Method and apparatus for measuring concentration of absorptive constituent in scattering medium

• US 5,694,931 A
• Filed: 09/21/1995
• Issued: 12/09/1997
• Est. Priority Date: 09/22/1994
• Status: Expired due to Fees

• Current Assignee: Hamamatsu Photonics KK
• Original Assignee: Hamamatsu Photonics KK
• Inventors: Tsuchiya, Yutaka
• Primary Examiner(s): Cohen, Lee S.
• Assistant Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US08/531,771
• Time in Patent Office: 810 Days
• Field of Search: 128/633, 128/664, 128/665
• US Class Current: 600/310
• CPC Class Codes: A61B 2562/0242 for varying or adjusting the optical path length in the tissue
• CPC Class Codes: A61B 5/14553 specially adapted for cerebral tissue
• CPC Class Codes: G01N 21/49 within a body or fluid

Abstract

A method for measuring a concentration of an absorptive constituent in a scattering medium comprises the steps of emitting of light having two or more predetermined wavelengths; making said light incident at a light incidence position into said scattering medium; detecting said light at one or more photodetection points to acquire one or more photodetection signals; detecting light quantities and average flight pathlengths for said respective wavelengths of light based on said signals; and obtaining a concentration of said absorptive constituent by arithmetic processing using said light quantities and average flight pathlengths based on a predetermined relationship. The method permits absolute-value measurement of the concentration of the absorptive constituent inside the scattering medium having various exterior shapes.

Claims

1. A method for measuring a concentration of an absorptive constituent in a scattering medium, comprising:

(a) a step of emitting light having two or more predetermined wavelengths at which scattering coefficients are equal or regarded as equal, toward a scattering medium which is a measured object;

(b) a step of making said light incident at a light incidence position into said scattering medium and letting said light propagate through said scattering medium;

(c) a step of detecting said light having propagated through said scattering medium, at one or more photodetection points different from said light incidence position, thereby acquiring one or more photodetection signals;

(d) a step of calculating light quantities and average flight pathlengths at said one or more photodetection points for said respective wavelengths of light, based on said one or more photodetection signals; and

(e) a step of obtaining said concentration of said absorptive constituent by arithmetic processing using said light quantities and said average flight pathlengths, based on a predetermined relationship among said light quantities, said average flight pathlengths, and a difference between absorption coefficients per unit concentration of said absorptive constituent, for said respective wavelengths of light;

wherein said predetermined relationship among said light quantities, said average flight pathlengths, and said difference between the absorption coefficients per unit concentration of said absorptive constituent, for said respective wavelengths of light, is a relationship derived from a partial differentiation of a natural logarithm of a light quantity detected with respect to an absorption coefficient, which is equal to an average flight pathlength.

2. The method according to claim 1, wherein said light having two or more predetermined wavelengths is pulsed light.

3. The method according to claim 1, wherein said light having two or more predetermined wavelengths is sinusoidally modulated light having a predetermined modulation frequency component,

wherein said light quantities are calculated from amplitudes of said predetermined modulation frequency component included in said photodetection signals, and

wherein said average flight pathlengths are calculated from phase delays of said predetermined modulation frequency component.

4. The method according to claim 1, wherein said light having two or more predetermined wavelengths is sinusoidally modulated light having a predetermined modulation frequency component,

wherein said light quantities are calculated from dc components of said photodetection signals, and

wherein said average flight pathlengths are calculated from phase delays of said predetermined modulation frequency component included in said photodetection signals.

5. The method according to claim 1, wherein said light having two or more predetermined wavelengths is modulated light having a predetermined repetitive modulation frequency component,

wherein said light quantities are calculated from amplitudes of one of said predetermined repetitive modulation frequency component and a frequency component of an integral multiple of said predetermined repetitive modulation frequency component included in said photodetection signals, and

wherein said average flight pathlengths are calculated from phase delays of one of said predetermined repetitive modulation frequency component and the frequency component of the integral multiple of said predetermined repetition modulation frequency component.

6. The method according to claim 1, wherein said light having two or more predetermined wavelengths is modulated light having a predetermined repetitive modulation frequency component,

wherein said light quantities are calculated from dc components of said photodetection signals, and

wherein said average flight pathlengths are calculated from phase delays of one of said predetermined repetitive modulation frequency component and a frequency component of an integral multiple of said predetermined modulation frequency component included in said photodetection signals.

7. The method according to claim 1, wherein said step (e) comprises obtaining the concentration of the absorptive constituent in said scattering medium by arithmetic processing using said light quantities and said average flight pathlengths, based on a relationship expressed by the following formula:

space="preserve" listing-type="equation">V=(ε

.sub.2 -ε

.sub.1).sup.-1 ×

p<

L.sub.1 (λ

.sub.1)>

+(1-p)<

L.sub.1 (λ

.sub.2)>

!.sup.-1 ×

ln I.sub.1 (λ

.sub.1)/I.sub.1 (λ

.sub.2)!

where

V;

the concentration of the absorptive constituent,

ε

₁ ;

the absorption coefficient per unit concentration of the absorptive constituent for the light of the wavelength λ

₁,

ε

₂ ;

the absorption coefficient per unit concentration of the absorptive constituent for the light of the wavelength λ

₂,

<

L₁ (λ

₁)>

;

the average flight pathlength for the light of the wavelength λ

₁ at a photodetection point r₁,

<

L₁ (λ

₂)>

;

the average flight pathlength for the light of the wavelength λ

₂ at the photodetection point r₁,

I₁ (λ

₁);

the quantity of detected light for the light of the wavelength λ

₁ at the photodetection point r₁,

I₁ (λ

₂);

the quantity of detected light for the light of the wavelength λ

₂ at the photodetection point r₁, and

p;

a predetermined value satisfying 0<

p<

1.

8. The method according to claim 1, wherein said step (e) comprises obtaining the concentration of the absorptive constituent in said scattering medium by arithmetic processing using said light quantities and said average flight pathlengths, based on a relationship expressed by the following formula:

##EQU10## where V;

the concentration of the absorptive constituent,

ε

.sub. ;

the absorption coefficient per unit concentration of the absorptive constituent for the light of the wavelength λ

₁,

ε

₂ ;

the absorption coefficient per unit concentration of the absorptive constituent for the light of the wavelength λ

₂,

<

L₁ (λ

₁)>

;

the average flight pathlength at a photodetection point r₁ for the light of the wavelength λ

₁,

<

L₁ (λ

₂)>

;

the average flight pathlength at the photodetection point r₁ for the light of the wavelength λ

₂,

<

L₂ (λ

₁)>

;

the average flight pathlength at a photodetection point r₂ for the light of the wavelength λ

₁,

<

L₂ (λ

₂)>

;

the average flight pathlength at the photodetection point r₂ for the light of the wavelength λ

₂,

I₁ (λ

₁);

the quantity of detected light at the photodetection point r₁ for the light of an intensity B₁ and the wavelength λ

₁,

I₁ (λ

₂);

the quantity of detected light at the photodetection point r₁ for the light of an intensity B₂ and the wavelength λ

₂,

I₂ (λ

₁);

the quantity of detected light at the photodetection point r₂ for the light of the intensity B₁ and the wavelength λ

₁,

I₂ (λ

₂);

the quantity of detected light at the photodetection point r₂ for the light of the intensity B₂ and the wavelength λ

₂,

p;

a predetermined value satisfying 0≦

p≦

1, and

q;

a predetermined value satisfying 0≦

q≦

1.

9. An apparatus for measuring a concentration of an absorptive constituent in a scattering medium, comprising:

(a) a light source for emitting light having two or more predetermined wavelengths at which scattering coefficients are equal or regarded as equal, toward a scattering medium which is a measured object;

(b) light incidence means for making said light incident at a light incidence position into said scattering medium and making said light propagate through said scattering medium;

(c) photodetection means for detecting said light having propagated through said scattering medium, at one or more photodetection points different from said light incidence position to acquire one or more photodetection signals;

(d) parameters calculation means for computing light quantities and average flight pathlengths at said one or more photodetection points for said respective wavelengths of light, based on said one or more photodetection signals; and

(e) arithmetic processing means for obtaining said concentration of said absorptive constituent by arithmetic processing using said light quantities and said average flight pathlengths, based on a predetermined relationship among said light quantities, said average flight pathlengths, and a difference between absorption coefficients per unit concentration of said absorptive constituent, for said respective wavelengths of light;

wherein said predetermined relationship among said light quantities, said average flight pathlengths, and said difference between the absorption coefficients per unit concentration of said absorptive constituent, for said respective wavelengths of light, is a relationship derived from a partial differentiation of a natural logarithm of a light quantity detected with respect to an absorption coefficient, which is equal to an average flight pathlength.

10. The apparatus according to claim 9, wherein said light having two or more predetermined wavelengths is pulsed light.

11. The apparatus according to claim 9, wherein said light having two or more predetermined wavelengths is sinusoidally modulated light having a predetermined modulation frequency component,

wherein said light quantities are calculated from amplitudes of said predetermined modulation frequency component included in said photodetection signals, and

wherein said average flight pathlengths are calculated from phase delays of said predetermined modulation frequency component.

12. The apparatus according to claim 9, wherein said light having two or more predetermined wavelengths is sinusoidally modulated light having a predetermined modulation frequency component,

wherein said light quantities are calculated from dc components of said photodetection signals, and

wherein said average flight pathlengths are calculated from phase delays of said predetermined modulation frequency component included in said photodetection signals.

13. The apparatus according to claim 9, wherein said light having two or more predetermined wavelengths is modulated light having a predetermined repetitive modulation frequency component,

wherein said light quantities are calculated from amplitudes of one of said predetermined repetitive modulation frequency component and a frequency component of an integral multiple of said predetermined repetitive modulation frequency component included in said photodetection signals, and

wherein said average flight pathlengths are calculated from phase delays of one of said predetermined repetitive modulation frequency component and the frequency component of the integral multiple of said predetermined repetition modulation frequency component.

14. The apparatus according to claim 9, wherein said light having two or more predetermined wavelengths is modulated light having a predetermined repetitive modulation frequency component,

wherein said light quantities are calculated from dc components of said photodetection signals, and

wherein said average flight pathlengths are calculated from phase delays of one of said predetermined repetitive modulation frequency component and a frequency component of an integral multiple of said predetermined modulation frequency component included in said photodetection signals.

15. The apparatus according to claim 9, wherein said arithmetic processing means (e) obtains the concentration of the absorptive constituent in said scattering medium by arithmetic processing using said light quantities and said average flight pathlengths, based on a relationship expressed by the following formula:

space="preserve" listing-type="equation">V=(ε

.sub.2 -ε

.sub.1).sup.-×

1 p<

L.sub.1 (λ

.sub.1)>

+(1-p)<

L.sub.1 (λ

.sub.2)>

!.sup.-1 ×

ln I.sub.1 (λ

.sub.1)/I.sub.1 (λ

.sub.2)!

where

V;

the concentration of the absorptive constituent,

ε

₁ ;

the absorption coefficient per unit concentration of the absorptive constituent for the light of the wavelength λ

₁,

ε

₂ ;

the absorption coefficient per unit concentration of the absorptive constituent for the light of the wavelength λ

₂,

<

L₁ (λ

₁)>

;

the average flight pathlength for the light of the wavelength λ

₁ at a photodetection point r₁,

<

L₁ (λ

₂)>

;

the average flight pathlength for the light of the wavelength λ

₂ at the photodetection point r₁,

I₁ (λ

₁);

the quantity of detected light for the light of the wavelength λ

₁ at the photodetection point r₁,

I₁ (λ

₂);

the quantity of detected light for the light of the wavelength λ

₂ at the photodetection point r₁, and

p;

a predetermined value satisfying 0≦

p≦

1.

16. The apparatus according to claim 9, wherein said arithmetic processing means (e) obtains the concentration of the absorptive constituent in said scattering medium by arithmetic processing using said light quantities and said average flight pathlengths, based on a relationship expressed by the following formula:

##EQU11## where V;

the concentration of the absorptive constituent,

ε

.sub. ;

the absorption coefficient per unit concentration of the absorptive constituent for the light of the wavelength λ

₁,

ε

₂ ;

the absorption coefficient per unit concentration of the absorptive constituent for the light of the wavelength λ

₂,

<

L₁ (λ

₁)>

;

the average flight pathlength at a photodetection point r₁ for the light of the wavelength λ

₁,

<

L₁ (λ

₂)>

;

the average flight pathlength at the photodetection point r₁ for the light of the wavelength λ

₂,

<

L₂ (λ

₁)>

;

the average flight pathlength at a photodetection point r₂ for the light of the wavelength λ

₂,

<

L₂ (λ

₂)>

;

the average flight pathlength at the photodetection point r₂ for the light of the wavelength λ

₁,

I₁ (λ

₁);

the quantity of detected light at the photodetection point r₁ for the light of an intensity B₁ and the wavelength λ

₁,

I₁ (λ

₂);

the quantity of detected light at the photodetection point r₁ for the light of an intensity B₂ and the wavelength λ

₂,

I₂ (λ

₁);

the quantity of detected light at the photodetection point r₂ for the light of the intensity B₁ and the wavelength λ

₁,

I₂ (λ

₂);

the quantity of detected light at the photodetection point r₂ for the light of the intensity B₂ and the wavelength λ

₂,

p;

a predetermined value satisfying 0≦

p≦

1, and

q;

a predetermined value satisfying 0≦

q≦

1.