Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-11-20
2001-05-08
Smith, Ruth S. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S476000, C356S432000, C250S341100
Reexamination Certificate
active
06230045
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: immersing the turbid medium in a calibration medium, irradiating the turbid medium, measuring intensities of a part of the light transported through the turbid medium and the calibration medium along a plurality of light paths, normalizing the measured intensities and reconstructing an image of the interior of the turbid medium from the normalized intensities. The invention also relates to a device for carrying out a method of this kind.
2. Description of the 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. Furthermore, attenuation coefficient is to be understood to mean the inverse diffuse absorption distance K which is given as , K={square root over (3&mgr;
a
+L &mgr;′
s
+L )} in which &mgr;′
s
is the reduced scatter coefficient and &mgr;
a
is the absorption coefficient.
A method of this kind is described in patent application EP 97202187.7. 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. In order to counteract edge effects, according to the known method an attenuation coefficient of the calibration medium is made equal to a predetermined mean attenuation coefficient of the breast tissue. This can be achieved, for example by choosing a calibration medium in the form of a liquid with a solution of, for example Intralipid, having an attenuation coefficient which is equal to the predetermined mean attenuation coefficient of the breast tissue. Another possibility is to add a dye to the liquid with the dissolved Intralipid with a predetermined fixed percentage, and to detune the wavelength of the light to be generated by the light source in such a manner that the attenuation coefficient of the calibration medium is equal to the predetermined mean attenuation coefficient of the breast tissue. Furthermore, in order to prevent artefacts in the reconstructed image which are due to transitions between the calibration medium and an environment of the calibration medium, the measured intensities are normalized.
It is a drawback of the known method that an actual mean attenuation coefficient of the breast tissue of the body to be examined varies relative to the predetermined mean attenuation coefficient. This is due, for example to the fact that a mean attenuation coefficient of breast tissue of a group of females of approximately the same age is taken for the value of the predetermined mean attenuation coefficient. Consequently, artifacts are liable to occur in the reconstructed image of the interior of the breast.
SUMMARY OF THE INVENTION
It is an object of the method according to the invention to counteract said artifacts in the reconstructed image. To this end, the method according to the invention is characterized in that it includes a step for performing a correction on the normalized intensities in which a corrected intensity of a light path to be selected is determined by a combination of the normalized intensity of the selected light path, the normalized intensities, lengths of the light paths and a length of the selected light path. As a result of the use of the corrected intensities in the reconstruction of the image, artifacts caused by mismatching of the attenuation coefficient of the calibration medium and the actual mean attenuation coefficient of the turbid medium are counteracted. The invention is based on the recognition of the fact that in an infinite medium for a first order approximation for a selected light path having a length r and an intensity I
ijk
a first attenuation coefficient K
1
of a calibration medium can be recalculated to an intensity I
ref
associated with a second attenuation coefficient K
2
of the reference medium. Furthermore, if the intensities for a given K
1
of a calibration medium are known, source strengths and photodetector sensitivities are also defined, so that artefacts in the reconstructed image which are caused by a change of the intensity of a light source to be used or by a change of the sensitivity of a photodetector are counteracted. Furthermore, it is assumed that edge effects which occur between the calibration medium and the turbid medium or the edge effects which occur between the calibration medium and a holder containing the calibration medium and the turbid medium will change in a predictable manner when the attenuation coefficient K of the calibration medium varies, so that the edge effects can be compensated after the reconstruction. A further advantage consists in that in order to perform the correction, a calibration measurement need be determined only once so as to determine the intensities associated with the light paths in the calibration medium. This calibration is subsequently used to determine the corrections of the normalized intensities for all subsequent measurements performed by means of the same device on different human or animal bodies by use of the formula
ln
(
I
ref
I
c
a
l
)
=
(
K
2
-
K
1
)
r
(
1
)
in which r is the length of a selected light path between a light source and a photodetector. As a result of the estimation of K
2
−K
1
from the combination of the normalized intensities and lengths of the plurality of light paths, a corrected intensity can be determined for a light path to be selected by means of I″=I′−(K
2
−K
1
)r, in which I′ represents the normalized intensity and r represents the length of the selected light path. Consequently, the results of intensity measurements of the object in the reference medium can be determined without performing a measurement of the object in the reference medium.
A special version of the method according to the invention is characterized in that a value of the combination comprises a function of the length of the selected light path and a derivative of the normalized intensity to the length of the light path. In conformity with the formula (1), it appears that the derivative constitutes an estimate of the difference K
2
−K
1
. The derivative is equal to a direction coefficient of a reference line representing the logarithm of the ratio of the measured intensity of the calibration medium to the measured intensity of the reference medium as a linear function a
1
.r of the selected light path, a first order parameter a
1
of which corresponds to an estimate of the difference K
2
−K
1
between the attenuation coefficients of the calibration medium and the reference medium, respectively.
A further version of the method according to the invention is characterized in that the normalized intensity of a light path to be selected is determined by a logarithm of a ratio of a first intensity of a selected light path in the turbid medium and the calibration medium to a second intensity of the selected light path in the calibration medium. As a result of this normalization, the corrected intensity I″ for a light path to be selected is determined from the measured intensity in conformity with the formula
I
″
=
ln
(
I
measurement
I
c
a
l
′
)
-
a
1
·
r
,
in which I
measurement
represents the measured intensity of the selected light path in the turbid medium and the calibration medium, I
cal
represents the measured intensity of the selected light path in the calibration medium, a
1
represents the direction coefficient of the reference line, and r represents the length of the selected light path.
By using a first constant b in the correction, a correction can be made for a change of a coupling
't Hooft Gert W.
Hoogenraad Johannes H.
Paasschens Jeroen C. J.
Smith Ruth S.
U.S. Philips Corporation
Vodopia John F.
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