Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-12-22
2001-03-20
Kamm, William E. (Department: 3762)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06205353
ABSTRACT:
BACKGROUND
1. Technical Field
The present invention relates generally to the imaging of objects in highly scattering turbid media and, more particularly, to a novel optical backscattering tomographic technique using optical radiation in visible, near infrared (NIR) spectral region for imaging objects in highly scattering turbid media.
2. Description of Related Art
There are many situations in which the detection of objects in a highly scattering turbid medium using backscattered light is highly desirable. For example, backscattered light may be utilized to detect a tumor embedded within tissue, such as breast tissue. Another example is using a laser source and detector located in an aircraft or a satellite to monitor the earth's atmospheric structure, such as cloud distribution, and land and water terrain. This method may also be used to detect hidden objects in a foggy or smoky environment. Various types of microscopes use backscattered light to display the surface image of a medium with high resolution. A confocal arrangement can extend the image to less than 200 &mgr;m below the surface.
The conventional Optical Coherent Tomography (OCT) technique, which uses backscattered light, can only image the internal structure of an eye and tissue down to about 600 &mgr;m below the skin surface. No clear image of the medium structure in a deeper depth, however, can be formed using the direct backscattered light signals. This is due to multiple light scattering within a medium, which contributes to noise, loss of coherence, and reduces the intensity of light directly backscattered from the hidden object.
Presently, diffusion optical tomography is a widely utilized optical image reconstruction tomographic technique. Examples of references which disclose this technique include: U.S. Pat. No. 5,813,988 to Alfano et al. entitled “Time-Resolved Diffusion Tomographic Imaging In Highly Scattering Turbid Media,” which issued Sep. 29, 1998; W. Cai et al., “Time-Resolved Optical Diffusion Tomographic Image Reconstruction In Highly Scattering Turbid Media,” Proc. Natl. Acad. Sci. USA, Vol. 93 13561-64 (1996); Arridge, “The Forward and Inverse Problems in Time Resolved Infra-red Imaging,” Medical Optical Tomography: Functional Imaging and Monitoring SPIE Institutes, Vol. IS11, G. Muller ed., 31-64 (1993); and Singer et al., “Image Reconstruction of Interior of Bodies That Diffuse Radiation,” Science, 248: 990-3 (1993), all of which are incorporated herein by reference.
The conventional diffusion optical tomography method has several disadvantages. For example, the diffusion method only uses diffusive photons which have suffered many scattering events. Therefore, the signals received by detectors are less sensitive to changes in the structure of the turbid medium, which makes it difficult to obtain high-resolution image reconstruction. Furthermore, the diffusion method requires that the source and detector be far enough apart such that diffusion is valid (e.g., larger than 5 l
t
where It is the transport mean free path). This leads to non-portable, costly equipment (in contrast to the backscattering arrangement where the sources and the detectors can be set near each other). Indeed, in many important applications it is virtually impossible to arrange the source and the detectors separately. Another disadvantage to this approach is that it requires the simultaneous imaging of a large volume of the medium, which, in many cases, is the entire volume of the turbid medium being tested. When solving the inverse problem, however, due to practical limitations in computation time, the number of voxels (a voxel is a division of the medium) can not be too large since the computation time is proportional to N
2.5-3
, where N is the number of voxels. In addition, imaging a large volume leads to a large volume of each voxel and low resolution. Consequently, the resolution obtained by using the conventional diffusion tomography method is on the order of a few centimeters.
The theoretical basis for diffusion tomography is the “diffusion approximation” to the more accurate Boltzmann photon transport equation. The above-mentioned disadvantages associated with diffusion tomography originate from failure of the “diffusion approximation” to describe the early-time migration of photons, which is when the photon distribution is highly anisotropic. Correspondingly, diffusion tomography can not be utilized in a backscattering arrangement, where sources and detectors are arranged near each other and, hence, early-time photon migration plays an important role.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a forward physical model of backscattering optical tomography for imaging objects in highly scattering turbid media.
It is another object of the present invention to provide an accurate analytical solution of the Boltzmann photon transport equation in an infinite uniform medium to serve as background Green's function in the forward physical model for the backscattering tomography method of the present invention.
It is another object of the present invention to provide a specific inverse algorithm (which is unique to the present backscattering tomography method) for determining the structure of a highly scattering turbid medium layer by layer to produce an internal map of the medium.
It is another object of the present invention to provide a tomographic method using laser sources with different wavelengths for producing an internal map of a specific material structure in a turbid medium.
It is another object of the present invention to provide experimental designs for using backscattering tomography for detecting breast cancer and to develop an optical mammography and/or tomography imaging system.
The present invention is directed to a novel optical backscattering tomographic method for imaging hidden objects in highly scattering turbid media. In one aspect of the present invention, a method for imaging objects in a highly scattering turbid medium includes the steps of: illuminating a highly scattering medium with light in visible and/or infrared spectral region; utilizing different time-gating techniques to acquire time-resolved signals of backscattered light emergent from the medium received by detectors located near the light source; applying an accurate physical model of photon migration based on solving the Boltzmann photon transport equation; and applying a specific inverse algorithm to form an image of the objects in the highly scattering turbid medium.
The optical inverse image reconstruction method of the present invention (which is based on knowledge of the physical principles of photon migration in a highly scattering turbid medium) utilizes a mathematical inverse algorithm to process intensity data of detected backscattered light to produce an image map of the internal structure the turbid medium. Advantageously, the deep internal structure of the turbid media can be imaged using the present method. For example, human tissue can be imaged to a depth on the order of several centimeters to tens of centimeters.
Preferably, an accurate analytical solution of the Boltzmann photon transport equation in an infinite uniform medium, first derived by the inventors, is described by equations (7) to (22) in the section of “detailed description of preferred embodiments”.
Preferably, the aforementioned physical model of photon migration for backscattering tomography is formed as follows. The optical parameters in a turbid medium (having hidden objects) are &mgr;
s
(r) the scattering rate, &mgr;
a
(r) the absorption rate, and &mgr;
s
(r)P(s′, s, r) the differential angular scattering rate. These parameters are position dependent, and represent the non-uniform structure of the highly scattering turbid medium. The values of these optical parameters change with different wavelength, &lgr;, of light sources. We define a change of scattering and absorption parameters as follows:
&Dgr;&mgr;
s
(
r
)=&mgr;
s
(
r
)−&mgr;
s
(0)
;
&Dgr;&mgr;
a
(
r
)=&mgr;
a
(
r
)&minus
Alfano R. R.
Cai Wei
Lax Melvin
F. Chau & Associates LLP
Kamm William E.
Research Foundation of CUNY
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