Patent Number: 
Section: claims

1. An X-ray computer tomography apparatus comprising:an X-ray tube which generates X-rays;a two dimensional array type X-ray detector which detects the X-rays;a rotating mechanism which rotates the X-ray tube and the X-ray detector around a rotation axis;a pair of collimators which form X-rays from the X-ray tube into a cone beam shape;a collimator moving mechanism which separately moves the pair of collimators in a direction substantially parallel to the rotation axis;a reconstruction processing unit which reconstructs image data in a reconstruction range set by an operator based on an output from the X-ray detector; anda collimator control unit which controls a position of each of the collimators,wherein the collimator control unit controls the position of each of the pair of collimators in accordance with a distance between a central plane of the X-rays which corresponds to a cone angle of substantially 0° and an end face of the reconstruction range, andthe collimator moving mechanism moves each of the pair of collimators in a range from an outermost position corresponding to a maximum cone angle corresponding to a width of the X-ray detector, beyond the central plane of the X-rays, at each edge of the reconstruction range, to an innermost position offset from a position corresponding to a cone angle of substantially 0° to an opposite side. 2. The apparatus according to claim 1, wherein the collimator control unit determines the position of each of the pair of collimators based on one of a distance end and a near point on an end face of the reconstruction range when viewed from the X-ray tube. 3. The apparatus according to claim 2, further comprising a bed top configured to have an object placed thereon, and wherein the collimator control unit switches, in a scan start interval, a reference for determination of the position of one of the pair of collimators from the distant end to the near point in accordance with movement of the bed top. 4. The apparatus according to claim 3, wherein the collimator control unit increases a moving speed of one of the pair of collimators in synchronism with switching from the distant end to the near point. 5. The apparatus according to claim 3, wherein the collimator control unit switches, in a scan end interval, a reference for determination of the position of the other of the pair of collimators from the near point to the distant end in accordance with movement of the bed top. 6. The apparatus according to claim 5, wherein the collimator control unit decreases a moving speed of the other of the pair of collimators in synchronism with switching from the near point to the distant end. 7. The apparatus according to claim 5, wherein the collimator control unit fixes both the positions of the pair of collimators at the outermost position corresponding at a maximum cone angle in an intermediate interval between the scan start interval and the scan end interval. 8. The apparatus according to claim 7, wherein the collimator control unit fixes the position of the other of the pair of collimators to the outermost position in the scan start interval. 9. The apparatus according to claim 1, further comprising a bed top configured to have an object placed thereon, and wherein the collimator control unit moves, in a scan start interval, the position of one of the pair of collimators from the innermost position to the outermost position in accordance with movement of the bed top. 10. The apparatus according to claim 9, wherein the collimator control unit moves, in a scan end interval, the position of the other of the pair of collimators from the outermost position to the innermost position in accordance with movement of the bed top on which the object is placed. 11. The apparatus according to claim 10, wherein the collimator control unit fixes the position of one of the pair of collimators to the outermost position in the scan end interval. 12. The apparatus according to claim 10, wherein the collimator control unit fixes both the positions of the pair of collimators at the outermost position corresponding to a maximum cone angle in an intermediate interval between the scan start interval and the scan end interval. 13. The apparatus according to claim 1, wherein the collimator control unit controls the position of each of the collimators in accordance with a distance obtained by subtracting a predetermined distance from a distance between the central plane of the X-rays and the end face of the reconstruction range. 14. An X-ray computer tomography apparatus comprising:an X-ray tube which generates X-rays;a two dimensional array type X-ray detector which detects the X-rays;a rotating mechanism which rotates the X-ray tube and the X-ray detector around a rotation axis;a bed including a movable top on which an object is placed;a pair of collimators which form X-rays from the X-ray tube into a cone beam shape;a collimator moving mechanism which separately moves the pair of collimators in a direction substantially parallel to the rotation axis;a reconstruction processing unit which reconstructs image data in a reconstruction range set by an operator based on an output from the X-ray detector;a collimator control unit which controls the collimator moving mechanism to separately set a position of each of the pair of collimators based on the position of the top; andthe collimator moving mechanism moves each of the pair of collimators in a range from an outermost position corresponding to a maximum cone angle corresponding to a width of the X-ray detector, beyond the central plane of the X-rays, at each edge of the reconstruction range, to an innermost position offset from a position corresponding to a cone angle of substantially 0° to an opposite side. 15. An X-ray computer tomography apparatus comprising:an X-ray tube which generates X-rays;a two dimensional array type X-ray detector which detects the X-rays;a rotating mechanism which rotates the X-ray tube and the X-ray detector around a rotation axis;a bed including a movable top on which an object is placed;a variable aperture collimator mechanism including a pair of collimators which forms X-rays from the X-ray tube into a cone beam shape;a reconstruction processing unit which reconstructs image data in a reconstruction range set by an operator based on an output from the X-ray detector;a collimator control unit which controls the collimator mechanism to set an asymmetrical aperture of the collimator mechanism based on the position of the top; anda collimator moving mechanism that moves each of the pair of collimators in a range from an outermost position corresponding to a maximum cone angle corresponding to a width of the X-ray detector, beyond the central plane of the X-rays, at each edge of the reconstruction range, to an innermost position offset from a position corresponding to a cone angle of substantially 0° to an opposite side. 16. An X-ray computer tomography apparatus comprising:an X-ray tube which generates X-rays;a two dimensional array type X-ray detector which detects the X-rays;a rotating mechanism which rotates the X-ray tube and the X-ray detector around a rotation axis;a reconstruction processing unit which reconstructs image data in a reconstruction range set by an operator based on an output from the X-ray detector;a pair of collimators which form X-rays from the X-ray tube into a cone beam shape; anda collimator moving mechanism which separately moves each of the pair of collimators in a range from an outermost position corresponding to a maximum cone angle corresponding to a width of the X-ray detector, beyond the central plane of the X-rays, at each edge of the reconstruction range, to an innermost position offset from a position corresponding to a cone angle of substantially 0° to an opposite side.