X-ray or gamma ray systems or devices – Beam control – Collimator
Reexamination Certificate
2002-07-23
2004-03-23
Glick, Edward J. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Collimator
C378S150000, C378S205000, C378S207000, C250S505100
Reexamination Certificate
active
06711236
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Application No. 2001-224725 filed Jul. 25, 2001.
BACKBROUND OF THE INVENTION
The present invention relates to an aperture position adjusting mechanism, a gantry apparatus and a method of controlling it in an X-ray CT (computerized tomography) system for obtaining an X-ray tomographic image of a subject by X-ray irradiation.
An X-ray CT system is mainly directed to the purpose of collecting projection data obtained from X-rays passing through a subject, and reconstructing an X-ray tomographic image from the projection data. Specifically, the following steps are taken: the subject is first laid on a table apparatus, and is carried to a cavity portion in a gantry apparatus. A rotating section (to which an X-ray tube and an X-ray detector are integrally attached) in the gantry apparatus is then rotated, X-rays are directed to the subject from different angles, and X-rays passing through the subject are detected at these angles. The detected data (projection data) is then received by an operating console, and an X-ray tomographic image is reconstructed by arithmetic operations. The sequence of steps for detecting X-rays as described above is generally referred to as a scan.
Also known is what is commonly referred to as a multi-slice X-ray CT system, in which the X-ray detecting section is comprised of a detector array comprising a plurality of rows arranged in the carrying direction of the table, enabling collection of data for a plurality of slices in one scan. The multi-slice X-ray CT system has an advantage that a plurality of X-ray tomographic images can be obtained in one scan.
It is generally known that during such a scan, heat gradually accumulated with the generation of X-rays from the X-ray tube leads to a shift of the focal position of the X-ray tube in the carrying direction of the table (which will be sometimes referred to as a z-axis hereinbelow). This point will be explained below.
FIG. 7
is a schematic diagram of the focal point of an X-ray tube, an aperture and an X-ray detector (comprised of two detector rows A, B) as viewed from a direction orthogonal to the z-axis.
In the drawing, the focal point is initially at a position indicated by a solid line. At this time, the position of the aperture in the z-direction is controlled so that the X-rays impinge equally on the detector rows A and B. In such an environment, the focal point shifts in the z-axis direction as indicated by a broken line by actually driving the gantry apparatus. If the position of the aperture remains at the illustrated position, the X-ray irradiation range in the z-axis direction shifts to a range indicated by broken lines. As can be seen from
FIG. 7
, the X-rays no longer impinge upon the overall surface of the detector row A in such a condition, and consequently, the relationship between signals a and b output from the respective detector rows becomes a<b, resulting in different CT values (pixel values in an X-ray tomographic image) to be reconstructed and different imaging ranges (thicknesses) between the detector rows.
Accordingly, a general X-ray CT system comprising a plurality of detector rows (multi-slice X-ray CT system) comprises a mechanism that employs detector elements typically at the ends among detector elements (also referred to as detector channels) as reference channels, determines that the focal point is shifted when the outputs from the reference channels are unequal between the rows, and adjusts the position of the aperture in the z-direction so that the outputs are equalized. Such a mechanism enables X-rays to constantly impinge equally upon all the detector rows.
FIG. 8
is a prior art mechanism for conducting such aperture position adjustment. The hatched portion on a component in the drawing indicates that the mechanism is mounted on a certain base within the rotating section in the gantry apparatus.
As shown, an aperture
80
is provided with a slit
80
a
for defining the X-ray irradiation range, and is supported by an aperture base plate
81
. A pair of parallel translating rails
82
a
and
82
b
are disposed along the z-axis direction, and the aperture base plate is slidably supported by the translating rails
82
a
and
82
b
via linear guides
83
a
and
83
b
formed at opposite ends of the aperture base plate. Moreover, a ball screw
84
is provided alongside of the translating rail
82
a
in parallel, and is attached to an output shank of a motor
86
via a coupling
85
. Finally, a nut
87
fitted on the ball screw
84
is fixed to the linear guide
83
a
, as shown.
A rotational motion of the ball screw
84
conducted by the driving motor
87
is transformed into a linear motion of the nut
87
. Since the nut
87
is fixed to the linear guide
83
a
, the aperture base plate
81
is moved following the linear guide
83
a
along the translating rails
82
a
and
82
b
(i.e., along the z-axis direction). The position adjustment of the aperture
80
is thus achieved.
In the aperture position adjustment, accuracy of the order of several micrometers is required, and therefore, the effect of backlash between the ball screw and nut needs to be eliminated as much as possible.
In the conventional case as described above, however, since the ball screw
84
is attached to an end side of the aperture base plate
81
, the backlash contained in the ball screw
84
and elastic deformation of the ball screw
84
may cause an inclination of the aperture
80
, leading to a problem that desired accuracy may not be attained.
The ball screw cannot be disposed in the center of the aperture to overcome this problem, however, because this is where the X-ray beam passes.
Alternatively, it can be contemplated that ball screws are disposed at opposite ends of the aperture base plate to reduce the effect of backlash; however, other problems arise that this increases the cost, and the two ball screws should be strictly adjusted during assembling.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an aperture position adjusting mechanism in an X-ray CT system with low cost and high accuracy, and to provide a gantry apparatus and a method of controlling it that enable stable reconstruction of X-ray tomographic images.
According to the present invention, there is provided an aperture position adjusting mechanism in an X-ray CT system capable of adjusting the position of an aperture (
6
) having an aperture opening for limiting an X-ray irradiation range, comprising: a pair of rails (
21
a
,
21
b
) disposed along a direction in which said aperture (
6
) is to be adjusted, for slidably mounting said aperture (
6
); a first shaft (
23
) that is hollow and is provided with a bore (
23
a
) passing in parallel with a center axis of said first shaft (
23
) at a position offset from said center axis, said first shaft (
23
) being rotatably supported by a base portion (
20
) of said aperture (
6
) orthogonally to said pair of rails (
21
a
,
21
b
); a second shaft (
25
) that is received and is rotatably supported within said bore (
23
a
) of said first shaft (
23
); and driving means (
7
) for rotating said second shaft (
25
) in reciprocal directions around an eccentric axis offset from a center axis of said second shaft (
25
), said mechanism characterized in that: said aperture (
6
) is moved along said rails (
21
a
,
21
b
) as said second shaft (
25
) is eccentrically rotated by said driving means (
7
) and, following said eccentric rotation and in a direction opposite to that of said rotation, said first shaft (
23
) is eccentrically rotated around a center axis of said bore (
23
a
).
Moreover, according to the present invention, there is provided a gantry apparatus in an X-ray system, comprising an aperture position adjusting mechanism.
The gantry apparatus preferably comprises an X-ray detecting section in which a plurality of detector rows are arranged in a carrying direction of a table for carrying a subject, each of said detector rows having a group of detector ele
Glick Edward J.
Ho Allen C
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