X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-09-01
2001-09-11
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S038000
Reexamination Certificate
active
06289074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) method and system for producing sectional images of local region and panoramic images of an object by radiating specific conical X-ray beam to the object to be examined.
2. Description of the Prior Art
A method of obtaining a sectional 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 three-dimensional absorption coefficient distribution information 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 an absorption coefficient distribution information from which the coefficient distribution information 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. In the light of high dose of radiation, the CT examination is limited to some once a year.
In the prior art panoramic X-ray system, the X-ray beam is scanned over the dental arch from behind roughly crosswise in relation to the dental arch. The rotary arm is so designed that the X-ray generator is moved continually around three different centers a, b and c in the front tooth area of the dental arch, on the right and left of the molar tooth area as shown in FIGS.
24
(
a
) to (
c
). That requires a complicated mechanism and control of the rotary arm (not shown) to rotate the X-ray generator
101
.
FIG. 25
shows loci of the rotary arm of another prior art panoramic X-ray system during the X-ray examination. The letter Lo designates the axis of symmetry and is the median line of the axial symmetry of a dental arch. The letter L is the X-ray beam bundle roughly perpendicular to the radiation area of the dental arch. The letter La is an envelope line formed by the bundle L of the X-ray beams. In this case, too, the rotary arm has to be so moved as to form an envelope line as shown in the figure. That makes moving mechanism and control complicated.
SUMMARY OF THE INVENTION
The present invention has been developed to solve those problems of the prior art X-ray CT technique.
Therefore, it is an object of the present invention to provide an X-ray CT method that substantially shortens the time required for tomography as compared with the prior art X-ray CT and which greatly reduces the exposure dose of X-ray radiation to the object and permits size reduction of the X-ray system and to provide an X-ray system for the method.
According to the X-ray method of the present invention, since the rotary arm is rotated with the center of the rotary arm fixed to irradiate the conical X-ray beam only on the local region of the object to be examined, the exposure dose of radiation is remarkably reduced from ½ to {fraction (1/100)} that of the prior art X-ray CT examination.
And since the image information is obtained by analyzing absorption coefficient distribution information, the method of the present invention has no problem of being adversely affected by bones or the like of the object. And therefore, it is possible to reduce the whole size of the system. In addition, it may be also possible to design such a vertical-type X-ray CT system as can rotate and irradiate the X-ray in the horizontal direction with the patient in a standing or sitting position. That is, the present invention can provide a small-size X-ray CT system suitable for use in such dentists as to have only a limited floor space.
It is another object of the present invention to provide an X-ray CT method that can produce panoramic images just by putting the rotary arm in a circular motion unlike the prior art panoramic image X-ray method in which the rotary arm had to be rotated while swing back and forth, right and left the center thereof and to provide an system for the method.
It is still another object of the present invention to provide an X-ray method that can produce panoramic images by substantially reducing the exposure dose of radiation to the object to be examined and to provide an X-ray system for the method.
After intensive researches, the inventors succeeded in developing this X-ray CT method and the system. According to the present invention, desired local radiated surface images and panoramic images can be obtained just by radiating the conical X-ray beam—row in the rotating direction and having a certain thickness in the vertical direction—as if wrapping such parts as the intended local region of the object and a specific region about in the center of the dental arch. The method and system according to the present invention is suitable for X-ray examination of such local regions as the denture and maxillofacial areas and nondestructive examination of small structures.
The X-ray CT method according to the present invention involves locally radiating the conical X-ray beam to only the local region of the object by rotating the rotary arm with the X-ray generator and the two-dimensional X-ray image sensor disposed opposed to each other. This radiographing is performed with the X-ray generator radiating the conical X-ray beam on only the local region to X-ray and with the rotary arm fixed with its rotation center on the central position of the local region to radiograph.
The rotary arm does not always have to be rotated around the local region but the circle to be covered may be from some 5 degrees to the full circle, that is, 360 degrees as necessary. This may be called an improvement on the prior art the present inventors disclosed in Japanese patent application laid on unexamined under No. H10-225455.
The prior art was a panoramic tomography and local CT technique and its feature included the fixing of the rotation center and partial CT of the local region. But that did not describe specifically the local radiation or the feature of the present invention. Also, the prior art called for the full circumferential radiation as the conventional CT systems.
By contrast, the present invention is characterized in that local radiation is specifically described and the scope to be covered by radiation is not limited to the full circle of 360 degrees but may be selected depending on the needs.
After extensive study, the inventors found that radiation of the conical X-ray beam in a scope of angle depending on the kind of required CT images could produce coefficient distribution information from which desired images could be obtained. The present invention is based on that discovery.
According to the present invention, radiation covering only 5 to 90 degrees will do in the radiographing in the direction perpendicular to the denture of the front teeth. Also, to produce panoramic images of the whole jaw of the dental arch, radiation should cover just the angular scope perpendicular to the dental arch, that is, about 180 degrees. This idea has been further developed into the X-ray CT method to obtain panoramic images of the dental arch.
In the present invention, furthermore, X-ray projection images produced on the two-dimensional image sensor is processed and reconstructed by a computer. That is, if an X-ray projection image is backprojected and put to a specific arithmetic processing, a three-dimensional absorption coefficient distribution information on the inside of the X-rayed local region can be taken out as an image information. If a specific object section is designated or a specific body section is set up before hand, its sectional image can be produced.
The radiographic method of the present invention is
Arai Yoshinori
Suzuki Masakazu
Tachibana Akifumi
Bruce David V.
J. Morita Manufacturing Corporation
Koda & Androlia
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