X-ray CT apparatus

Details X-ray CT apparatus X-ray CT apparatus

2001-07-05

2003-06-17

Dunn, Drew A. (Department: 2882)

X-ray or gamma ray systems or devices

Specific application

Computerized tomography

C378S015000

Reexamination Certificate

active

06580777

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an x-ray CT (Computed Tomography) apparatus and, more particularly, to a technique of precisely measuring a three-dimensional (3D) CT image (3D image) of a human body or the like in a large field of view.
2. Description of the Related Prior Arts
As a conventional x-ray CT apparatus, a cone-beam CT apparatus for measuring a 3D CT image of the object is known. The cone-beam CT apparatus is an apparatus for detecting a projection of the object of an x-ray emitted radially (conically) from an x-ray source by a two-dimensional (2D) detector, and reconstructing a 3D CT image of the object. A general cone-beam CT apparatus, as shown in
FIG. 22A
, rotates a pair of an x-ray tube
1
and a 2D detector
3
around the object
8
and scans the object with the x-ray to obtain a projection. In this case, the orbit of the x-ray source
1
is a circle orbit indicated by a circle
2200
. It is, however, known that data necessary for reconstructing a 3D CT image of the object cannot be completely collected with the circle orbit. It causes distortion, artifact, or the like in a reconstructed image, and a problem such that the image quality of an obtained 3D CT image deteriorates arises.
A necessary condition to collect complete data was first taught by a paper of Tuy, “An inversion formula for cone-beam reconstruction”, J. Appl. Math., Vol. 43, 1983, pp. 546-552 (Literature
1
). Necessary and sufficient conditions were proved by a paper of B. D. Smith, “Image reconstruction from cone-beam projections: Necessary and sufficient conditions and reconstruction methods”, IEEE Trans. Med. Imag., Vol. MI-4, 1985, pp. 14 to 25 (Literature
2
).
In the following description, the condition disclosed in Literature
1
will be referred to as “Tuy's condition”. The Tuy's condition is that “the necessary and sufficient condition to collect complete data is that each of all the planes crossing an object crosses an orbit of an x-ray source at least one point”. An example of an x-ray orbit satisfying the Tuy's condition is described in a paper of H. Kudo and T. Saito, “Feasible cone beam scanning methods for exact reconstruction in three-dimensional tomography”, J. Opt. Soc. Am. A., Vol. 7, 1990, pp. 2169 to 2183 (Literature
3
). In the example of the x-ray orbit described in Literature
3
, assuming that the object is a human body, it is difficult to use two circle orbits which perpendicularly cross each other. Preferably, a helical orbit is used.
The inventors of the present invention examined the conventional arts and found the following problems. The scanning method realizing the helical orbit described in Literature
3
can be, as shown in
FIG. 21
, easily realized by rotating a set of the x-ray source
1
and the detector
3
around the object
8
and, simultaneously, moving the set of the x-ray source
1
and the detector
3
in the direction Z of the rotation axis. However, since the field of view of the detector
3
is also deviated in the direction Z of the rotation axis, a problem such that a shaded common region
601
is limited occurs. In other words, there is a problem such that the 3D region which can be reconstructed is small.
An example of the scanning method for solving the problem that the common region
601
is limited is disclosed in Literature
3
. According to the scanning method described in Literature
3
, as shown in
FIG. 22B
, a scan is performed while moving the x-ray source
1
along a helical orbit
2202
on a spherical shell
2201
having a center point O of rotation. By always fixing the center of the view field of the detector
3
at the point O, a large common region is assured. In the following description, the scanning method described in Literature
3
will be referred to as “spherical-helical scan”. According to a method of realizing the spherical-helical scan described in Literature
3
, as shown in
FIG. 22C
, the object
8
is rotated around the Z-axis, and the x-ray source
1
is moved on a circular arc
2203
having the point O as a center. In the method described in Literature
3
, however, the object
8
has to be rotated. In the case where a human body is assumed as the object
8
, a problem such that it is difficult to realize the method occurs. To be specific, due to a centrifugal force accompanying the rotation of the object
8
, the object
8
moves during a scan, and a problem such that the picture quality of a 3D CT image deteriorates arises. There is also a problem such that a patient being operated or a patient required to rest in bed cannot be rotated.
SUMMARY OF THE INVENTION
An object of the invention is to provide a technique of collecting complete data necessary to reconstruct an image of the object in a stationary state, further, a technique of acquiring a 3D CT image of a high picture quality, a technique of easily enlarging a common region, and an x-ray CT apparatus capable of realizing improved accuracy of diagnosis of a lung cancer or the like.
The objects and novel features of the invention will become apparent from the description of the specification and the appended drawings.
A representative invention disclosed in the application will be briefly described as follows. As shown in
FIG. 6
, a scanner (rotary plate) on which a scanning system including an x-ray source
1
and a two-dimensional detector (serving as image acquiring means)
3
is mounted is rotated around an object. Simultaneously, the axis Z′ of rotation of the scanner (which coincides with the axis of rotation of the scanning system) is tilted with respect to the body axis direction Z of the object
8
. The tilting is performed around the center O of rotation (which coincides with the center of rotation of the scanning system) of the scanner as a center. By carrying out the rotation and tilting simultaneously, a helical scan as shown in
FIG. 22B
can be realized without rotating the object
8
. Since the view field O of the two-dimensional detector
3
is always fixed to the object
8
, a common region
601
can be enlarged.
In the configuration of the representative x-ray CT apparatus of the invention, a scanner on which a scanning system is mounted, the scanning system having an x-ray source for generating radial x-rays emitted to an object and image acquiring means provided so as to face the x-ray source and acquiring a projection of the object is rotated around the object by rotating means, a tilt angle formed between a surface of rotation of the scanner and a body axis of the object is changed while rotating the scanner, projections are acquired from a plurality of directions, and a CT image of the object is generated and displayed. The tilting means tilts the surface of rotation of the scanner to change the tilt angle while holding the center of rotation of the scanner almost in a predetermined position. Since the tilt angle is changed so that the upper part of the gantry storing the scanner is moved apart from the head of the object, the upper part of the gantry is moved apart from the field of view of the object and the object does not feel fear. The shortest distance between the rotation center of the scanner and the rotational axis of the tilting means is set to be smaller than spatial resolution of the image acquiring means. Consequently, the spatial resolution of a CT image does not deteriorate.
Further, as functions of the tilting means, various functions exist as follows. The tilt angle is changed by using the straight line almost orthogonal to the axis of center of rotation of the scanner as a central axis. The tilt angle is changed by using the straight line almost orthogonal to the axis of center of rotation of the scanner and the body axis of the object as a central axis. A change amount of the tilt angle with respect to a unit rotation amount of the scanner is held to be constant. A period of rotation of (360°×N) (where, N denotes a natural number) of the scanner by the rotating means is synchronized with a period from start until end of a chang

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