X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-10-19
2001-10-16
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000
Reexamination Certificate
active
06304626
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a two-dimensional array type of X-ray detector and a computerized tomography apparatus equipped with that X-ray detector.
The principal scan scheme in current X-ray computerized tomography apparatuses is the so-called third-generation type which collects projection data repeatedly while an X-ray tube and an X-ray detector revolve around a living body under examination. The principal X-ray detector is the so-called one-dimensional type in which a number of X-ray detecting elements are arranged in the channel direction.
In recent years, a scintillation type of X-ray detecting element has been put into practical use which consists of a combination of a scintillator and a photodiode. Having a good X-ray-to-electricity conversion efficiency and being compact and light, this type of detecting element surpasses an ionization chamber type of detector.
This compact, light X-ray detecting element has contributed greatly to the practical use of a two-dimensional array type of X-ray detector (also referred to as the multislice type of X-ray detector) in which a plurality of X-ray detecting elements are arrayed into a matrix. The two-dimensional array X-ray detector is manufactured roughly in two stages as shown in FIG.
1
: in the first stage, a plurality of X-ray detecting elements corresponding in number to slices, for example, three X-ray detecting elements are arrayed along the Y direction (slice direction) to produce a module and, in the second stage, a plurality of modules corresponding in number to channels are arranged along the x direction (channel direction) so as to form a circular arc.
Here, a problem is that a module arrangement error arises in the second stage. That is, the X-ray detecting elements cannot be arranged accurately in a line in the channel direction. As a result, an artifact may be produced on a reconstructed image.
In addition, the scintillation type of X-ray detecting element has a property that the sensitivity drops abruptly in the vicinity of the edges of the scintillator as shown in
FIG. 2
because the effective area of the photodiode is smaller than the plane of X-ray incidence of the scintillator. The half shadow of the collimator falls mainly on the edge portions of the scintillator. The half shadow greatly varies with variations in the geometrical position relationship of a collimator with the X-ray tube and/or the detector due to thermal expansion of parts of the X-ray tube. Thus, the sensitivity at the edges is very unstable, which may produce an artifact on the reconstructed image.
A method to solve the above problem is disclosed in Japanese Unexamined Patent Publication No. 5-184563. According to this method, a plurality of collimators are arranged at regular intervals along the slice direction between a body under examination and a two-dimensional X-ray detector. The collimator allows X-rays to be formed smaller than the dimension of the effective area of each X-ray detecting element in the slice direction. The resulting X-rays fall on only a portion
100
in the vicinity of the center of the effective area of each X-ray detecting element as shown in
FIG. 1
, thus allowing the effective area of the X-ray detecting element to have a margin. The module arrangement error is therefore allowed by the amount corresponding to that margin.
In Japanese Unexamined Patent Publication No. 9-224929, a collimator that is adjustable during scanning, called a dynamic pre-patient collimator, is provided between an X-ray tube and a body under examination, which allows the position of X-ray exposure to shift so as to reduce the effect of module arrangement errors.
However, the above-described two methods are accompanied by two problems that the half shadow is produced by the collimator and errors of collimator arrangement with respect to the X-ray tube and the X-ray detector are unavoidable. This means that the cause of the artifact is only changed from the module arrangement error to the two problems.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a two-dimensional array type of X-ray detector which cancels out the instability of sensitivity of X-ray detection of the edges of the X-ray detecting elements and arrangement errors of X-ray detecting element modules.
A two-dimensional array type of X-ray detector of the present invention, incorporated into an X-ray computerized tomography apparatus, has a plurality of X-ray detecting elements arranged in the x and Y directions for detecting incident X-rays into electric signals. A mask is placed adjacent to the X-ray detecting elements in the X-ray incidence side. The mask is formed with a pattern of shield portions in line form each extending substantially parallel to the channel direction (X direction) to shield edges of all the X-ray detecting elements arranged in the X direction from incident X-rays. Thereby, the instability of sensitivity of X-ray detection of the edges of the X-ray detecting elements and arrangement errors of the X-ray detecting elements in the slice direction (Y direction) are canceled out.
Another two-dimensional array type of X-ray detector of the present invention, incorporated into an X-ray computerized tomography apparatus, has a plurality of X-ray detecting elements arranged in the X and Y directions for detecting incident X-rays into electric signals. A mask is placed adjacent to the X-ray detecting elements in the X-ray incidence side. The mask has a plurality of shield portions and a transparent portion substantially transparent to X-rays for supporting the shield portions. The shield portions are patterned in lines substantially parallel to the X direction. The edges of all the X-ray detecting elements arranged in the channel direction (X direction) are shielded from incident X-rays. Thereby, the instability of sensitivity of X-ray detection of the edges of the X-ray detecting elements and arrangement errors of the X-ray detecting elements in the slice direction (Y direction) are canceled out.
Still another two-dimensional array type of X-ray detector of the present invention, incorporated into an X-ray computerized tomography apparatus, has a plurality of X-ray detecting elements arranged in the X and Y directions for detecting incident X-rays into electric signals. A mask is placed adjacent to the X-ray detecting elements in the X-ray incidence side. The mask shields the edges of each X-ray detecting element which are opposed to each other in the Y direction from incident X-rays. Thereby, the instability of sensitivity of X-ray detection of the edges of each X-ray detecting element is canceled out.
A further two-dimensional array type of X-ray detector of the present invention, incorporated into an X-ray computerized tomography apparatus, has a plurality of X-ray detecting elements arranged in the X and Y directions for detecting incident X-rays into electric signals. A mask is placed adjacent to the X-ray detecting elements in the X-ray incidence side. The mask has a plurality of shield portions for shielding the edges of each X-ray detecting element which are opposed to each other in the Y direction from incident X-rays and a transparent portion substantially transparent to X-rays for supporting the shield portions. Thereby, the instability of sensitivity of X-ray detection of the edges of the X-ray detecting elements is canceled out.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
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patent: 4731534 (1988-03-01), Klein et al.
patent: 4982096 (1991-01-01), Fujii et al.
patent: 5131021 (1992-07-01), Gard et al.
patent: 5400378 (1995-03-01), Pfoh et al.
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patent: 5666395 (1997-09-01), Tsukamoto et al.
patent: 6018566 (2000-01-01),
Adachi Akira
Igarashi Kenji
Kabushiki Kaisha Toshiba
Kiknadze Irakli
Kim Robert H.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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