Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-02-22
2001-12-04
Smith, Ruth S. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S473000, C600S476000, C250S341100, C250S358100, C356S432000
Reexamination Certificate
active
06327489
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of localizing an object in a turbid medium, which method includes the following steps:
irradiation of the turbid medium,
measurement of intensities of a part of the light transported through the turbid medium along a plurality of light paths from a source point where the light enters the turbid medium to a measuring point where the light leaves the turbid medium,
reconstruction of an image of the interior of the turbid medium from a combination of the measured intensities.
The invention also relates to a device for localizing objects in turbid media, which device includes:
a light source for irradiating the turbid medium,
means for coupling the light to be generated from the light source into the turbid medium at different angles,
a photodetector for converting the light transported through the turbid medium into a photodetector signal,
means for converting the photodetector signal in measured intensities, and
a control unit for reconstructing an image of the interior of the turbid medium from the measured intensities.
2. Description of Related Art
In the context of the present application the term light is to be understood to mean electromagnetic radiation of a wavelength in the range of from 400 to 1400 nm. Furthermore, a turbid medium is to be understood to mean a substance consisting of a material having a high light scattering coefficient. Examples in this respect are an Intralipid solution or biological tissue.
A method of this kind is known from the article “The forward and inverse problems in time resolved infra-red imaging”, by S. R. Arridge, SPIE, IS11:35, 1993. The known method can be used for in vivo breast examinations. to determine the presence of tumors in breast tissue of a human or animal female. According to the known method the turbid medium is irradiated from different positions, intensities of light transported through the turbid medium being measured for one irradiation position in different positions where the light leaves the turbid medium. The known algebraic reconstruction technique constitutes an iterative method in which per iteration step a next image is determined in dependence on a sensitivity matrix and an intensity measurement and a previously determined image which, after the first iteration, is chosen to be equal to the image determined during the previous iteration step. Herein, a sensitivity matrix is to be understood to mean a matrix in which a matrix element (i, j) contains a sensitivity function, the row number (i) of the matrix element corresponding to a light source and a measuring position of an intensity measurement i whereas the column number (j) of the matrix element corresponds to a position in space of a volume element j. The sensitivity function of a matrix element A(i,j) is given by the formule:
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in which
V represents the volume of a volume element j in the turbid medium,
s(i)−d(i) represents a distance between a light source in a position s(i) and a measuring position d(i) for a measurement (i),
s(i)−po) represents a distance between the position of the light source s(i) during the measurement (i) and the position p(j) of a volume element (j) for which the variation of the attenuation coefficient &kgr; is determined,
p(j)−d(i) represents the distance between the position of said volume element j and the measuring position d(i) during the measurement (i), and
F(s(i),d(i),&kgr;) represents the relative variation of the measured intensity in a position d(i) of a light source in a position s(i).
Furthermore, an attenuation coefficient is to be understood to mean the inverse diffuse absorption distance &kgr;, given by the formule
&kgr;={square root over (3&mgr;
a
+L &mgr;′
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+L )},
in which
&mgr;′
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is the inverse transport-corrected free path length, and
&mgr;
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It is a drawback of the known method that the image suffers from artefacts in the form of stripes which correspond to a path from a first position where light enters the turbid medium to a second position where light leaving the turbid medium has been measured. Furthermore, artefacts in the form of a more or less regular structure occur at the center of the image.
Citation of a reference herein, or throughout this specification, is not to construed as an admission the such reference is prior art to the Applicants' invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
It is an object of the invention to counteract said artefacts in the image. To achieve this, the method according to the invention is characterized in that the combination contains a weighting factor which reduces an effect exerted on the reconstructed image by the measured intensities with a high noise factor relative to the effect exerted thereon by the measured intensities with a low noise factor. The invention is based on the recognition of the fact that the density of possible light paths near the edge of the turbid medium is higher than the density of possible light paths at the center of the turbid medium, because long as well as short light paths extend through a volume element near the edge of the turbid medium whereas only long light paths extend through a volume element at the center of the turbid medium. Furthermore, the measured intensities of light having traveled along a long light path through the turbid medium are generally lower than the measured intensities of light having traveled along a short light path through the turbid medium. Due to both factors, the effect of noise on a reconstruction of the center of the image is greater than that on the reconstruction of the edges of the image. In order to counteract the effects of noise, for example corrections can be applied in which intensities which are less than 10 times an absolute value of the noise in the intensity measurement itself are excluded from the reconstruction. However, when a weighting factor having a value 1 or 0 is chosen, it may occur that subsequent to the reconstruction the center of the image contains an area for which it may be impossible to determine a value because no measurements are available for the relevant area. Furthermore, this area may become ragged and may contain extreme values because only few measurements are available for reconstruction in comparison with the remainder of the image. On the basis of, for example an absolute measured intensity and an absolute measured intensity of a reference measurement, a weighting factor can be assigned to each intensity measured, the weighting factor being in a range from the value 0 to the value 1. Because all known reconstruction techniques, such as said algebraic reconstruction technique or back-transformation, assign location-dependent weights to the measured intensities, analogously a weighting factor is introduced in the reconstruction.
A special version of the method according to the invention is characterized in that for a light source and a measuring position in which an intensity is measured the method also includes the following steps:
determination of estimated intensities from a transformation of a predetermined first image,
determination of a difference between the measured intensities and the estimated intensities, and
determination of a next image from the first image, a convergence factor and a back-transformation of the difference, the convergence factor being chosen to be depending on the weighting factor. These step
't Hooft Gert W.
Hoogenraad Johannes H.
Smith Ruth S.
U.S. Philips Corporation
Vodopia John F.
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