Optics: measuring and testing – For light transmission or absorption
Reexamination Certificate
2002-01-25
2004-01-13
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
C356S434000, C128S126100
Reexamination Certificate
active
06678049
ABSTRACT:
TECHNICAL FIELD
The present invention relates in general to an apparatus and associated method for detecting light transmitted through a scattering medium. More particularly, the present invention relates to a detection system and method using time-resolved tissue transillumination. The invention may be employed for tumor detection using a time-resolved breast transillumination technique.
BACKGROUND OF THE INVENTION
Optical imaging of turbid media such as the human breast has been the subject of much research activity and has seen an increase in interest since the early 1990s. This type of imaging is based on the fact that the propagation of light in a turbid medium depends on the absorption and scattering properties of the medium. Absorption results from energy level transitions of the constituent atoms and molecules in the medium. The absorption property of the medium is quantified by its absorption coefficient &mgr;
a
, defined as the probability of a photon being absorbed per infinitesimal pathlength. Scattering results from variations in the index of refraction of the different structures present in the medium. In a highly diffusive medium, scattering is quantified by the reduced scattering coefficient &mgr;
s
′ defined as the probability of a photon being isotropically scattered per infinitesimal pathlength. Characteristics such as intensity, coherence and polarization of the incident light change as it is absorbed and scattered by the medium resulting in diffuse transmittance of the light. In particular, scattering causes a collimated laser beam to spread over a sizeable volume element. This complicates the imaging of a turbid medium. Special imaging modalities must be implemented to offset the detrimental light diffusion. For example, time-resolved methods use ultra-short laser pulses to illuminate the medium. The emergent light is collected by a fast detector capable of reproducing its time variation, which can provide further information about the turbid medium. A simple data processing approach in this case is time-gating, by which only the earliest part of the output light pulses is used to produce an image. Refer to article S. K. Gayen and R. R. Alfano, “Emerging optical biomedical imaging techniques,”
Opt
. &
Photon. News
, 7, pp. 7-22 (1996). This amounts to using only the light with the straightest trajectory through the scattering medium, thus improving spatial resolution. See the articles: J. C. Hebden, “Evaluating the spatial resolution performance of a time-resolved optical imaging system,”
Med. Phys
, 19, pp. 1801-1087 (1992) and J. C. Hebden, D. J. Hall, and D. T. Delpy, “The spatial resolution performance of a time-resolved optical imaging system using temporal extrapolation”
Med. Phys
., 22, pp. 201-208 (1995).
The strong interest in optical imaging of scattering media stems from the need for biomedical diagnostic techniques that are safe and non-invasive. The optical properties of biological tissues are at the heart of optically based biomedical diagnostic techniques. As for the general case of a turbid medium, the manner in which light propagates through tissue depends on its absorption and scattering properties. Thus, if abnormal tissue can be said to differ from normal in its absorption or scattering of light for some physiological or morphological reason, it then becomes possible to optically differentiate between normal and abnormal conditions. A specific application is optical mammography where tumors could be differentiated from normal breast tissue on the basis of optical properties.
Mainly two types of biomedical optical imaging exist: tomography and transillumination. Tomography is typically based on a multi-point geometry involving a large number of detectors and allows the reconstruction of 3D images. Refer to the article S. B. Colak, D. G Papaioannou, G. W. Hooft, M. B. Van der Mark, H. Schomberg, J. C. J. Paasschens, J. B. M. Melissen, and N. A. A. J. Van Asten, “Tomographic image reconstruction from optical projections in light-diffusing media,”
Appl. Opt
., 36, 180-213 (1997). Obtaining 3D information is an important advantage of tomography, however, measurements and reconstructions are potentially time-consuming.
Transillumination (or 2D projection imaging) refers to a scanning procedure in which each image pixel is determined from the detection of the light that enters the medium through a certain entrance area, that propagates through it and that exits over a certain detection area usually facing the entrance area. The light entering the medium is generated by an excitation light source, typically a laser source emitting a laser beam at a wavelength in the range of 700-850 nm. The absorption of light by the tissues is minimal within this wavelength range.
A typical apparatus for obtaining transillumination images is illustrated in FIG.
1
. This system typically includes an excitation light source
10
, an input fiber
12
, the scattering medium
14
, an output fiber
16
, and the detection system
18
.
FIG. 1
also shows at
20
, the scanning direction.
The gray zone
22
in
FIG. 1
indicates the volume through which the photon travel before emerging to the output surface, while the dark zone
24
indicates the same but for the limited fraction of photon that are detected. As shown in
FIG. 1
, optical fibers can be used to facilitate the scanning of the input beam and the area over which the output light is collected.
The adverse effect of light scattering can be alleviated by detecting the light in a time-resolved manner. For example, at each point of a transillumination scan, an ultra-short laser pulse (typically <0.5 ns) can be injected at an input surface of the scattering medium. The light emerging from the opposite surface can be detected as a function of time, on a nanosecond range. This can be repeated a large number of cycles within a certain time integration in order to accumulate a sufficient photon statistics. The resulting time-resolved measurement typically appears as a vector of light intensity for different time values.
From each vector, a scalar must be calculated and transformed into a pixel value. A different method can be used for the calculation of such a scalar number. The sum of all the vector values can be performed and the resulting Continuous Wave (CW) image would be the same as obtained with a CW optical imaging transillumination technique. The sum can also be limited to the first time values which correspond to considering only the first arrival photons which experienced less diffusion and providing a less blurred image compared to a CW image. See the article: B. B. Das, K. M. Yoo, and R. R. Alfano, “Ultrafast time-gating imaging in thick tissues: a step toward optical mammography”
Opt. Lett
., 18, pp. 1092-1094 (1993). More sophisticated techniques can also be used as described in the article: Y. Painchaud, A. Mailloux, M. Morin, S. Verreault, and P. Beaudry, “Time-domain opticalimaging: discrimination between scattering and absorption,”
Appl. Opt
., 38, pp. 3686-3693 (1999).
For obtaining a good spatial resolution, the detection of the emerging light is typically done over a detection area which is small compared to the area from which the light emerges at the output surface. However, the detection of light over a large area is beneficial in order to increase the number of detected photons and thus the signal-to-noise ratio. A detection area on the order of 10 mm
2
appears to be a good compromise. The detection system has to measure the emerging light intensity over a certain area and with a certain time resolution. Assuming that the entire time-distribution of the emerging light is of interest and that the injected laser power is 50 mW at 800 nm, the requirements of an ideal detection system are the following:
Large area (~10 mm
2
);
High numerical aperture (~0.4);
Fast time-response (<0.5 ns);
High quantum efficiency at 800 nm (as high as possible);
High dynamic range (10
−8
to 10
31 3
W/cm
2
).
Two main detection systems have been used in reported studies involving tim
Anglehart James
ART Advanced Research Technologies Inc.
Nguyen Sang H.
Olilvy Renault
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