Optics: measuring and testing – By particle light scattering – With photocell detection
Reexamination Certificate
2000-09-29
2003-05-20
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
By particle light scattering
With photocell detection
C356S343000
Reexamination Certificate
active
06567165
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a concentration measuring method and apparatus for an absorption component in a scattering medium. More specifically, the present invention relates to a concentration measuring method and apparatus for an absorption component in a scattering medium, in which at least two light rays with predetermined wavelengths, whose scattering coefficients are different and have a known ratio, are made incident on a scattering medium such as living bodies having various shapes, light which has a predetermined wavelength and diffuses and propagates inside the scattering medium and comes to the surface is detected to obtain the light intensity and mean flight pathlength (mean optical pathlength) at the detection position, and on the basis of the light intensity and mean flight pathlength, a relative value or an absolute value of concentration of a specific absorption component in the scattering medium, oxygen saturation of hemoglobin, and a change in time or a spatial distribution thereof can be highly accurately and noninvasively measured without any influence of the shape of the scattering medium.
2. Related Background Art
There is a strong demand for highly accurate and noninvasive measurement of a relative value and an absolute value of concentration of a specific absorption component in a scattering medium such as a living body, and a change in time as well as a spatial distribution thereof. Various methods or examinations have been used or made so far, including a method using continuous light (CW light) or modulated light (e.g., pulse light, rectangular waveform light, or sine-wave-modulated light) and a method using light components with different wavelengths.
In these conventional techniques, a method and apparatus for sufficiently accurately measuring the concentrationof a specific absorption component in an object such as living bodies whose regions have different shapes or shapes have individual differences even in identical regions have not been developed yet. This poses a serious problem in noninvasive measurement of a living body using light, and a strong demand has arisen for improvement thereof.
Light incident on a scattering medium such as a living body diffuses and propagates inside the scattering medium while being scattered and absorbed, and partially comes to the surface. Since the scattering medium is normally surrounded by air, the light coming to the surface dissipates through the free space.
In the measurement of internal information of a scattering medium, light that has come to the surface in the above way is detected. In this case, if the boundary condition (shape) of the scattering medium changes, e.g., depending on whether the scattering medium has a spherical shape or a rectangular parallelopiped shape, the intensity and behavior of light coming to a predetermined position of the surface changes greatly.
Hence, to increase the accuracy of such measurement, the behavior of light in the scattering medium must be understood well. As is recently known, the behavior of light in a scattering medium can be relatively accurately described and analyzed by analysis, experiments, and examinations of Monte Carlo simulation using a computer, or photon diffusion theory.
As described above, to understand the behavior of light in a scattering medium, Monte Carlo simulation or photon diffusion theory is conventionally used. However, Monte Carlo simulation takes a verylong time for calculation and cannot calculate the concentration of a specific absorption component in a scattering medium from the result of simulation.
To use the photon diffusion theory, a boundary condition must be set to actually solve a photon diffusion equation. However, the boundary condition largely depends on the shape of a scattering medium. For this reason, for accurate measurement, a new boundary condition must be set to solve a photon diffusion equation every time the shape of the scattering medium changes. Additionally, a relatively accurate boundary condition can be set for only a scattering medium with a very simple shape, such as an infinite space, semi-infinite space, infinite circular cylinder, or infinitely spreading slab having a limited thickness. As a result, to measure a living body having a complex shape using the photon diffusion theory, it is indispensable to use an approximate boundary condition, resulting in a large measurement error.
As a solution to these problems, the present inventor has already developed and filed a patent application (Japanese Patent Application Laid-Open Gazette No. Hei 8-94517) for a method of measuring the absorption coefficient of a scattering material and the concentration of an absorber on the basis of the Micro-Beer-Lambert law.
SUMMARY OF THE INVENTION
The method in Japanese Patent Application Laid-Open Gazette No. Hei 8-94517 is excellent because it can quantitatively measure an absorption coefficient independently of the boundary condition (shape) of an object to be measured. However, this method uses a plurality of light components with different wavelengths whose scattering characteristics are equal or can be regarded to be equal for a scattering medium to be measured. Hence, the method disclosed in Japanese Patent Application Laid-Open Gazette No. Hei 8-94517 is not satisfactory because it can use only limited wavelengths, and as the difference in scattering characteristics between the plurality of wavelengths of light components in use increases, the measurement error increases, or if the difference further increases, measurement is disabled.
As described above, a diffused light handling method which can be systematically applied, without any limitation on usable wavelengths, to a scattering medium having scattering characteristics depending on a wavelength and a different boundary condition has not been developed yet. For this reason, it is conventionally impossible to systematically accurately measure the concentration of an internal specific absorption component in such a scattering medium without limiting a wavelength to be used.
The present invention has been made to solve the above problems of the prior art, and has as its object to provide a concentration measuring method and apparatus for an absorption component in a scattering medium, in which the basic relationship associated with the behavior of light in a scattering medium having scattering characteristics depending on a wavelength and having a different boundary condition is newly disclosed, even when the scattering characteristics depend on the wavelength, a relative value or an absolute value of concentration of a specific absorption component in scattering media having various shapes can be accurately measured using that relationship without any limitation on the wavelength to be used and any influence of the wavelength dependence of such scattering characteristics, and a change in time or a spatial distribution thereof can also be accurately measured without any influence of the wavelength dependence of the scattering characteristics.
In the present invention, at least two light rays having predetermined wavelengths and a known ratio of transport scattering coefficients are made incident on a scattering medium having various boundary conditions (shapes), a light intensity and mean flight pathlength of each light ray having the predetermined wavelength at the light detection position are obtained, and on the basis of these values, a relative value or an absolute value of concentration of a specific absorption component are obtained by arithmetic processing without any influence of the boundary condition of the scattering medium or wavelength dependence of scattering characteristics.
Specifically, a concentration measuring method for an absorption component in a scattering medium according to the present invention comprises
(a) a light generation step of generating at least two light rays having predetermined wavelengths (light rays having wavelengths of two types or more), the light rays having di
Tsuchiya Yutaka
Urakami Tsuneyuki
Hamamatsu Photonics K.K.
Stafira Michael P.
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