Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2003-04-15
2004-03-09
Ruhl, Dennis (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C378S062000, C378S043000, C382S132000
Reexamination Certificate
active
06704591
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method, an apparatus and a program for restoring phase information, which are used for constituting an image on the basis of image information obtained by radiation imaging or the like. In this application, the word “radiation” is used in a wide sense so as to include a corpuscular beam such as an electron beam, or an electromagnetic wave, in addition to a general radiation including X-rays and the like.
2. Description of a Related Art
Conventionally, an imaging method using X-rays or the like is utilized in various fields, and employed as one of the most important means for diagnosis, particularly, in a medical field. Since a first X-ray photograph was realized, X-ray photography has been repeatedly improved and a method using a combination of a fluorescent screen and an X-ray film is predominantly used at present. On the other hand, in recent years, various digitized devices such as X-ray CT, ultrasonic or MRI are in practical use and establishment of a diagnostic information processing system and the like in hospitals is being promoted. As for X-ray images, many studies have also been made for digitizing an imaging system. The digitization of the imaging system not only enables a long-term preservation of a large amount of data without incurring deterioration in image quality but also contributes to development into the medical diagnostic information system.
Incidentally, thus obtainable radiation images are generated by converting intensity of radiation transmitted through an object into brightness of the image. For example, in the case of imaging a region including a bone part, the radiation transmitted through the bone part is largely attenuated, and the radiation transmitted through a region other than the bone part, namely, a soft part is slightly attenuated. In this case, since the difference in the intensity of the radiation transmitted through different tissues is large, the radiation image with high contrast can be obtained.
On the other hand, for example, in the case of imaging a region of the soft part such as a breast, since the radiation apt to transmit wholly in the soft part, the difference between tissues in the soft part hardly appears as the difference in the intensity of the transmitted radiation. Because of this, as for the soft part, only a radiation image with low contrast can be obtained. Thus, the radiation imaging method is not suitable as a method of visualizing slight difference between tissues in the soft part.
Herein, information contained in radiation transmitted through an object includes phase information in addition to intensity information. In recent years, a phase contrast method has been studied in which an image is generated by using the phase information. The phase contrast method is an image construction technique for converting the phase difference resulted by transmitting X-rays or the like through the object into the brightness of the image.
Examples of the phase contrast method include a method of obtaining the phase difference on the basis of interference light generated by using an interferometer or a zone plate, and a method of obtaining the phase difference on the basis of diffracted light. Among them, in the method of obtaining the phase difference on the basis of the diffracted light, which method is called as a diffraction method, the phase difference is obtained on the basis of the following principle. For example, X-ray propagates through substance by travel of waves similar to light. Propagation velocity thereof varies depending on a refractive index of the substance. Therefore, when X-rays having a uniform phase are irradiated toward an object to be inspected, a difference is made in a propagation way of the X-ray, depending on the difference between tissues in the object. For this reason, a wave front of the X-ray transmitted through the object is distorted and, as a result, diffraction fringes are produced on an X-ray image obtained on the basis of the transmitted X-ray. A pattern of the diffraction fringes varies depending on the distance between a screen on which the X-ray image is formed and the object, or wavelength of the X-ray. Accordingly, by analyzing two or more sheets of X-ray images having different diffraction fringe patterns, phase difference of X-rays, which is produced at each position of the screen, can be obtained. By converting the phase difference into the brightness, the X-ray image, in which difference between tissues in the object clearly appears, can be obtained.
In particular, in the radiation transmitted through a soft part of an object, the phase difference is larger than the intensity difference depending on the difference of tissues through which the radiation has transmitted, and therefore, delicate difference between tissues can be visualized by using the phase contrast method.
For the purpose of using such a phase contrast method, imaging conditions in the radiation imaging or techniques for restoring the phase from the diffraction fringe pattern are being studied.
For example, B. E. Allman et al. “Noninterferometric quantitative phase imaging with soft x rays”, J. Optical Society of America A, Vol. 17, No. 10 (October 2000) pp. 1732-1743 discloses that the phase restoration is performed on the basis of image information obtained by imaging with soft X-rays to constitute an X-ray image.
In this reference, TIE (transport of intensity equation), which is the basic equation of the phase restoration, is used.
κ
∂
I
(
r
⇀
)
∂
z
=
-
∇
⊥
·
{
I
(
r
⇀
)
∇
⊥
φ
(
r
⇀
)
}
(
1
)
where
∇
⊥
=
(
∂
∂
x
,
∂
∂
y
)
and k is a wave number.
Here, principle of the phase restoration is described by referring to FIG.
8
. As shown in
FIG. 8
, the X-ray having wavelength of &lgr; emits from the left side of the figure, transmits through an object plane
101
and enters a screen
102
at a distance of z from the object plane
101
. At this time, when assuming intensity of the X-ray and phase thereof at a position (x,y) on the screen
102
to be I (x,y) and &phgr; (x,y) respectively, relationship represented by the following expression holds between the intensity I (x,y) and the phase &phgr; (x,y). Here, the intensity I is square of amplitude of the wave.
2
π
λ
∂
I
(
x
,
y
)
∂
z
=
-
∇
·
{
I
(
x
,
y
)
∇
φ
(
x
,
y
)
}
(
2
)
In the expression (2), by substituting k=2&pgr; and rewriting (x,y) component into a vector r, the TIE represented by the expression (1) is derived.
Further, T. E. Gureyev et al. “Hard X-ray quantitative non-interferometric phase-contrast imaging”, SPIE Vol. 3659 (1999) pp. 356-364, discloses that the phase restoration is performed on the basis of image information obtained by imaging with hard X-rays to constitute an X-ray image.
In this reference, the TIE represented by the expression (1) is approximated as follows.
First, the expression (1) is developed as follows:
-
κ
∂
I
(
x
,
y
)
∂
z
=
(
∂
∂
x
,
∂
∂
y
)
·
(
I
(
x
,
y
)
∂
φ
(
x
,
y
)
∂
x
,
I
(
x
,
y
)
∂
φ
(
x
,
y
)
∂
y
)
=
∂
∂
x
(
I
(
x
,
y
)
∂
φ
(
x
,
y
)
∂
x
)
+
∂
∂
y
(
I
(
x
,
y
)
∂
φ
(
x
,
y
)
∂
y
)
=
I
(
x
,
y
)
(
∂
2
φ
(
x
,
y
)
∂
x
2
+
∂
2
φ
(
x
,
y
)
∂
y
2
)
+
∂
I
(
x
,
y
)
∂
x
∂
φ
(
x
,
y
)
∂
x
+
∂
I
(
x
,
y
)
∂
y
∂
φ
(
x
,
y
)
∂
y
=
I
(
x
,
y
)
∇
2
φ
&a
Pass Barry
Ruhl Dennis
LandOfFree
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