X-ray or gamma ray systems or devices – Beam control – Antiscatter grid
Reexamination Certificate
2001-07-06
2003-12-30
McCall, Eric S. (Department: 2855)
X-ray or gamma ray systems or devices
Beam control
Antiscatter grid
C378S070000, C378S086000
Reexamination Certificate
active
06671348
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray image detecting apparatus for detecting an X-ray image of a subject, such as a person to be examined for diagnosis or the like.
2. Description of the Related Art
Recently, X-ray image acquisition systems for taking X-ray images of subjects being examined for diagnosis using semiconductor sensors have been developed.
When compared with conventional X-ray radiographic systems employing ordinary silver halide photography, these X-ray image acquisition systems have such advantages in practical use that images can be recorded which have a very wide dynamic range corresponding to a very wide range in the amount of radiation, to which the sensor is exposed. That is, X-ray images can be obtained which are unlikely to be affected by variations in the amount of exposure of radiation; after X-rays with a very wide dynamic range are read with a detector including a photoelectric transducer and converted into an electric signal, the electric signal is processed so as to output X-ray images on recording materials such as a photosensitive material, and the like and on display units such as a CRT, and the like, as visible images. In this radiography, an X-ray grid, which removes scattered X-rays generated in subjects, are used in many cases in order to improve contrast in a radiographic image.
FIG. 1
is a sectional view of an X-ray grid and a detector used in a conventional radiographic apparatus. An X-ray image detector
1
is arranged such that a plurality of photoelectric conversion elements
3
are two-dimensionally disposed on an insulation substrate
2
, and further, fluorescent substance
4
is laminated on the photoelectric conversion elements
3
. In addition, a grid
5
is disposed above the X-ray image detector
1
with a predetermined space therebetween. The grid
5
is arranged such that foils
7
, which are composed of lead or the like, having a high X-ray absorption ratio, and intermediate materials
8
, which are composed of aluminum or the like, having a low X-ray absorption ratio, are held by a cover member
6
. Using the grid
5
arranged as described above permits primary X-rays L
1
, which have passed through a subject without being scattered thereby, to pass through the grid
5
and to reach the fluorescent substance
4
of the X-ray image detector
1
. When X-rays L
1
are irradiated onto the fluorescent substance
4
, the optical materials (light emitting materials) in the fluorescent substance
4
are excited and emit fluorescence L
2
having a wavelength within the spectral sensitivity wavelength range of the photoelectric conversion elements
3
. Further, X-rays which are incident on the grid
5
with a large angle with respect to the primary X-rays L
1
, such as a scattered X-ray component L
3
generated by the subject, are absorbed by the foils
7
.
During exposure of radiation, the grid
5
is moved in a direction B or C by a drive unit (not shown). With this operation, an excellent image can be obtained by the X-ray image detector
1
which has no image component of stripes of the grid
5
as well as no moires or aliasing caused by a difference between the pitch of the foils
7
and the pitch of the pixels of the X-ray image detector
1
.
Radiography is required to satisfy contradictory conditions (1) that an excellent image with a high contrast is to be obtained while (2) reducing the dosage of the subjects as much as possible by reducing the amount of X-rays with which they are irradiated. However, the grid
5
shown in
FIG. 1
may act as a factor for deteriorating an image by reducing the intensity of X-rays on the X-ray image detector
1
.
One reason for this reducing of the intensity of X-rays is that the X-rays L
1
, which reach the X-ray image detector
1
, must pass through the intermediate materials
8
. While the intermediate materials
8
are composed of aluminum or the like having a high X-ray transmission ratio as described above, that transmittance is not 100% as a matter of fact. When, for example, the thickness &Dgr;1 of the foils
7
is set to 43 &mgr;m at a time a grid density is 40 lines/cm and a grid ratio is 10:1, the intermediate materials
8
have a thickness &Dgr;2 of 207 &mgr;m (=1 cm/40−&Dgr;1) and a height &Dgr;3 of 2070 &mgr;m (=&Dgr;2×10).
When the intermediate materials
8
are composed of aluminum, the aluminum in the above case has a thickness of about 2 mm, and the primary X-rays L
1
have a transmittance of about 70%. Accordingly, about 30% of the intensity of the X-rays will be lost. Further, when viewed from the direction from which the X-rays are incident, 17% (=&Dgr;1/(&Dgr;2+&Dgr;1)) of the grid
5
is composed of lead through which X-rays do not pass. Accordingly, the total X-ray transmittance of the grid
5
is about 60% (0.7×(1−0.17)) when the loss of the intermediate materials
8
is also taken into consideration, which means that the reduction of the intensity of X-rays caused by the grid
5
is large and cannot be ignored.
Further, the fluorescence L
2
generated in the fluorescent substance
4
by the primary X-rays which have passed through the grid
5
, radiates in various directions because the fluorescent substance
4
is formed in a continuous flat shape so as to entirely cover the photoelectric conversion elements
3
. Accordingly, this fluorescence L
2
reaches not only a photoelectric conversion element
3
a
located just below a position where it emits but also other photoelectric conversion elements, for example,
3
b
, and the like adjacent to the photoelectric conversion element
3
a.
Therefore, as described below, the grid
5
reduces the intensity of X-rays, while it does remove the incident scattered X-ray component L
3
. Further, the continuous flat-shaped fluorescent substance
4
may deteriorate the MTF (modulation transfer function) of the X-ray image detector because the fluorescence L
2
generated in the fluorescent substance
4
reaches a plurality of adjacent photoelectric conversion elements. Furthermore, when the intensity of the emitting fluorescence L
2
is increased by increasing the thickness of the fluorescent substance
4
to improve the intensity of signals outputted from the photoelectric conversion elements
3
, the above tendency becomes stronger, and improvement of the sensitivity of X-ray image detectors may be impeded.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention, which was made based on the above recognition of the problem, to provide an excellent X-ray image detecting apparatus capable of obtaining a good image having a high contrast while reducing the dosage received by a subject.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.
REFERENCES:
patent: 5801385 (1998-09-01), Endo et al.
patent: 5956382 (1999-09-01), Wiener-Avnear et al.
patent: 6034406 (2000-03-01), Kobayashi et al.
patent: 6118851 (2000-09-01), Endo et al.
patent: 6429578 (2002-08-01), Danielsson et al.
patent: 2002/0117626 (2002-08-01), Danielsson
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