Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2000-09-18
2003-07-22
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S322000, C600S328000
Reexamination Certificate
active
06597931
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to an apparatus and methods for determining absolute values of various properties of a physiological medium. In particular, the present invention relates to non-invasive optical systems and methods for determining absolute values of concentrations of oxygenated and deoxygenated hemoglobins (and/or their ratios) in the physiological medium. The present invention also relates to apparatus and methods for obtaining such absolute values by solving a generalized photon diffusion equation as well as its variations such as a modified Beer-Lambert equation.
BACKGROUND OF THE INVENTION
Near-infrared spectroscopy has been used for non-invasive measurement of various physiological properties in animal and human subjects. The basic principle underlying the near-infrared spectroscopy is that physiological tissues include various highly-scattering chromophores to the near-infrared waves with relatively low absorption. Many substances in a medium may interact or interfere with the near-infrared light waves propagating therethrough. Human tissues, e.g., include numerous chromophores such as oxygenated hemoglobin, deoxygenated hemoglobin, water, lipid, and cytochrome, where the hemoglobins are the dominant chromophores in the spectrum range of 700 nm to 900 nm. Accordingly, the near-infrared spectroscope has been applied to measure oxygen levels in the physiological medium such as tissue hemoglobin oxygen saturation and total hemoglobin concentrations.
Various techniques have been developed for the near-infrared spectroscopy, e.g., time-resolved spectroscopy (TRS), phase modulation spectroscopy (PMS), and continuous wave spectroscopy (CWS). In a homogeneous and semi-infinite model, both of TRS and PMS have been used to obtain spectra of an absorption coefficient and reduced scattering coefficient of the physiological medium by solving a photon diffusion equation, and to calculate concentrations of oxygenated and deoxygenated hemoglobins as well as tissue oxygen saturation. CWS has generally been designed to solve a modified Beer-Lambert equation and to measure changes in the concentrations of oxygenated and deoxygenated hemoglobins.
Despite their capability of providing the hemoglobin concentrations as well as the oxygen saturation, one major drawback of TRS and PMS is that the equipment is bulky and expensive. CWS may be manufactured at a lower cost but limited in its utility because it cannot compute the oxygen saturation from the changes in the concentrations of oxygenated and deoxygenated hemoglobins. Accordingly, there is a need for novel CWS systems and methods thereof for measuring absolute value of concentrations of the hemoglobins as well as the oxygen saturation in the physiological medium.
SUMMARY OF THE INVENTION
The present invention generally relates to an apparatus and method for obtaining absolute values of concentrations of chromophores of a medium and/or absolute values of their ratios. More particularly, the present invention relates to non-invasive optical systems and methods for determining absolute values of oxygenated and/or deoxygenated hemoglobins in a physiological medium.
In general, wave propagation or photon migration in a medium is described by a generalized diffusion equation:
I
=
α
·
β
·
γ
·
I
0
·
ⅇ
{
-
B
·
L
·
δ
·
Σ
i
(
ϵ
i
C
i
)
+
σ
}
(
1
)
where “I
o
” is a variable representing an intensity of electromagnetic waves irradiated by a wave source and “I” is a variable for an intensity of electromagnetic waves detected by a wave detector. Parameter “&agr;” is generally associated with the wave source and/or medium and accounts for, e.g., characteristics of the wave source such as power and configuration thereof, mode of optical coupling between the wave source and medium, and/or coupling loss therebetween. Parameter “&bgr;” is generally associated with the wave detector and/or medium and accounts for, e.g., characteristics of the wave detector, optical coupling mode between the wave detector and medium, and the associated coupling loss. Parameters “&agr;” and “&bgr;” may also depend upon, to some extent, other system characteristics and optical properties of the medium, including those of chromophores included therein. Parameter “&ggr;” may be either a proportionality constant (including, e.g., 1.0) or a system parameter which may change its value according to the characteristics of the wave source, wave detector, and/or medium. Parameter “B” generally accounts for lengths of optical paths of photons or electromagnetic waves through the medium, and is predominantly determined by the optical properties of the medium. However, an exact value of parameter “B” may also depend on the characteristics of the wave source and/or wave detector as well. A typical example of such parameter “B” is conventionally known as a path length factor. It is appreciated that the parameter “B” may also take the value of 1.0 where the generalized diffusion equation (1) is reduced to the Beer-Lambert equation. To the contrary, parameter “L” is generally geometry-dependent and accounts for a linear distance between the wave source and wave detector. Parameter “&dgr;” may be either a proportionality constant (including, e.g., 1.0) or a system parameter which may be associated with the wave source, wave detector, and/or medium. Parameter “&egr;
i
” accounts for an optical interaction or interference of photons or electromagnetic waves with an i-th chromophore included in the medium. It is appreciated that, depending upon the definition and value of the parameter “&dgr;”, the parameter “&egr;
i
” may represent an extinction coefficient, an absorption coefficient, and/or a (reduced) scattering coefficient of the medium or the chromophores included therein. Variable “C
i
” represents concentration of the i-th chromophore included the medium, and parameter “&sgr;” is either a proportionality constant (including, e.g., 0.0) or a parameter which may be associated with the wave source, wave detector, and/or medium. Despite the numerous parameters of the generalized diffusion equation (1) and various modified versions thereof which will be described in greater detail below, the optical systems and methods of the present invention enable direct determination of absolute values of the chromophore concentrations and/or ratios thereof.
In one aspect of the present invention, a method is provided to solve a set of wave equations applied to an optical system having at least one wave source and at least one wave detector. Photons or electromagnetic waves are irradiated by the wave source, transmitted through the physiological medium including at least one chromophore, and detected by the wave detector. The wave equation, e.g., the generalized diffusion equation (1), expresses the intensity of electromagnetic waves detected by the wave detector (i.e., “I”) as a function of system variables (e.g., “I
o
” and “C
i
”) and system parameters (e.g., “&agr;,” “&bgr;,” “&ggr;,” “B,” “L,” “&dgr;,” “&egr;
i
,” and “&sgr;”). The method generally includes the steps of obtaining multiple sets of equations by applying the wave equation to the optical system capable of irradiating multiple sets of electromagnetic waves having different wave characteristics, eliminating the source-dependent parameters (e.g., “&agr;”) and detector-dependent parameter (e.g., “&bgr;”) therefrom to obtain a set of intermediate equations, providing at least one correlation of the medium-dependent and geometry-dependent parameters (e.g., “B” and “L,” respectively) with the chromophore concentrations (and/or their ratios), incorporating the correlation into the set of intermediate equations, and obtaining absolute values of the concentrations of the chromophores (and/or ratios thereof) based on the intensities of electromagnetic waves (e.g., “I” and “I
o
”) and the medium- or chromophore-dependent parameters (e.g., “&egr;
i
”).
This embodiment of the prese
Cheng Xuefeng
Wang Lai
Xu Xiaorong
Zhou Shuoming
Photonify Technologies, Inc.
Ritter Lang & Kaplan LLP
Winakur Eric F.
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