the second spectrum which has the frequency of the other harmonic;

an optimizer (120) for determining the spectral ratio while using the first and second wave ratios, the optimizer being configured to determine the spectral ratio such that it differs from the first wave ratio and the second wave ratio, and meets an optimization criterion.

2. The apparatus (100) as claimed in claim 1, wherein the period of the periodic signal is independent of the biological quantity.

3. The apparatus as claimed in one of claims 1 or 2, wherein the first signal and the second signal depend on the biological quantity in different ways.

4. The apparatus (100) as claimed in any of claims 1 to 3, wherein the first and second signals exhibit dependencies on the biological quantity in the plurality of harmonics.

5. The apparatus (100) as claimed in any of claims 1 to 4, wherein the two signals are based on optical signals, the wavelengths of which are selected such that the attenuation of the one optical signal depends on the biological quantity, and the attenuation of the other optical signal does not depend on the biological quantity.

6. The apparatus (100) as claimed in any of claims 1 to 5, wherein the biological quantity is a blood oxygen saturation or a heart frequency.

7. The apparatus (100) as claimed in any of claims 1 to 6, wherein the computer (110) is configured to compute the first wave ratio at a fundamental wave frequency of the periodic signal, and the second wave ratio at a harmonic frequency of the periodic signal.

8. The apparatus (100) as claimed in any of claims 1 to 7, wherein the computer (110) is configured to form wave ratios between a plurality of spectral values of the first and second signals, and wherein, in addition, the optimizer (120) is configured to form the spectral ratio from the plurality of wave ratios.

9. The apparatus (100) as claimed in any of claims 1 to 8, wherein the signals originate from a transillumination of a tissue of a test person.

10. The apparatus (100) as claimed in any of claims 1 to 9, wherein one signal is based on an infrared light receive signal, and one signal is based on a red light receive signal.

11. The apparatus (100) as claimed in any of claims 1 to 10, wherein the optimizer (120) is configured to optimize a cumulated distance measure between the spectral ratio and the wave ratios.

12. The apparatus (100) as claimed in any of claims 1 to 11, wherein the computer (110) is configured to pass on a plurality of the spectral values of the two spectra to the optimizer (120) as tuples of a frequency, and the optimizer (120) is configured to determine and to output a regression line from the plurality of tuples, the horizontal, vertical or perpendicular, the linear or the square distance being minimized.

13. The apparatus (100) as claimed in any of claims 1 to 12, wherein the optimizer (120) is configured to determine the regression line in accordance with the least squares fit or the total squares fit methods.

14. The apparatus (100) as claimed in any of claims 1 to 13, wherein the optimizer (120) is configured to determine the regression line using a singular-value decomposition of a matrix, the two signals forming the columns of the matrix, and that singular value which is smaller, in magnitude, of the singular-value decomposition being set to zero for linearizing the equation system.

15. The apparatus (100) as claimed in any of claims 1 to 14, further comprising a means (130) for providing the first and second signals, and further being configured to reduce frequency portions on the basis of previous knowledge of the spectral curve of a signal portion.

16. The apparatus (100) as claimed in claim 15, wherein the means (130) for providing the first and second signals is configured to reduce other spectral portions on the basis of known intensity ratios between the harmonics.

17. The apparatus (100) as claimed in one of claims 15 or 16, wherein the means (130) for providing the first and second signals is configured to provide only spectral portions of the fundamental wave, of the first harmonic, and of the second harmonic of the periodic signal.

18. The apparatus (100) as claimed in any of claims 15 to 17, wherein the means (130) for providing the first and second signals is configured to provide only those signal portions of signals which correspond to a pulse signal of a living being in terms of their frequency and amplitude ratios.

19. The apparatus (100) as claimed in any of claims 15 to 18, wherein the means (130) for providing the first and second signals is configured to determine, on the basis of a reference spectrum, pulse signal portions of a living being, and to mask out other frequencies from the signals.

20. The apparatus (100) as claimed in any of claims 1 to 19, wherein the computer (110) is configured to pass the signals on to the optimizer (120), and wherein the optimizer (120) is configured to determine, on the basis of the spectra of the signals, a reference spectrum for which a distance measure in relation to the spectra of the signals is optimized.

21. The apparatus (100) as claimed in any of claims 1 to 20, wherein the two signals originate from a plethysmogram.

22. The apparatus (100) as claimed in any of claims 1 to 21, comprising a second means for providing a first time-discrete reference signal which exhibits a first interference portion, and a second time-discrete reference signal which exhibits a second interference portion, the second interference portion being shifted in phase relative to the first interference portion;

and a subtraction means for generating a differential signal from the two reference signals, the differential signal comprising a frequency component being caused by the first and second interference portions, and

a means for manipulating the two signals, on the basis of the differential signal, such that the frequency component is reduced in a manipulated signal; and

the manipulated time-discrete signal is made available to the computer.

23. The apparatus (100) as claimed in any of claims 1 to 22, comprising a first means for providing the time-discrete signal, the means being adapted to sample an iterating optical signal which corresponds to a bright period during which a transmit light source adopts an on state, and comprising a second means for providing a time-discrete reference signal adapted to sample two optical signals which correspond to dark periods during which no transmit light source adopts an on state.

24. The apparatus (100) as claimed in any of claims 1 to 23, comprising a first means for providing a time-discrete signal, the means being configured to process an iterating optical signal, the optical signal comprising sequences and comprising a sequence of at least two bright periods during which a transmit light source adopts an on state, and comprising at least one dark period during which no transmit light source adopts the on state, and wherein the at least two bright periods are irregularly arranged within a sequence, and the means for providing the time-discrete signal further being configured to make available the time-discrete signal in accordance with a bright transmit signal on the basis of the information about the arrangement of the bright periods in the sequence.

25. The apparatus (100) as claimed in claim 24, wherein the sequence of an optical signal is based on a clock, in accordance with which the bright and dark periods occur.

26. The apparatus (100) as claimed in claim 25, wherein the means for providing the time-discrete signals are configured to operate at a clock higher than 800 Hz.

27. The apparatus (100) as claimed in any of claims 1 to 26, wherein the signals originate from a spread spectrum modulation.

28. A method for determining a spectral ratio between a first signal having a first spectrum which depends on a biological quantity, and a second signal having a second spectrum which depends on a biological quantity, the first signal and the second signal having a plurality of harmonics of a periodic signal, the method comprising:

computing a first wave ratio between a spectral value of the first spectrum which has a frequency of a harmonic of the plurality of harmonics, and a spectral value of the second spectrum which has the frequency of the harmonic;

computing a second wave ratio between a spectral value of the first spectrum which has a frequency of another harmonic of the plurality of harmonics, and a spectral value of the second spectrum which has the frequency of the other harmonic;

determining the spectral ratio while using the first and second wave ratios, the spectral ratio being determined such that it differs from the first and second wave ratios and meets an optimization criterion.