X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-12-28
2002-12-10
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S038000, C378S039000
Reexamination Certificate
active
06493415
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) method and apparatus for obtaining panoramic X-ray images of an object to be examined seen from an optional direction by radiating a conical X-ray beam on a part of the object to be examined.
2. Prior Art
A method of obtaining a panoramic X-ray image known as X-ray computer tomography (CT) has been widely used in medical practice as for diagnosis. This method involves radiating X-rays on the object from all around, followed by analyzing a three-dimensional absorption coefficient distribution information, namely a three-dimensional distribution information of an X-ray absorption coefficient, on the X-rayed object from the projection data by the Radon theory as widely known, thereby obtaining a sectional image of object.
The prior art X-ray CT is a technique in which a fan-shaped X-ray beam relatively wide in the direction of rotation and thin is radiated on the object from around at a depth and this is repeated at different depths.
Accordingly, in the event that only a part inside the object is to be put to tomography examination, the fan-shaped wide X-ray beam is radiated over the whole object to obtain a three-dimensional distribution information of an X-ray absorption coefficient from which a three-dimensional distribution information of an X-ray absorption coefficient on that part or region is taken out for analysis. That is, the object is exposed to a substantial dose of the X-ray beam. In addition, it takes long to radiograph and analyze the test data. Accordingly in the light of high dose of radiation, the CT examination is limited to some once a year.
The applicant of the present invention proposes an X-ray computer tomography (CT) method and apparatus for solving the problem of exposed dose of the X-ray beam, namely by radiating conical X-ray beams of a small section area onto a local region, the object to be examined, only once, the three-dimensional distribution information of an X-ray absorption coefficient of the local region is calculated out.
FIG. 24
a
is a plane view conceptually showing the area of the three-dimensional distribution information of an X-ray absorption coefficient obtained by the X-ray CT apparatus,
FIG. 24
b
is a partial plane view for conceptually showing the method for producing panoramic X-ray images from the three-dimensional distribution information of an X-ray absorption coefficient and
FIG. 24
c
is a partial view of the panoramic X-ray image produced by this method.
In
FIG. 24
a
, S is a dental arch and SA is a curved sectional area along the dental arch S and is covered by oblique lines. According to the X-ray CT apparatus, a distribution information of an X-ray absorption coefficient is obtained for the curved sectional area SA, which is an object to be examined. In this area SA, if a region is designated, the three-dimensional distributed information of an X-ray absorption coefficient at the region can be obtained. SB is a panoramic image layer of the curved sectional area SA. Lc is a normal line to the panoramic image layer SB. In
FIG. 24
b
, H
1
and H
2
conceptually show foreign objects existing inside of a part of the dental arch S.
The X-ray CT apparatus enables the production of normal panoramic X-ray images as such shown in
FIG. 24
c
by processing the three-dimensional distribution information of an X-ray absorption coefficient on the normal line Lc to the panoramic image layer SB shown in
FIG. 24
b
. It also enables the production of sectional images of selected teeth, wherein the X-ray exposed dose is decreased to about 1 to 5% of that in case of using conventional systems.
But the panoramic X-ray images produced by the above mentioned method has one problem: as the production method of panoramic X-ray images is limited to the method shown in
FIG. 24
b
, if there are any foreign materials H
1
and H
2
in the direction of a normal line LC, they are overlapped in the obtained panoramic X-ray image shown in
FIG. 24
c
and cannot be distinguished. In addition, how a tooth is overlapped with another is sometimes invisible on a panoramic view produced from one direction.
The applicant of the present invention has proposed a panoramic X-ray picturing apparatus applicable to various dental arches in a Japanese patent application laid on unexamined under No. S60-103942. With this apparatus, the loci of the rotation of panoramic X-ray radiation can be adjusted depending on male, female or children examinees. The direction of normal lines to the dental arches is changeable depending on the size of the dental arch, and the problem derived from the fixed normal line mentioned above has been solved with that apparatus.
This apparatus, however, requires the loci of the rotation of panoramic X-ray radiation to be changed in order to obtain different panoramic X-ray images, so that the setting condition should be set before picturing. Another problem is that as this apparatus was not made for a computer tomography but for a conventional picturing of sectional images, it also requires the rotating center of the rotary arm to be moved along the specified loci during picturing, which needs a complex driving mechanism.
The applicant of this invention has also introduced in a Japanese patent application laid on unexamined under No. H4-144549 a digital panoramic X-ray picturing apparatus with which panoramic X-ray images of sectional image layers obliquely crossing the curved sectional image layers are produced.
This apparatus enabled the production of panoramic X-ray images seen from the oblique direction of the dental arch. It uses a frame image comprising the projection images and obtains partial images by shifting at a predetermined distance in accordance with the direction of the image shifting in a specific time period. Then the partial images are added to obtain a panoramic X-ray image seen from obliquely.
With this apparatus, however, the data from the obtained projection images is recalculated only after a digital processing and does not utilize the CT method computing a three-dimensional distribution information of an X-ray absorption coefficient by means of backprojection processing, so that the overlapped foreign materials H
1
and H
2
can not be distinguished.
SUMMARY OF THE INVENTION
The present invention has been proposed to solve the above-mentioned problems. According to which, a projection image taken by one X-ray picturing can be effectively utilized with all sorts of merits of X-ray computed tomography method and apparatus using a conical X-ray beam, and a panoramic X-ray image of one panoramic sectional image layer seen from an optional direction can be further gained.
The present invention has been developed to solve those problems as a result of intensive researches by the inventors and following (1)-(11) propose its method and (12)-(29) propose its apparatus.
In the X-ray CT of the present invention, a dental arch being an object to be examined is considered as a curved sectional area, an X-ray projection image is obtained for the curved sectional area by local radiation, the obtained X-ray projection image is processed by a backprojection method, and a three-dimensional distribution information of an X-ray absorption coefficient of the curved sectional area is calculated. Then the three-dimensional distribution information of an X-ray absorption coefficient is reconstructed by several methods, namely the three-dimensional distribution information of an X-ray absorption coefficient on a projection line intersecting a normal line of an area at an optional angle is calculated for the area on a panoramic image layer of the curved sectional area, the calculated result is expanded on a two-dimensional plane, and a panoramic X-ray image wherein the curved sectional area is seen from an optional projection line can be produced.
(1) An X-ray computed tomography method of the present invention is comprised of: sequentially obtaining on a two-dimensional X-ray image sensor an X-ray projection image in
Arai Yoshinori
Suzuki Masakazu
Bruce David V.
Hobden Pamela R.
Koda & Androlia
Nihon University
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