Patent Number: 
Section: claims

1. A radiation computed tomographic imaging apparatus comprising:a radiation source for emitting radiation while rotating around a predefined axis of rotation, wherein said radiation source has an X-ray focal spot;a radiation detector for detecting said radiation passing through a subject around said axis of rotation, said radiation detector having a plurality of radiation detector elements for detecting said radiation, extending in a two-dimensional manner in first and second arrangement directions, said first arrangement direction being contained in a plane of rotation of said radiation source, said second arrangement direction being orthogonal to said first arrangement direction and aligned along said axis of rotation; anda reconstructing device for arithmetically reconstructing tomographic image data for a tomographic image of said subject based on projection data of said subject obtained from said radiation detected by said radiation detector, wherein said radiation detector comprises collimators for confining an angle at which said radiation impinges upon said radiation detector elements, said collimators being provided at borders between said radiation detector elements adjoining in said second arrangement direction, and wherein a first one of said collimators has a first height determined based on a drift of said X-ray focal spot. 2. The radiation computed tomographic imaging apparatus of claim 1, wherein:said plurality of radiation detector elements are arranged in said first arrangement direction to form a curve along the direction of rotation of said radiation source, and flatly arranged in said second arrangement direction. 3. The radiation computed tomographic imaging apparatus of claim 1, wherein:said radiation is emitted from said focal spot of said radiation source;said radiation source and said radiation detector are disposed symmetrically with respect to a line connecting said focal spot and a midpoint in said second arrangement direction; andsaid radiation computed tomographic imaging apparatus further comprises a moving device for rotating said radiation source and said radiation detector around said axis of rotation while maintaining their positional relationship relative to each other. 4. The radiation computed tomographic imaging apparatus of claim 3, wherein:said collimators are provided at positions other than said midpoint in said second arrangement direction. 5. The radiation computed tomographic imaging apparatus of claim 4, wherein:said collimators are provided at regular intervals in said second arrangement direction. 6. The radiation computed tomographic imaging apparatus of claim 1, wherein:the first height is determined based on efficiency of radiation usage by at least one of said radiation detector elements. 7. The radiation computed tomographic imaging apparatus of claim 6, wherein:the first height is smaller than a second height of a second one of said collimators, and wherein said first collimator lying farther from said midpoint in said second arrangement direction than said second collimator. 8. The radiation computed tomographic imaging apparatus of claim 1, wherein:said radiation detector further comprises said collimators at borders between said radiation detector elements adjoining in said first arrangement direction as well. 9. The radiation computed tomographic imaging apparatus of claim 1, wherein the first height is determined based on a threshold exceeded by an efficiency of X-ray usage of at least one of said radiation detector elements. 10. The radiation computed tomographic imaging apparatus of claim 1, wherein said collimators are located at a position other than a center of said radiation detector. 11. A radiation computed tomographic imaging apparatus comprising:a radiation source for emitting radiation while rotating around a predefined axis of rotation;a radiation detector for detecting said radiation passing through a subject around said axis of rotation, said radiation detector having a plurality of radiation detector elements for detecting said radiation, extending in a two-dimensional manner in first and second arrangement directions, said first arrangement direction being contained in a plane of rotation of said radiation source, said second arrangement direction being orthogonal to said first arrangement direction and aligned along said axis of rotation; anda reconstructing device for arithmetically reconstructing tomographic image data for a tomographic image of said subject based on projection data of said subject obtained from said radiation detected by said radiation detector, wherein said radiation detector comprises collimators for confining an angle at which said radiation impinges upon said radiation detector elements, said collimators being provided at borders between said radiation detector elements adjoining in said second arrangement direction, and wherein said reconstructing device corrects a difference in radiation detection sensitivity among said radiation detector elements due to a shadow of said collimators created in emission of a beam of said radiation. 12. The radiation computed tomographic imaging apparatus of claim 11, wherein:said reconstructing device makes said correction based on a value of a sensitivity correction vector selected based on a ratio between a detected value by a first one of reference channels in which said radiation detection sensitivity varies and a detected value by a second one of reference channels in which said radiation detection sensitivity is invariant, said reference channels being those among said plurality of radiation detector elements that always detect said radiation not passing through said subject. 13. The radiation computed tomographic imaging apparatus of claim 12, wherein:elements in said sensitivity correction vector have individual values respectively corresponding to said radiation detector elements in said first arrangement direction. 14. A radiation detector for use in a radiation computed tomographic imaging apparatus for generating tomographic image data for a tomographic image of a subject based on projection data of said subject obtained from radiation emitted from a radiation source rotating around a predefined axis of rotation and passing through said subject, wherein said radiation detector comprises:a plurality of radiation detector elements for detecting said radiation for acquiring said projection data, extending in a two-dimensional manner in first and second arrangement directions, said first arrangement direction being contained in a plane of rotation of said radiation source, said second arrangement direction being orthogonal to said first arrangement direction and aligned along said axis of rotation; andcollimators for confining an angle at which said radiation impinges upon said radiation detector elements, provided at borders between said radiation detector elements adjoining in said second arrangement direction, wherein a first one of said collimators has a first height determined based on a drift of an X-ray focal spot of the radiation source. 15. The radiation detector of claim 14, wherein:said plurality of radiation detector elements are arranged in said first arrangement direction to form a curve along the direction of rotation of said radiation source, and flatly arranged in said second arrangement direction. 16. The radiation detector of claim 14, wherein:said radiation detector is configured to be symmetric with respect to a line connecting the focal spot of said radiation source and a midpoint in said second arrangement direction; andsaid collimators are provided at positions other than said midpoint in said second arrangement direction. 17. The radiation detector of claim 16, wherein:said collimators are provided at regular intervals in said second arrangement direction. 18. The radiation detector of claim 14, wherein:the first height is determined based on efficiency of radiation usage by at least one of said radiation detector elements. 19. The radiation detector of claim 18, wherein:the first height is smaller than a second height of a second one of said collimators, and wherein said first collimator lying farther from said midpoint in said second arrangement direction than said second collimator. 20. The radiation detector of claim 14, further comprising:said collimators at borders between said radiation detector elements adjoining in said first arrangement direction as well.