Measuring Tissue Oxygenation

• US 20080097173A1
• Filed: 05/30/2007
• Published: 04/24/2008
• Est. Priority Date: 05/30/2006
• Status: Active Grant

• Current Assignee: University Of Massachusetts
• Original Assignee: University Of Massachusetts
• Inventors: Soller, Babs, Soyemi, Olusola, Yang, Ye
• Granted Patent: US 7,532,919 B2
• Time in Patent Office: Days
• US Class Current: 600/310
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/412 Detecting or monitoring sepsis
• CPC Class Codes: G01N 2021/3144 for oxymetry
• CPC Class Codes: G01N 21/35 using infrared light G01N21/39 takes precedence
• CPC Class Codes: G01N 33/4925 measuring blood gas content, e.g. O2, CO2 , HCO3

Abstract

Methods and systems for calculating tissue oxygenation, e.g., oxygen saturation, in a target tissue are disclosed. In some embodiments, the methods include: (a) directing incident radiation to a target tissue and determining reflectance spectra of the target tissue by measuring intensities of reflected radiation from the target tissue at a plurality of radiation wavelengths; (b) correcting the measured intensities of the reflectance spectra to reduce contributions thereto from skin and fat layers through which the incident radiation propagates; (c) determining oxygen saturation in the target tissue based on the corrected reflectance spectra; and (d) outputting the determined value of oxygen saturation.

Claims

1. A method for calculating oxygen saturation in a target tissue, the method comprising:

directing incident radiation to a target tissue and determining reflectance spectra of the target tissue by measuring intensities of reflected radiation from the target tissue at a plurality of radiation wavelengths;

correcting the measured intensities of the reflectance spectra to reduce contributions thereto from skin and fat layers through which the incident radiation propagates;

determining oxygen saturation in the target tissue based on the corrected reflectance spectra; and

outputting the determined value of oxygen saturation.

2. The method of claim 1, wherein determining oxygen saturation comprises determining light attenuation spectra from the corrected reflectance spectra, and calculating oxygen saturation based on concentrations of oxygenated and deoxygenated heme in the target tissue that are derived from the light attenuation spectra, wherein heme comprises hemoglobin and myoglobin in the target tissue.

3. The method of claim 2, wherein the concentrations of oxygenated and deoxygenated heme are derived from the light attenuation spectra by fitting the light attenuation spectra to a model light attenuation equation.

4. The method of claim 3, wherein the light attenuation equation comprises a Beer'"'"'s Law equation comprising terms that correspond to incident light absorption by oxygenated heme, deoxygenated heme, and water in the target tissue.

5. The method of claim 4, wherein the light attenuation equation comprises a series expansion of light attenuation in a plurality of terms that correspond to Beer'"'"'s Law absorption terms.

6. The method of claim 5, wherein the series expansion of light attenuation comprises a Taylor series expansion of light attenuation.

7. The method of claim 3, wherein the light attenuation equation comprises a term that varies linearly with a wavelength of the incident light, the term having a functional form aλ

where a is a constant and λ

is the wavelength of the incident light.

8. The method of claim 3, wherein the light attenuation equation comprises a constant term independent of a wavelength of the incident light.

9. The method of claim 3, wherein fitting the light attenuation spectra to a model comprises performing a two-stage fitting procedure wherein, in a first stage, initial values of one or more model parameters are determined, and in a second stage, the light attenuation spectra are fitted to the model, wherein the model comprises the initial parameter values determined in the first stage.

10. The method of claim 9, wherein the light attenuation spectra are fitted to the model by minimizing a sum of squared differences between the light attenuation spectra and light attenuation values determined from the model.

11. The method of claim 3, wherein the fitting is performed automatically by a processor.

12. The method of claim 7, wherein the value of a is constrained during fitting so that a assumes only values that are less than or equal to zero.

13. The method of claim 4, wherein the light attenuation equation further comprises a baseline function derived from a difference between light attenuation values determined from the light attenuation equation and the light attenuation spectra.

14. The method of claim 4, wherein the light attenuation equation further comprises a differential path length factor that varies directly with a scattering coefficient of the target tissue and inversely with an absorption coefficient of the target tissue.

15. The method of claim 3, wherein the light attenuation equation comprises a diffuse reflectance equation derived from a radiation diffusion model of incident light in the target tissue.

16. The method of claim 1, wherein measuring intensities of reflected radiation comprises:

measuring, along a first optical path from a light source to a detector, reflected radiation from the target tissue that corresponds to a first source-detector spacing; and

measuring, along a second optical path from the light source to the detector, reflected radiation from the target tissue that corresponds to a second source-detector spacing different from the first source-detector spacing.

17. The method of claim 16, wherein the reflected radiation measured at the first source-detector spacing comprises a first weighting of contributions from the target tissue and from tissue layers disposed between the light source and the target tissue, and the reflected radiation measured at the second source-detector spacing comprises a second weighting of contributions from the target tissue and from the tissue layers disposed between the light source and the target tissue different from the first weighting.

18. The method of claim 17, wherein the tissue layers disposed between the light source and the target tissue are skin and fat layers.

19. The method of claim 18, wherein correcting the measured intensities of the reflectance spectra comprises reducing contributions from the skin and fat layers to the reflected radiation measured at the second source-detector spacing based on the reflected radiation measured at the first source-detector spacing.

20. The method of claim 1, further comprising determining oxygen tension in the target tissue based on oxygen saturation in the target tissue.

21. The method of claim 2, further comprising assessing a level of vasoconstriction in a patient based on a measurement of total hemoglobin in a target tissue of the patient, wherein total hemoglobin is determined based on the concentrations of oxygenated and deoxygenated heme in the target tissue.

22. The method of claim 1, wherein the target tissue is within a human.

23. The method of claim 1, wherein the target tissue is within an animal.

24. The method of claim 1, wherein the plurality of wavelengths comprises at least 100 wavelengths or more.

25. The method of claim 1, wherein the plurality of wavelengths comprises wavelengths from 700 nm to 1000 nm.

26. The method of claim 22, wherein the plurality of wavelengths comprises wavelengths from 725 nm to 880 nm.

27. The method of claim 1, wherein the target tissue is a muscle tissue.

28. A method of monitoring blood volume in a patient, the method comprising:

directing incident radiation to a target tissue of the patient and determining reflectance spectra of the target tissue by measuring intensities of reflected radiation from the target tissue at a plurality of wavelengths;

correcting the measured intensities of the reflectance spectra to reduce contributions thereto from skin and fat layers through which the incident radiation propagates;

determining total heme concentration in the target tissue based on the corrected reflectance spectra;

assessing a blood volume in the patient based on the total heme concentration; and

outputting the assessed blood volume.

29. The method of claim 28, further comprising assessing a stage of progress of at least one of hemorrhage, sepsis, heart disease, and diabetes in the patient based on the assessed blood volume.

30. A method for calculating oxygen saturation in a target tissue, the method comprising:

directing incident radiation to a target tissue and determining reflectance spectra of the target tissue by measuring intensities of reflected radiation from the target tissue at a plurality of radiation wavelengths;

determining light attenuation spectra of the target tissue from the reflectance spectra, and fitting the light attenuation spectra to a model light attenuation equation; and

determining oxygen saturation in the target tissue based on the fitting of the light attenuation spectra,

wherein fitting the light attenuation spectra to a model comprises performing a two-stage fitting procedure wherein, in a first stage, initial values of one or more model parameters are determined, and in a second stage, the light attenuation spectra are fitted to the model, wherein the model comprises the initial parameter values determined in the first stage.

31. The method of claim 30, wherein the model comprises a term having a functional form aλ

, and wherein the value of a is constrained during the fitting to be less than or equal to zero.

32. A system, comprising:

a light source configured to direct incident radiation to a target tissue;

a detector; and

a processor coupled to the detector and configured to;

determine reflectance spectra of the target tissue;

correct the reflectance spectra to reduce contributions thereto from skin and fat layers through which the incident radiation propagates; and

determine oxygen saturation in the target tissue based on the corrected reflectance spectra.

33. The system of claim 32, wherein the processor is configured to determine reflectance spectra of the target tissue by directing the detector to measure intensities of reflected radiation from the target tissue at a plurality of radiation wavelengths.

34. The system of claim 32, wherein the processor is configured to determine oxygen saturation by calculating light attenuation spectra from the corrected reflectance spectra, and calculating oxygen saturation based on concentrations of oxygenated and deoxygenated heme in the target tissue that are derived from the light attenuation spectra, wherein heme comprises hemoglobin and myoglobin in the target tissue.

35. The system of claim 32, further comprising:

a first radiation path between the light source and the detector, and corresponding to a first distance between the light source and the detector; and

a second radiation path between the light source and the detector, and corresponding to a second distance between the light source and the detector different from the first distance,

wherein incident radiation from the light source is directed along each of the first and second radiation paths to the target tissue, and reflected radiation from the target tissue is directed along each of the first and second radiation paths to the detector.

36. The system of claim 35, wherein the processor is configured to reduce contributions to the measured reflectance spectra from skin and fat layers by measuring reflectance spectra along each of the first and second light paths, and combining the reflectance spectra to produce corrected reflectance spectra.

37. The system of claim 35, wherein each of the first and second radiation paths comprises an optical fiber.

38. The system of claim 34, wherein the processor is configured to derive the concentrations of oxygenated and deoxygenated heme in the target tissue by fitting the light attenuation spectra to a model light attenuation equation.

39. The system of claim 38, wherein the model light attenuation equation comprises a Beer'"'"'s Law equation comprising terms that correspond to absorption of incident radiation by oxygenated heme, deoxygenated heme, and water in the target tissue.

40. The system of claim 32, wherein the processor is further configured to determine oxygen tension in the target tissue from oxygen saturation.

41. The system of claim 34, wherein the processor is further configured to determine total heme concentration in the target tissue from the concentrations of oxygenated and deoxygenated heme in the target tissue.

42. The system of claim 41, wherein the processor is further configured to assess a blood volume in the target tissue based on the total heme concentration.