Optics: measuring and testing – For light transmission or absorption
Reexamination Certificate
2000-11-21
2002-01-01
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
C356S343000, C600S310000, C600S322000, C600S473000
Reexamination Certificate
active
06335792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for measuring an internal property distribution in a scattering medium and an apparatus therefor. More specifically, the present invention concerns a measuring method of internal property distribution and is applicable to equipment for obtaining internal information while moving a light injection position and a light detection position along a surface of a measured object, and an apparatus therefor.
2. Related Background Art
Optical CT (computer tomography) means a technique or apparatus of measuring an optical property distribution or a concentration distribution of an absorptive constituent in an organism, and makes use of light (signal light) detected after injection of light into a living tissue and migration therethrough.
Three types of optical CT techniques making use of rectilinearly propagating light, quasi-rectilinearly propagating light, and scattered light have been reported heretofore. Among these, the method making use of rectilinearly propagating light has extremely poor utilization efficiency of light and is thus applicable only to very small media. For example, where the near-infrared light is used, transmitted light demonstrates attenuation to about 10
−5
times against a standard living tissue having the thickness of 1 cm. In contrast with it, the method making use of scattered light utilizes all light emerging from the medium, so as to increase signal-to-noise ratios, and is thus expected to be applied to larger media. The method making use of quasi-rectilinearly propagating light stands intermediate between these two methods.
The practical optical CT needs to utilize the scattered light because of the limitations including maximum permissible incidence power to organisms, measurement sensitivity, required measurement time, and so on, but the above optical CT does not have been put into practical use yet because of the following technological problems.
The first problem is that no method has been developed for describing the behavior of light or a photon in a scattering medium with sufficient accuracy. For analyzing the behavior of a photon migrating in a scattering medium, it has been common practice heretofore to employ approximation to the transport equation, or the photon diffusion equation resulting from application of diffusion approximation to the transport theory. The diffusion approximation, however, holds only in sufficiently larger media than the mean free path length of photons therein and is thus incapable of handling relatively small media, tissues having complicated internal shapes, and media having complicated shapes. In addition, the diffusion approximation is predicated on isotropic scattering; therefore, when applied to measurement of actual living tissues having anisotropic scattering characteristics, it gives rise to unignorable errors due to the anisotropic scattering. Further, the diffusion equation does not allow us to find its solution by either of analytical or numerical techniques (such as the finite element method or the like) unless boundary conditions are preliminarily set. Namely, it is necessary to set the boundary conditions at each of the light injection and detection positions, i.e., the shape of the medium and reflection characteristics at interfaces, prior to measurement. If these conditions vary depending upon individual differences etc., computation must be redone under boundary conditions modified according to the variation. Therefore, the optical CT making use of the relation between signal light and optical properties of a scattering medium derived from the approximate expression of the transport equation or the photon diffusion equation still has a significant problem in accuracy and operability.
Besides the above, there is another method for deriving the relation between signal light and optical properties of a scattering medium by applying the perturbation theory to the approximate expression of the transport equation or to the photon diffusion equation, and for reconstructing an optical CT image using this relation. This method, however, makes how to handle its nonlinear effects (terms of the second and higher degrees) very complex. On this occasion, it is theoretically possible to perform the computation including the terms of the second and higher degrees by a computer, but operation time is enormous even with use of the presently fastest computer; therefore, practical use thereof is impossible. It is thus usual practice to ignore the terms of the second and higher degrees. Therefore, when this method is applied to reconstruction of an optical CT image of a medium containing a plurality of relatively strong absorption regions, interaction becomes unignorable between the absorption regions and a large error can be made due to it.
The second problem is that the conventional optical CT makes use of a weight function in a narrow sense, i.e., a mean path length or a phase lag equivalent thereto. For this reason, it becomes extremely complicated to handle the mean path length of detected light varying depending upon absorption coefficients. It is thus usual practice to employ approximation, but use of approximation poses a significant problem of increase in errors. Such methods making use of the weight function in a narrow sense are described, for example, in references listed below. (1) S. Arridge: SPIE Institutes for Advanced Optical Technologies, Vol. IS11, Medical Optical Tomography: Functional Imaging and Monitoring, 35-64 (1993); (2) R. L. Barbour and H. L. Graber: ibid. 87-120 (1993); (3) H. L. Graber, J. Chang, R. Aronson and R. L. Barbour: ibid. 121-143 (1993); (4) J. C. Schotland, J. C. Haselgrove and J. S. Leigh: Applied optics, 32, 448-5453 (1993); (5) Chang, R. Aronson, H. L. Graber and R. L. Barbour: Proc. SPIE, 2389, 448-464 (1995); (6) B. W. Pogue, M. S. Patterson, H. Jiang and K. D. Paulsen: Phys. Med. Biol. 40, 1709-1729 (1995); (7) S. R. Arridge: Applied Optics, 34, 7395-7409 (1995); (8) H. L. Graber, J. Chang, and R. L. Barbour: Proc. SPIE, 2570, 219-234 (1995); (9) A. Maki and H. Koizumi: OSA TOPS, Vol. 2, 299-304 (1996); (10) H. Jiang, K. D. Paulsen and Ulf L. Osterberg: J. Opt. Soc. Am. A13, 253-266 (1996); (11) S. R. Arridge and J. C. Hebden: Phys. Med. Biol. 42, 841-853 (1997); (12) S. B. Colak, D. G. Papaioannou, G. W. It Hooft, M. B. van der Mark, H. Schomberg, J. C. J. Paasschens, J. B. M. Melissen and N. A. A. J. van Astten: Applied Optics, 36, 180-213 (1997).
In image reconstruction of optical CT, it is most important to know which part the signal light has passed in the scattering medium, i.e., to know a path distribution of the signal light in the medium. From this viewpoint, there is also a method for deriving the path distribution of signal light by the random walk theory or the like, but the aforementioned problem is not solved yet.
As described above, the conventional optical CT techniques do not allow us to obtain a reconstructed image with sufficient accuracy and still have significant issues in terms of spatial resolution, image distortion, quantitation, measurement sensitivity, required measurement time, and so on.
In order to break through the circumstances as described above, the inventors have been conducting a series of studies with the focus on the following points. Namely, the important points for realizing optical CT are to clarify the behavior of light migrating in living tissues of strong scattering media, to clarify the relation between signal light detected and optical properties of scattering media (scattering absorbers) containing absorptive constituents, and to develop an algorithm for reconstructing an optical CT image by making use of the signal light and the relation.
Then the inventors proposed {circle around (1)} a model based on Microscopic Beer-Lambert Law (hereinafter referred to as “MBL”), derived {circle around (2)} analytic expressions to indicate the relation between optical properties of scattering media and signal light, and reported them in the fol
Hamamatsu Photonics K.K.
Morgan & Lewis & Bockius, LLP
Pham Hoa Q.
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