Method and apparatus for measuring blood oxygen saturation within a retinal vessel with light having several selected wavelengths

• US 5,776,060 A
• Filed: 02/20/1997
• Issued: 07/07/1998
• Est. Priority Date: 02/20/1997
• Status: Expired due to Fees

• Current Assignee: Vectranetics, Inc.
• Original Assignee: University of Alabama in Huntsville (University of Alabama System)
• Inventors: Smith, Matthew H., Chipman, Russell A., Minnich, Thomas E.
• Primary Examiner(s): Bahr, Jennifer
• Assistant Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US08/803,065
• Time in Patent Office: 502 Days
• Field of Search: 600/300, 600/301, 600/318, 600/320, 600/322, 600/323, 600/340, 128/920, 128/923, 128/924
• US Class Current: 600/340
• CPC Class Codes: A61B 5/14555 specially adapted for the eye fundus
• CPC Class Codes: Y10S 128/92 Computer assisted medical diagnostics

Abstract

The method and apparatus for measuring the oxygen saturation of blood within a retinal vessel illuminates the retinal vessel with light having a combination of wavelengths selected to reduce the error in the measured blood oxygen saturation. The oxygen saturation measuring method and apparatus illuminates a retinal vessel with light having the selected combination of wavelengths. For example, the oxygen saturation measuring method and apparatus can illuminate the retinal vessel with light having a first wavelength between 460 nm and 503 nm, a second wavelength between 600 nm, and 770 nm and a third wavelength between 770 nm and 1000 nm. The oxygen saturation measuring method and apparatus then measures the transmittance of the blood within the retinal vessel in response to the illumination at each of the selected wavelengths. Based upon the respective transmittances of the blood within the retinal vessel that are measured at each of the selected wavelengths, the oxygen saturation measuring method and apparatus determines the oxygen saturation of the blood within the retinal vessel. By appropriately selecting the combination of wavelengths with which the retinal vessel is illuminated, the corresponding error in the measured blood oxygen saturation can be reduced.

Claims

1. A method of measuring blood oxygen saturation within a retinal vessel based upon a transmittance of the blood within the retinal vessel, the method comprising the steps of:

determining a relationship between an error in a blood oxygen saturation measurement and measurement errors in the respective transmittances of the blood within the retinal vessel for light at a plurality of wavelengths;

determining, for each of a plurality of different combinations of wavelengths, the error in the blood oxygen saturation measurement for retinal vessels having a range of vessel diameters and a range of blood oxygen saturation values;

selecting a combination of wavelengths based upon the corresponding error in the blood oxygen saturation measurement;

illuminating the retinal vessel with light having the selected combination of wavelengths;

measuring the transmittance of the blood within the retinal vessel in response to illumination at each of the selected wavelengths; and

determining the blood oxygen saturation within the retinal vessel based upon the respective transmittances of the blood within the retinal vessel measured at each of the selected wavelengths.

2. A method according to claim 1 wherein said step of determining a relationship comprises the step of determining a relationship between the blood oxygen saturation of the retinal vessel and the respective transmittances of the blood within the retinal vessel for light at the plurality of wavelengths.

3. A method according to claim 1 wherein said step of determining a relationship comprises the step of determining the error Δ

s in the blood oxygen saturation measurement as follows;

##EQU9## wherein n is an integer designating respective ones of the plurality of wavelengths, wherein T.sup.λ

.sbsp.n represents the transmittance of the blood within the retinal vessel for light at the n^th wavelength, wherein Δ

T.sup.λ

.sbsp.n represents the measurement error in the transmittance of the blood within the retinal vessel for light at the n^th wavelength, and wherein ##EQU10## is a partial derivative of the calculated blood oxygen saturation s with respect to the transmittance of blood within the retinal vessel.

4. A method according to claim 3 wherein said step of determining the error in the blood oxygen saturation measurement comprises the step of determining, for each of the plurality of different combinations of wavelengths, the error in the blood oxygen saturation measurement arising from respective measurement errors Δ

T.sup.λ

.sbsp.n in the transmittance of the blood within the retinal vessel for light at each of the different wavelengths.

5. A method according to claim 4 wherein said step of determining the error in the blood oxygen saturation measurement comprises the step of setting the predetermined transmittance error to a constant value for light having each of the different wavelengths.

6. A method according to claim 4 wherein said step of determining the error in the blood oxygen saturation measurement comprises the step of setting the predetermined transmittance error to a value which varies for light having different wavelengths.

7. A method of selecting a combination of wavelengths of light to reduce errors in measuring blood oxygen saturation within a retinal vessel based upon a transmittance of the retinal vessel, the method comprising the steps of:

determining a relationship between an error in a blood oxygen saturation measurement and measurement errors in the respective transmittances of the blood within the retinal vessel for light at a plurality of wavelengths;

determining, for each of a plurality of different combinations of wavelengths, the error in the blood oxygen saturation measurement for retinal vessels having a range of vessel diameters and a range of blood oxygen saturation values; and

selecting a combination of wavelengths based upon the corresponding error in the blood oxygen saturation measurement, wherein said selecting step comprises selecting a combination of wavelengths such that the corresponding error in the blood oxygen saturation measurement is no greater than a predetermined maximum error across the range of vessel diameters and the range of blood oxygen saturation values.

8. A method according to claim 7 wherein said step of determining a relationship comprises the step of determining a relationship between the blood oxygen saturation of the retinal vessel and the respective transmittances of the blood within the retinal vessel for light at the plurality of wavelengths.

9. A method according to claim 7 wherein said step of determining a relationship comprises the step of determining the error Δ

s in the blood oxygen saturation measurement as follows;

##EQU11## wherein n is an integer designating respective ones of the plurality of wavelengths, wherein T.sup.λ

.sbsp.n represents the transmittance of the blood within the retinal vessel for light at the n^th wavelength, wherein Δ

T.sup.λ

.sbsp.n represents the measurement error in the transmittance of the blood within the retinal vessel for light at the n^th wavelength, and wherein ##EQU12## is a partial derivative of the calculated blood oxygen saturation s with respect to the transmittance of blood within the retinal vessel.

10. A method according to claim 9 wherein said step of determining the error in the blood oxygen saturation measurement comprises the step of determining, for each of the plurality of different combinations of wavelengths, the error in the blood oxygen saturation measurement arising from respective measurement errors Δ

T.sup.λ

.sbsp.n in the transmittance of the blood within the retinal vessel for light at each of the different wavelengths.

11. A method according to claim 10 wherein said step of determining the error in the blood oxygen saturation measurement comprises the step of setting the predetermined transmittance error to a constant value for light having each of the different wavelengths.

12. A method according to claim 10 wherein said step of determining the error in the blood oxygen saturation measurement comprises the step of setting the predetermined transmittance error to a value which varies for light having different wavelengths.

13. A computer program product for facilitating a selection of a combination of wavelengths of light to reduce errors in measuring blood oxygen saturation within a retinal vessel based upon a transmittance of the retinal vessel, the computer program product comprising a computer-readable storage medium having computer-readable program code means embodied in said medium, said computer-readable program code means comprising:

first computer-readable program code means for determining a relationship between an error in a blood oxygen saturation measurement and measurement errors in the respective transmittances of the blood within the retinal vessel for light at a plurality of wavelengths;

second computer-readable program code means, responsive to said first computer-readable program code means, for determining, for each of a plurality of different combinations of wavelengths, the error in the blood oxygen saturation measurement for retinal vessels having a range of vessel diameters and a range of blood oxygen saturation values; and

third computer-readable program code means, responsive to said second computer-readable program code means, for displaying, for each of a plurality of different combinations of wavelengths, the error in the blood oxygen saturation measurement for retinal vessels having a range of vessel diameters and a range of blood oxygen saturation values to thereby facilitate selection of a combination of wavelengths of light.

14. A computer program product according to claim 13 wherein said first computer-readable program code means comprises computer-readable program code means for determining a relationship between the blood oxygen saturation of the retinal vessel and the respective transmittances of the blood within the retinal vessel for light at the plurality of wavelengths.

15. A computer program product according to claim 13 wherein said first computer-readable program code means comprises fourth computer-readable program code means for determining the error Δ

s in the blood oxygen saturation measurement as follows;

##EQU13## wherein n is an integer designating respective ones of the plurality of wavelengths, wherein T.sup.λ

.sbsp.n represents the transmittance of the blood within the retinal vessel for light at the n^th wavelength, wherein Δ

T.sup.λ

.sbsp.n represents the measurement error in the transmittance of the blood within the retinal vessel for light at the n^th wavelength, and wherein ##EQU14## is a partial derivative of the calculated blood oxygen saturation s with respect to the transmittance of blood within the retinal vessel.

16. A computer program product according to claim 15 wherein said fourth computer-readable program code means further comprises computer-readable program code means for determining, for each of the plurality of different combinations of wavelengths, the error in the blood oxygen saturation measurement arising from respective measurement errors Δ

T.sup.λ

.sbsp.n in the transmittance of the blood within the retinal vessel for light at each of the different wavelengths.

17. An apparatus for measuring blood oxygen saturation within a retinal vessel based upon a transmittance of the blood within the retinal vessel, the apparatus comprising:

means for selecting a combination of wavelengths of light based upon a corresponding error in a blood oxygen saturation measurement, said selecting means comprising;

means for determining a relationship between the error in the blood oxygen saturation measurement and measurement errors in the respective transmittances of the blood within the retinal vessel for light at a plurality of wavelengths; and

means for determining, for each of a plurality of different combinations of wavelengths, the error in the blood oxygen saturation measurement for retinal vessels having a range of vessel diameters and a range of blood oxygen saturation values;

an optical source for illuminating the retinal vessel with light having the selected combination of wavelengths;

a detector for measuring the transmittance of the blood within the retinal vessel in response to illumination at each of the selected wavelengths; and

means, responsive to said detector, for determining the blood oxygen saturation within the retinal vessel based upon the respective transmittances of the blood within the retinal vessel measured by said detector at each of the selected wavelengths.

18. An apparatus according to claim 17 wherein said means for determining a relationship comprises means for determining a relationship between the blood oxygen saturation of the retinal vessel and the respective transmittances of the blood within the retinal vessel for light at the plurality of wavelengths.

19. An apparatus according to claim 17 wherein said means for determining a relationship comprises means for determining the error Δ

s in the blood oxygen saturation measurement as follows;

##EQU15## wherein n is an integer designating respective ones of the plurality of wavelengths, wherein T.sup.λ

.sbsp.n represents the transmittance of the blood within the retinal vessel for light at the n^th wavelength, wherein Δ

T.sup.λ

.sbsp.n represents the measurement error in the transmittance of the blood within the retinal vessel for light at the n^th wavelength, and wherein ##EQU16## is a partial derivative of the calculated blood oxygen saturation s with respect to the transmittance of blood within the retinal vessel.

20. An apparatus according to claim 19 wherein said means for determining the error in the blood oxygen saturation measurement comprises means for determining, for each of the plurality of different combinations of wavelengths, the error in the blood oxygen saturation measurement arising from respective measurement errors Δ

T.sup.λ

.sbsp.n in the transmittance of the blood within the retinal vessel for light at each of the different wavelengths which are each less than a predetermined transmittance error.

21. An apparatus for measuring blood oxygen saturation within a retinal vessel based upon a transmittance of the blood within the retinal vessel, the apparatus comprising:

a first optical source for illuminating the eye with light having a first wavelength between 460 nm and 503 nm;

a second optical source for illuminating the eye with light having a second wavelength between 600 nm and 770 nm;

a third optical source for illuminating the eye with light having a third wavelength between 770 nm and 1100 nm;

a detector for measuring the transmittance of the blood within the retinal vessel in response to illumination by each of said first, second and third optical sources; and

means, responsive to said detector, for determining the blood oxygen saturation within the retinal vessel based upon the respective transmittances of the blood within the retinal vessel measured by said detector at each of the first, second and third wavelengths.