X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2003-05-01
2004-04-20
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S098000
Reexamination Certificate
active
06724857
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method, an apparatus and a program for radiation imaging, which are used for constituting an image on the basis of radiation image information obtained by radiation imaging. 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 such as X-rays, &agr;-rays, &bgr;-rays, &ggr;-rays, ultraviolet 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, a radiation image obtained as described above is 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 is easily transmitted through the soft part as a whole, 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 irradiating an object with an X-ray that has a uniform phase, the way the X-ray propagates varies in accordance with 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. 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 performing the radiation imaging or techniques for restoring the phase from the diffraction fringe patterns are being studied. For example, T. E. Gureyev et al. “Quantitative In-Line Phase-Contrast Imaging with Multienergy X Rays”, PHYSICAL REVIEW LETTERS Vol. 86, No. 25 (2001), pp. 5827-5830 discloses that the phase restoration is performed on the basis of image information obtained by X-ray imaging with three types of X-rays having different wavelength respectively.
In the reference, relationship between phase and intensity of the X-ray just after having transmitted through an object to be inspected, and intensity of the X-ray at a predetermined distance from the object is noticed. That is, in the reference, as shown in
FIG. 8
, the X-ray imaging is performed on the assumption of such configuration that three types of X-rays having wavelength of &lgr;
0
, &lgr;
1
and &lgr;
2
respectively transmit through an object
100
to be inspected and enter a screen
102
disposed at a distance of R from an object plane
101
.
In this case, relationship represented by the following expression stands up between intensity I(r
⊥
,0,&lgr;
0
) and phase &phgr;(
⊥
,0,&lgr;
0
) of the X-ray just after having transmitted through the object
100
to be inspected, and intensity I(r
⊥
,R,&lgr;
m
) of the X-ray diffraction light detected on the screen
102
, provided that in the following expression (1), I(r
⊥
,0,&lgr;0)=exp{−M(r
⊥
,0,&lgr;
0
)}
A
(
M
(
r
⊥
,
0
,
λ
0
)
-
∇
2
φ
(
r
⊥
,
0
,
λ
0
)
∇
M
·
∇
φ
(
r
⊥
,
0
,
λ
0
)
)
=
(
g
0
g
1
g
2
)
(
1
)
where
A
=
(
-
1
γ
0
γ
0
-
σ
1
3
σ
1
γ
1
σ
1
4
λ
1
-
σ
2
3
σ
2
γ
2
σ
2
4
γ
1
)
provided that
σ
m
=
λ
m
λ
0
,
γ
m
=
R
λ
m
2
π
,
g
m
=ln{I
(
r
⊥
,R,&lgr;
m
) (
m
=0,1,2)
In the expression (1), when ∇M·∇&phgr;(r
⊥
,0,&lgr;
0
) is sufficiently small, it can be approximated as follows.
(
-
1
γ
0
-
σ
1
3
σ
1
γ
1
)
(
M
(
r
⊥
,
0
,
λ
0
)
-
∇
2
φ
(
r
⊥
,
0
,
λ
0
)
)
=
(
g
0
g
1
)
(
2
)
Further, from the expression (2), the intensity and the phase of the X-ray just after having transmitted through the object
100
to be inspected are represented as follows.
M
(
r
⊥
,
0
,
λ
0
)
=
λ
0
Δ
λ
(
g
0
-
σ
-
2
g
1
)
(
3
)
-
∇
2
φ
(
r
⊥
,
0
,
λ
0
)
=
2
π
R
Δ
λ
(
σ
g
0
-
σ
-
2
g
1
)
(
4
)
By performing an inverse Laplace operation on Laplace ∇&phgr;
2
(r
⊥
,0,&lgr;
0
) of phase in the expression (4), the phase &phgr;(r
⊥
,0,&lgr;
0
)can be obtained. Further, by conver
Bruce David V.
Fuji Photo Film Co. , Ltd.
Sughrue & Mion, PLLC
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